There is something special about

coral reefs. The warm, clearwater, spectacular colors, andmultitude of living things captivate almosteveryone who sees a reef. Coral reefs rivalthat other great tropical community, therain forest, in their beauty, richness, andcomplexity. Tropical rain forests and coralreefs are also similar in that the basic physicalstructure of both communities is producedby organisms. Both reef-buildingcorals and the giant trees of a rain forestcreate a three-dimensional framework thatis home to an incredible assortment of organisms.Coral reefs are such massivestructures, in fact, that they must be considerednot only biological communitiesbut geological structures, the largest geologicalfeatures built by organisms.A few organisms other than corals,like oysters, polychaete worms, and redalgae can form reefs (see Fig. 12.22).Such reefs are important in some places,but they are minor features compared tocoral reefs. Chapter 14 looks exclusivelyat coral reefs.THE ORGANISMSTHAT BUILD REEFSCoral reefs are made of vast amountsof calcium carbonate (CaCO3), limestone,that is deposited by living things.Of the thousands of species in coralreef communities, only a fraction producethe limestone that builds thereef. The most important of these reefbuildingorganisms, as you might guess,are corals.A coral reef in Indonesia.

Coral Reefsc h a p t e r

14Coral reefCastro−Huber: MarineBiology, Fourth Edition

III. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003

Reef CoralsCorals are cnidarians. Nearly all are inthe class Anthozoa, making them closelyrelated to sea anemones. Unlike manyother cnidarians, they lack a medusa stageand live only as polyps. In reef-buildingcorals, which are sometimes called hermatypiccorals, the polyps produce calciumcarbonate skeletons. Billions ofthese tiny skeletons build a massive reef.There are a few kinds of reef-buildingcorals that are not anthozoans. Fire corals(Millepora), for example, are hydrozoans,more closely related to jellyfishes than seaanemones. They do have a medusa stageduring their life cycle, but like other reefbuildingcorals they grow on the reef incolonies of polyps that have limestoneskeletons. They are called “fire corals” becausetheir powerful nematocysts cause aburning sensation if you touch them.Not all corals help build reefs. Nearlyall soft corals (order Alcyonacea) lack ahard skeleton and, although they are abundanton coral reefs (see Fig. 14.28), theydon’t help build them. Black corals (orderAntipatharia) and gorgonians (orderGorgonacea), including sea fans (seeFig. 7.9) and sea whips, are hard but theirskeletons are made mostly of protein andcontribute little to reef formation. Thesenon-reef-building, or ahermatypic, coralsare common in other habitats as well ascoral reef. Precious corals live mostly indeep-water habitats rather than coral reefs,though they do have a calcareous skeleton.Precious corals are gorgonians.•Reef-building corals are the primary architectsof coral reefs.

•The Coral PolypYou have to look closely to see thelittle polyps that build coral reefs. Coralpolyps are not only small but deceptivelysimple in appearance. They look muchlike little sea anemones, consisting of anupright cylinder of tissue with a ring oftentacles on top (Figs. 14.1 and 14.2b).Like anemones and other cnidarians, theyuse their nematocyst-armed tentacles tocapture food, especially zooplankton. The

tentacles surround the mouth, the onlyopening to the sac-like gut.Most reef-building corals are coloniesof many polyps, all connected by a thinsheet of tissue. The colony starts when aplanktonic coral larva, called a planula,settles on a hard surface. Coral larvaegenerally do not settle on soft bottoms.Immediately after settling, the larvatransforms, or metamorphoses, into apolyp. This single “founder” polyp, if itsurvives, divides over and over to formthe colony. Thus, all the polyps in a coralcolony are genetically identical copies, orclones, of the founder polyp. The digestivesystems of the polyps usually remainconnected, and they share a commonnervous system (Fig. 14.3). A few reefbuildingcorals consist of only a singlepolyp (Fig. 14.2a).298 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyTissue connectionsbetween polyps Tentacles PolypsGut cavityof polypMesenterialfilamentsMouthBareskeletonof polyp

FIGURE 14.1 Cutaway view of one of the polyps in a coral colony and of the calciumcarbonate skeleton underneath. The polyps are interconnected by a very thin layer of tissue.FIGURE 14.2 Coral polyps. (a) A fewcorals, like the mushroom coral (Fungia) fromthe Indo-West Pacific, consist of a single polyp.Most are colonies of many individual polyps.(b) In some of these, like the large-cuppedboulder coral (Montastrea cavernosa) from theCaribbean, each polyp has its own individualcup, called a corallite. (c) In coral colonies likethe brain coral (Diploria) from the Caribbean,meandering lines of polyps lie in a commoncorallite.(a) (b)(c)Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003

•Most corals are colonies of many identical polyps.

•Coral polyps lie in a cup-like skeletonof calcium carbonate that they makethemselves. Over the years a series ofpolyps, each laying down a new layer of

calcium carbonate, builds up the skeleton(Fig. 14.4). The skeleton forms the bulkof the colony (Fig. 14.5), which can takemany different shapes (Fig. 14.6). Theactual living tissue is only a thin layer onthe surface. It is the calcium carbonatecoral skeletons, growing upward and outwardwith the coral, that create theframework of the reef.Nearly all reef-building corals containsymbiotic zooxanthellae (Fig. 14.7).In fact, hermatypic corals are sometimesdefined as those with zooxanthellaerather than as those that build reefs.Chapter 14 Coral Reefs 299Corals are amazingly adaptable animals. They come in all shapes andsizes and have many ways to feed themselves. It should come as nosurprise, then, that they also have more than one way to reproduce.In one way, growth and reproduction are the same thing incorals. The coral colony grows as its individual polyps divide to formnew polyps. Thus, the colony grows as the polyps reproduce. Theprocess crosses the fine line between growth and reproduction whena piece of coral breaks off and continues to grow. It is now a separatecolony, though it is a genetically identical clone of its “parent.”Certain species of coral may depend heavily on this form of reproductionand may even be adapted to break easily. After a reef isdamaged by a severe storm, an important part of its recovery is thegrowth of pieces of shattered coral colonies.Corals can also reproduce sexually. Like other animals, theyproduce eggs and sperm, which fuse and eventually develop intoplanula larvae, the characteristic larvae of cnidarians. Some coralsare hermaphrodites, and make both eggs and sperm, whereas otherspecies have separate sexes. The method of fertilization also variesamong corals. In some, whether or not they are hermaphrodites,the egg is fertilized and develops inside the polyp. Most corals,however, are broadcast spawners and release the eggs and sperminto the water.One of the most spectacular findings about coral reefs in recentyears was the discovery of annual mass spawning of corals on theGreat Barrier Reef. For a few nights a year between October andearly December, a whole variety of coral species spawns at once. Theevent takes place just after a full moon. At a given place, the time ofmass spawning can usually be predicted down to the night. It mayseem surprising that this was only recently discovered—the first scientificreport appeared in 1984—but no one was really looking!Since its discovery on the Great Barrier Reef, mass spawning hasalso been found to occuron a number of other reefsaround the world.The eggs and spermmay be released directlyinto the water or enclosedin little bundles that are releasedthrough the mouth.Depending on the coralspecies, the bundles maycontain both eggs andsperm, or only one or theother. The bundles float to

the surface and break up,allowing the eggs andsperm to mix.Nobody knows whythe corals all spawn together.Maybe the eggpredators get so full thatmost of the eggs go uneaten.Maybe it has somethingto do with the tides.There may be an explanationthat no one hasthought of. Another interestingthing is that althoughthe mass spawninghappens on some reefs it does not occur on others. Is there somethingdifferent about these reefs? The answers to these questions arelikely to occupy coral reef biologists for some time to come.CORAL REPRODUCTIONThe release of sperm and egg bundlesduring the mass coral spawning on theGreat Barrier Reef. Photographscourtesy of the Great Barrier ReefMarine Park Authority.Cnidarians Animals in the phylumCnidaria; they have radial symmetry,tissue-level organization, and tentacleswith nematocysts, specialized stingingstructures.Life stages of cnidarians:Polyp The sessile, sac-likestage, with the mouth andtentacles on top.Medusa The swimming,bell-like stage, with the mouthand tentacles underneath.Chapter 7, p. 120Plankton Primary producers(phytoplankton) and consumers(zooplankton) that drift with the currents.Chapter 10, p. 230; Figure 10.19Zooxanthellae Dinoflagellates(single-celled, photosynthetic algae)that live within animal tissues.Chapter 5, p. 99Symbiosis The living together in closeassociation of two different species,often divided into parasitism, where onespecies benefits at the expense of theother; commensalism, where one speciesbenefits without affecting the other; andmutualism, where both species benefit.Chapter 10, p. 220Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003

300 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyFIGURE 14.4 This coral (Lobophyllia hemprichii) shows particularly well how corals buildup their skeleton. Each of the irregular rings is a single polyp that has built up a column ofcalciumcarbonate skeleton.FIGURE 14.3 The polyps in a coralcolony are interconnected. If you touch anextended polyp, it will contract, and so will itsneighbors, followed by their neighbors, and soon, so that a wave of contraction passes overthe colony. In this colony of the coralGoniopora lobata the wave of contraction ismoving up from the bottom.FIGURE 14.5 The calcium carbonateskeleton makes up most of a coral colony. Alive colony (Pocillopora verrucosa) is shown onthe right. On the left is a colony with the livingtissue removed. You can see that the maindifference is the color; the live part is only athin layer of tissue on the surface. The “corals”sold in shell and aquarium shops are actuallyonly the skeletons of live corals that were takenfrom the reef and bleached. Some reefs havebeen devastated by the collectors who supplythese shops in order to feed their families.EncrustingBranchingMassiveColumnarFoliaceousPlate-likeFree-living

FIGURE 14.6 Corals come in a multitude of shapes, or growth forms.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003A few reef-building corals, however, lackzooxanthellae. There are also some deepwatercorals without zooxanthellae thatslowly build up calcium carbonate moundson the ocean floor. These mounds aretechnically not reefs, though they are oftenreferred to as such.Without zooxanthellae corals producetheir skeletons very slowly, notnearly fast enough to build a reef. Thezooxanthellae enable the coral to depositcalcium carbonate much faster. It is thezooxanthellae as much as the corals thatconstruct the reef framework; withoutthem there would be no coral reefs.Coral NutritionZooxanthellae also provide vital nourishmentto the coral. They perform photosynthesisand pass some of the organicmatter they make on to the coral. Thus,

the zooxanthellae feed the coral from theinside. Many corals can survive and growwithout eating, as long as the zooxanthellaehave enough light.•Almost all reef-building corals contain symbioticzooxanthellae. The zooxanthellae nourishthe corals and help them produce their skeletons.

•Although corals get much of theirnutrition from their zooxanthellae, mosteat when they get the chance. They preyvoraciously on zooplankton. The billionsof coral polyps on a reef, along with allthe other hungry reef organisms, are veryefficient at removing zooplanktonbrought in by currents. Indeed, the reefhas been called a “wall of mouths.”Coral polyps catch zooplankton withtheir tentacles or in sheets of mucus thatthey secrete on the colony surface. Tiny,hair-like cilia gather the mucus intothreads and pass them along to themouth. Some corals hardly use their tentaclesand rely on the mucus method. Afew corals have even lost their tentaclesaltogether.Corals have still other ways of feedingthemselves. There are a number oflong, coiled tubes called mesenterial filamentsattached to the wall of the gut(Fig. 14.1). The mesenterial filaments secretedigestive enzymes. The polyp canextrude the filaments through the mouthor body wall to digest and absorb foodparticles outside the body. Corals also usethe mesenterial filaments to digest organicmatter from the sediments. In addition,corals are among the relatively fewanimals that can absorb dissolved organicmatter (DOM) (see “The TrophicPyramid,” p. 223).•Corals nourish themselves in a remarkablenumber of ways. Zooxanthellae are the mostimportant source of nutrition. Corals can alsocapture zooplankton with tentacles or mucusnets, digest organic material outside the bodywith mesenterial filaments, or absorb dissolvedorganic matter (DOM) from the water.

•Other Reef BuildersAlthough they are the chief architects,corals cannot build a reef alone. Many

other organisms help make a coral reef.The most important of these are algae,which are essential to reef growth. Infact, some marine biologists think thatcoral reefs should be called “algal reefs”or, to be fair to both, “biotic reefs.” Onereason for this is that zooxanthellae,which are algae, are essential to thegrowth of corals. There are other algae,however, that also have key roles inbuilding the reef. Like corals, corallinered algae produce a “skeleton” of calciumcarbonate. Encrusting coralline algae(Porolithon, Lithothamnion) grow in rockhardsheets over the surface of the reef.They deposit considerable amounts ofcalcium carbonate, sometimes more thancorals, and thus contribute to reefgrowth. Coralline algae are more importanton Pacific reefs than Atlantic ones.Encrusting coralline algae not onlyhelp build the reef, they also help keep itfrom washing away. The stony pavementformed by these algae is tough enough towithstand waves that would smash eventhe most rugged corals. The algae form adistinct ridge on the outer edge of manyreefs, especially in the Pacific. This algalridge absorbs the force of the waves andprevents erosion from destroying the reef.Encrusting algae do yet another jobthat is vital to reef growth. Coral skeletonsand fragments create an open network,full of spaces, that traps coarsecarbonate sediments (Fig. 14.8). Sediment,especially fine sediment, damagescorals when it settles directly on them,but the buildup of coarse sediment in thereef framework is an essential part of reefgrowth. The structure of a reef is formedas much by the accumulation of calcareoussediment as by the growth of corals.Encrusting algae grow over the sedimentas it builds up, cementing the sediment inplace. Thus, encrusting coralline algae arethe glue that holds the reef together.Some invertebrates, notably sponges andbryozoans, also form encrusting growthsthat help bind the sediments.•Encrusting coralline algae help build the reefby depositing calcium carbonate, by resistingwave erosion, and by cementing sediments.

•Chapter 14 Coral Reefs 301

FIGURE 14.7 The microscopic,golden-brown zooxanthellae that pack thesepolyp tentacles help corals build reefs. Theyalso occur in sea anemones, giant clams, andother animals.Photosynthesis CO2 H2O sun energy →glucose O2Chapter 4, p. 71Bryozoans Small, colonial, encrustinganimals that make delicate, often lacelike,skeletons.Chapter 7, p. 139; Figure 7.40Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003Nearly all the sediment that accumulatesto help form the reef comes fromcoral fragments, or coral rubble, and theshells or skeletons of other organisms. Inother words, nearly all the sediment isbiogenous. Probably the most importantof all the sediment-forming organisms isa calcareous green alga called Halimeda(Fig. 14.9). Halimeda deposits calciumcarbonate within its tissues to providesupport and to discourage grazers—amouthful of limestone is pretty unappetizing.The remnants of Halimeda accumulateon reefs in huge amounts, to bebound together by encrusting organisms.Many other organisms make calciumcarbonate sediments and thus contributeto the growth of the reef. The shells offorams, snails, clams, and other molluscsare very important. Sea urchins, bryozoans,crustaceans, sponges, bacteria anda host of other organisms add or helpbind carbonate sediments. Reef growth istruly a team effort.•The accumulation of calcium carbonate sedimentsplays an important role in reef growth.A calcareous green alga, Halimeda, and coralrubble account for most of the sediment, butmany other organisms also contribute.

•Conditions for Reef GrowthOther organisms may be important, butcoral reefs do not develop without corals.Corals have very particular requirementsthat determine where reefs develop.Reefs are rare on soft bottoms, for example,because coral larvae need to settle ona hard surface.

302 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyLoose sedimentand coral rubble(a)(b)(c)Consolidationof sedimentReefgrowthHard substrateHard substrateHard substrate

FIGURE 14.8 Reef growth involves several processes. (a) The framework is made whenreef-building corals settle and grow on some hard surface, usually a preexisting reef. (b) Thespacesin this framework are partially filled in by coarse carbonate sediments. (c) When the sediments areglued together by encrusting organisms, new reef “rock” is formed and the reef has grown. On areal reef all three steps go on at the same time. Reefs are not actually completely solid as depictedhere but are porous, with many holes and crevices that serve as home to a host of organisms.FIGURE 14.9 This calcareous greenalga (Halimeda) is one of the main sedimentformingorganisms on most reefs. About 95%of the plant’s weight is calcium carbonate, andthere is only a thin layer of live tissue on theoutside. When the tissue dies the segmentsseparate, each leaving a piece of limestone.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003Light and TemperatureCorals can grow only in shallow water,where light can penetrate, because thezooxanthellae on which they dependneed light. Calcareons algae also requiresunlight. Particular types of coral andalgae have different depth limits—somecan live deeper than others—but reefsrarely develop in water deeper thanabout 50 m (165 ft). Because of this,coral reefs are found only on the continentalshelves, around islands, or on topof seamounts. Many types of coral live indeep water and do not need light, butthese corals do not contain zooxanthellaeor build reefs. Corals also prefer clearwaters, since water clouded with sedimentor plankton does not allow light topenetrate very well.Reef-building corals are limited towarm water and can grow and reproduceonly if the average water temperature isabove about 20°C (68°F). Most reefs growin considerably warmer areas. Figure 14.10illustrates the relationship between coral

reefs and water temperature.•Corals need light and warm temperatures, soreefs grow only in shallow, warm waters.

•Water that is too warm is also bad forcorals. The upper temperature limitvaries, but is usually around 30° to 35°C(86° to 95°F). The first outward sign ofheat stress, or stress of other kinds, isbleaching, in which the coral expels itszooxanthellae (see Fig. 18.3). It is calledbleaching because the golden-brown orgreenish zooxanthellae give the coral mostof its color; without them, the coral is almostwhite. Corals also slough off largeamounts of slimy mucus when stressed. Ifthe warm conditions last too long or thetemperature gets too high the coral dies.The exact temperature range preferredby corals differs from place to placebecause corals from a particular locationadapt to the normal temperatures there(Fig. 14.11). Corals from places with verywarm water, for example, are able to toleratehigher temperatures. In someplaces, corals must adapt to drastic fluctuationsin temperature. For instance,reefs grow in parts of the Persian Gulfwhere the water temperature ranges from16° to 40°C (60° to 104°F).The upper temperature tolerances ofcorals are usually not far above the normaltemperature range at the place where theylive. Corals suffer when exposed to temperaturesoutside this normal range. Thissometimes happens during extreme lowtides, when shallow pools on the reef maybe cut off from circulation. Heated by thesun, the water can warm up to fatal temperatures.By discharging heated water,electric power plants can also kill corals.El Niño (see “The El Niño-SouthernOscillation Phenomenon,” p. 351), bringsunusually warm water to many parts ofthe ocean. Widespread coral bleachingChapter 14 Coral Reefs 30303060306018015012090603006030030606090120150609012015018015012090603000

0 1000 2000 3000 Kilometers1000 2000 3000 MilesBermudaWestIndiesBelizeHawaiianMarshall IslandsIslandsFijiPapuaNew GuineaGuamJapanIndonesiaSuvadivaMaldiveIslandsNew CaledoniaGreatBarrierReefGalápagosIslandsFrenchPolynesiaBrazilPersian GulfRedSeaIndianOceanPacificOceanAtlanticOceanReef coral communities Average 20C isothermAmazon R.

FIGURE 14.10 The geographic distribution of reef-building corals, like that of kelps, isrelated to temperature. Reef corals require warmwater, however, whereas kelps need cold water. Compare the distribution of reefs with that of kelps shown in Figure 13.21. Note the effects of warmsurface currents: Reefs extend farther north and south on the east sides of continents than onthe west sides.Foraminiferans (Forams) Protozoans,often microscopic, with a calciumcarbonate shell.Chapter 5, p. 101; Figure 5.10Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003and mortality also occur during El Niñoevents. During the unusually strong ElNiño of 1997–98, severe bleaching occurredon many reefs around the world,probably as a direct result of the unusuallywarm water. In some places, includingthe Caribbean and parts of theGreat Barrier Reef, bleaching did notkill many corals, and reefs quickly recoveredafter the water cooled. In otherareas, such as the Indian Ocean, SoutheastAsia, and the far western Pacific,many corals died after bleaching andsome reefs were severely damaged.Some of these may be recovering as newcoral larvae settle and grow, but it is tooearly to tell.High water temperatures have ledto several other major, though more localized,

bleaching events since the 1997–98El Niño. It is possible that such events occurredin the past but were not reportedbecause many reefs are in remote locationsthat were rarely observed by scientists.Today, scientists continually monitor seawatertemperatures by satellite and can immediatelyreport bleaching when it occursvia the Internet. It is also possible that thehigh water temperatures and coral bleachingof the past several years are just normal,random fluctuations. Both El Niñoand coral bleaching are natural events thathave occurred for millenia. Increasingly,however, coral reef scientists are concernedthat warm-water episodes and bleachingevents are becoming more frequent, andmore intense, as a result of global climatechange (see “Living in a Greenhouse: OurWarming Earth,” p. 410). If so, thisthreatens coral reefs around the globe, especiallybecause they are already faced withmany other human-induced stresses (see“Coral Reefs,” p. 408).Sediments, Salinity, and PollutionFine sediment like silt is very harmful tocorals. It clouds the water, cutting downlight for the zooxanthellae. Even a thinlayer of sediment on the colony surfacesmothers the coral. To remove the sediment,corals use mucus opposite to theway in which they use it to feed. Insteadof being brought to the mouth, the mucusis sloughed off, carrying sediment with it.Even with this defense, corals generallydon’t do well in places where there is alot of sediment, unless there is enoughwave or current action to wash the sedimentaway (Fig. 14.12). Reefs tend to bepoorly developed, for example, near rivermouths. This is not only because of thesediment brought in by rivers but becausecorals are also quite sensitive to reducedsalinity caused by the freshwater input.Human activities like mining, logging,construction, and dredging can greatly increasethe amount of sediment and freshwaterrunoff. These factors may have veryharmful effects on local reefs.304 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyN o r t hP a c i f i cO c e a nPhilippinesJapanNorthAmerica

MarshallIslandsHawaiianIslandsAlaskaHawaiianIslandsMarshallIslands95TemperatureF C3586307725Mean highwatertemperatureLethal limitsfor coralsEquator

FIGURE 14.11 The upper temperature limit that corals can stand is related to thetemperature at their home sites. For example, the water in the Marshall Islands is warmer onaverage than that in Hawai‘i. During the hottest months of the year the average high temperature isseveral degrees higher in the Marshalls. Corals from the Marshalls can tolerate correspondinglyhigher temperatures. Why do you think the highest temperature that the corals can tolerate wouldbe higher than the average high temperature at their location?FIGURE 14.12 Corals often flourishwhere there is plenty of wave action. The watermotion keeps sediment from settling on thecorals and brings in food, oxygen, and nutrients.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003Corals are also sensitive to pollutionof many kinds. Even low concentrationsof chemicals like pesticides and industrialwastes can harm them. The larvae are especiallysensitive. In high concentrations,nutrients also can be harmful to reefgrowth. Humans release tremendousamounts of nutrients in sewage and infertilizers that are washed from farmlandand carried to the sea. The nutrients mayharm the corals directly by interferingwith the formation of their skeletons.More importantly, increased nutrientscan alter the ecological balance of thecommunity. Most coral reefs grow inwater that is naturally low in nutrients. Insuch nutrient-poor water, seaweeds donot grow very rapidly and are kept undercontrol by grazers. This allows corals tocompete successfully for space and light.When nutrients are added, seaweeds maygrow much faster and shade and chokeout the slow-growing corals. This is aparticular problem when, as is often the

case, grazing fishes have been removed byfishing (see “Grazing,” p. 318).•Corals are very sensitive to fine sediment, freshwater, and pollution, including pollution byhigh nutrient levels.

•The Kane‘ohe Bay StoryOne of the best-known examples of theharmful effects of nutrient enrichmentoccurred in a partially enclosed bay in theHawaiian Islands. Kane‘ohe Bay, locatedon the northeast shore of the island ofO‘ahu, once had some of the most luxuriantreefs in Hawai‘i. Until the 1930s thearea around the bay was sparsely populated.In the years leading up to WorldWar II, with the military buildup ofO‘ahu, the population began to increase.This increase continued after the war asthe shores of the bay were developed forresidential use.The sewage from this expandingpopulation was dumped right into thebay. By 1978 about 20,000 m3 (over5 million gallons) of sewage were beingdumped into the bay every day. Long beforethen, by the mid-1960s in fact, marinebiologists began to notice disturbingchanges in the middle part of the bay.Loaded with nutrients, the sewage actedas a fertilizer for seaweeds. A green alga,the bubble alga (Dictyosphaeria cavernosa),found the conditions particularly to itsliking and grew at a tremendous rate, literallycovering the bottom in many partsof the bay. Bubble algae began to overgrowand smother the corals. Phytoplanktonalso multiplied with the increase innutrients, clouding the water. Kane‘oheBay’s reefs began to die. Such acceleratedalgal growth due to nutrient input iscalled eutrophication (see “Eutrophication,”p. 410).For a time it appeared that the storyhad a happy ending. As the once beautifulreefs smothered, scientists and the generalpublic began to cry out. It took a while,but in 1978 public pressure finally managedto greatly reduce the discharge ofsewage into the bay, and the sewage wasdiverted offshore. The result was dramatic.Bubble algae died back in much of the bay,and the bay’s corals began to recover muchfaster than anyone had expected. By the

early 1980s, bubble algae were fairly scarceand corals had started to grow again. Thereefs were not what they once were, butthey seemed to be on track to recover.Then the ghost of pollution pastreared its ugly head. In November 1982,Hurricane Iwa struck Kane‘ohe Bay. Duringthe years of pollution a layer of thecoral skeleton had weakened, becomingfragile and crumbly. When the hurricanehit, this weak layer collapsed and manyreefs were severely damaged. Fortunately,the corals were already beginning to recover,and the broken pieces were able togrow back. If the hurricane had hit duringthe years of pollution, the reefs of Kane‘oheBay—and the benefits of fishing, tourism,and recreation—might have disappearedforever.The rapid recovery of Kane‘ohe Bay’sreefs that was seen during the early 1980sdid not continue. By 1990 the recoveryseemed to have leveled off. Some areaseven began to decline again, with bubblealgae once more becoming abundant.There are a number of possible explanationsfor this. Even though most sewageis now discharged outside the bay, somenutrients continue to enter from boats,the septic tanks and cesspools of privatehomes, and other sources. Not only that,nutrients from the old sewage outfallswere stored in the sediments, and are stillbeing released even after 30 years. Thereis also evidence that fishing has reducedpopulations of fishes that graze on bubblealgae. Furthermore, the grazing fishesthat remain prefer to eat other species ofseaweed that have been introduced fromoutside Hawai‘i, and have shifted awayfrom eating bubble algae. Thus, bubblealgae may be abundant because they arenot being eaten as fast as they once were.Another introduced seaweed, which likebubble algae is not a preferred food ofherbivorous fishes, has also started toproliferate and smother corals in the bay(Fig. 14.13). It will be much harder to restorethe coral gardens of Kane‘ohe Baythan it was to destroy them.The Kane‘ohe Bay story is far fromunique. Most of the world’s tropicalcoasts are undergoing increasing developmentand population growth. As a result,more and more nutrients are ending up inthe waters that support coral reefs, and

there are many reports of reefs beingthreatened by eutrophication. On theother hand, new research is showing thatthe effects of added nutrients on coralreefs are much more complicated than weonce thought. A few experiments have indicatedthat algal growth is not nutrientlimitedon at least some reefs. There areeven suggestions that in some cases addednutrients may be good for the zooxanthellaeand help corals grow faster. The bulkof the evidence, however, is that eutrophicationis damaging, especially when populationsof algal grazers are reduced. Manyreef biologists consider it one of the mostserious threats to the world’s coral reefs.KINDS OFCORAL REEFSCoral reefs are usually divided into threemain categories: fringing reefs, barrierreefs, and atolls. Some reefs do not fitneatly into any particular category, andsome fall between two categories. Still,the division of reefs into the three majortypes works well for the most part.•The three main types of reefs are fringing reefs,barrier reefs, and atolls.

•Chapter 14 Coral Reefs 305Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003

Fringing ReefsFringing reefs are the simplest and mostcommon kind of reef. They develop nearshore throughout the tropics, whereverthere is some kind of hard surface for thesettlement of coral larvae. Rocky shorelinesprovide the best conditions forfringing reefs. Fringing reefs also grow onsoft bottoms if there is even a small hardpatch that lets the corals get a foothold.Once they get started, the corals createtheir own hard bottom and the reefslowly expands.As their name implies, fringing reefsgrow in a narrow band or fringe alongthe shore (Fig. 14.14). Occurring closeto land, they are especially vulnerable tosediment, freshwater runoff, and humandisturbance. Under the right conditions,

however, fringing reefs can be impressive.In fact, the longest reef in the world(though not the one with the largestcoral area) is not the famous Great BarrierReef in Australia but a fringing reefthat runs some 4,000 km (2,500 mi)along the coast of the Red Sea. Part ofthe reason that this reef is so well developedis that the climate is dry and thereare no streams to bring in sediment andfresh water.The typical structure of a fringingreef is shown in Figure 14.14. Dependingon the place, the shore may be steepand rocky or have mangroves or a beach.The reef itself consists of an inner reefflat and an outer reef slope. The reefflat is the widest part of the reef. It isshallow, sometimes exposed at low tide(Fig. 14.15), and slopes very gently towardthe sea. Being closest to land, it isthe part of the reef most strongly affectedby sediments and freshwaterrunoff. The bottom is primarily sand,mud, or coral rubble. There are someliving corals, but neither as manycolonies nor as many different kinds ason the reef slope. Seaweeds, seagrasses,and soft corals may also occupy the reefflat, sometimes in dense beds.The reef slope can be quite steep,nearly vertical in fact. It is the part of thereef with the densest cover (Fig. 14.16)and the most species of coral, becausethe slope is away from shore and thereforeaway from the effects of sedimentand fresh water. Also, the waves thatbathe the slope provide good circulation,bring in nutrients and zooplankton, andwash away fine sediments. The reefcrest is the shallow edge of the reefslope. The crest usually has even moreluxuriant coral growth than the rest ofthe reef slope. If there is intense waveaction, however, the crest may consist ofan algal ridge, with the richest coralgrowth just below. Because there is lesslight in deep water, the deepest part ofthe reef slope usually has less live coraland fewer coral species.•Fringing reefs grow close to shore and consist ofan inner reef flat and an outer reef slope.

•

306 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyBeachReef flatReefslopeSediment andrubbleReef crestMangrovesRocky shore

FIGURE 14.14 Typical structure of a fringing reef. Fringing reefs, like this one in theBismarck Archipelago in the southwest Pacific (photo), can grow right up to the shore.FIGURE 14.13 The red alga Kappaphycus striatum, introduced to Kane‘ohe Bay from thePhilippines, has gotten out of control. The arrow shows one of many clumps of K. striata in thispicture that are growing over and smothering reef corals.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003Large amounts of sediment and coralrubble tumble down the reef slope andsettle at the base. As this material buildsup, reef organisms may begin to grow onit, depending on the water depth andother factors. Thus, the reef can growoutward, as well as upward. Beyond thebase of the slope, the bottom is usuallyfairly flat and composed of sand or mud.On many Caribbean reefs, turtle grass(Thalassia testudinum) dominates the bottombeyond the slope.Barrier ReefsThe distinction between barrier reefs andfringing reefs is sometimes unclear becausethe two types grade into one another. Likefringing reefs, barrier reefs lie along thecoast, but barrier reefs occur considerablyfarther from shore, occasionally as far as100 km (60 mi) or more. Barrier reefs areseparated from the shore—which may alsohave a fringing reef—by a relatively deeplagoon (Fig. 14.17). Largely protectedfrom waves and currents, the lagoon usuallyhas a soft sediment bottom. Inside thelagoon, coral formations variously knownas patch reefs, coral knolls, or pinnaclesdepending upon their size and shape maygrow up nearly to the surface.The barrier reef consists of a backreefslope, a reef flat, and a fore-reefslope, which corresponds to the reefslope of a fringing reef and has a reefcrest (Fig. 14.17). The back-reef slopemay be gentle or as steep as the fore-reefslope. It is protected from waves by therest of the reef, but waves wash large

amounts of sediment from the reef downthe slope. As a result, coral growth isoften not as vigorous on the back-reefslope as on the fore-reef slope. This is notalways true; some back-reef slopes, especiallygentle ones, have luxuriant coralgrowth (Fig. 14.18).The reef flat, like that on fringingreefs, is a shallow, nearly flat platform.Sand and coral rubble patches are interspersedwith seagrass or seaweed beds,soft corals, and patches of dense coralcover. Waves and currents may pile upsand to form small sand islands calledsand cays or, in the United States, keys.The richest coral growth is usuallyat the outer reef crest. There may be aChapter 14 Coral Reefs 307FIGURE 14.15 The upward growth of most reefs is limited by the tides. Whenextreme low tides occur, shallow areas like this reef flat on the Great Barrier Reef are exposed. Ifthe corals are only exposed for a short time, they can survive, but they will die if exposed for toolong. It is this occasional exposure at extreme low tides that keeps reef flats flat because all thecorals above a certain depth are killed. Photograph courtesy of the Great Barrier Reef MarinePark Authority.FIGURE 14.16 The elkhorn coral (Acropora palmata) is a dominant coral on the reefslopes of fringing reefs in the Caribbean and Florida. Its broad branches rise parallel to thesurface to collect light. This colony suffers from a disease known as white-band disease (see “CoralReefs,” p. 408).Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003well-developed algal ridge if the reef isexposed to wave action, with coralgrowth most luxuriant just below thecrest. Exposed fore-reef areas often havea series of finger-like projections alternatingwith sand channels (Figs. 14.17 and14.19). There is still debate about whatcauses these formations, known as spurand-groove formations or buttresses.The wind, waves, or both, are definitelyinvolved, because spur-and-groove formationsdevelop primarily on reef slopesthat are exposed to consistent strongwinds. These formations are found onatolls and some fringing reefs as well asbarrier reefs.Fore-reef slopes vary from relativelygentle to nearly vertical. The steepnessdepends on the action of wind and waves,the amount of sediment flowing down theslope, the depth and nature of the bottom

at the reef base, and other factors. As withother types of reefs, the abundance anddiversity of corals on the fore-reef slopegenerally decreases with depth. Thegrowth form of the corals also changesdown the slope. At the crest, under thepounding of the waves, the corals aremostly stout and compact; many are massive(see Fig. 14.6). Below the crest thereis great variety in form. Whether theyform branches, columns, or whorls, coralsin this zone often grow vertically upward.This may be an adaptation for competition.Corals that grow upward like skyscrapersrather than outward need lessspace to attach. They are also less likely tobe shaded, and if they spread out at thetop, can shade out other corals. Deeper onthe reef slope, corals tend to grow in flatsheets, which probably helps them collectmore light (see Fig. 10.1).308 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyLagoonBack-reefslopePatch reefsReefflatReefcrestFore-reefslopeSand caySpur and groove

FIGURE 14.17 Typical structure ofa barrier reef.FIGURE 14.18 A rich back-reef slope on a Pacific barrier reef.FIGURE 14.19 A portion of the spur-and-groove formations atEnewetak, an atoll in the Marshall Islands, at low tide.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003The largest and most famous barrierreef is the Great Barrier Reef. It runsmore than 2,000 km (1,200 mi) along thenortheastern coast of Australia, varyingin width between about 15 and 350 km(10 to 200 mi) and covering an area ofover 225,000 km2 (80,000 mi2). Thoughnot the longest reef in the world, it coverssuch a large area and is so complex andwell developed that it is generally regardedas the largest reef structure. Actually,the Great Barrier Reef is not a singlereef but a system of more than 2,500smaller reefs, lagoons, channels, islands,

and sand cays (Fig. 14.20).The largest barrier reef in theCaribbean lies off the coast of Belize,Central America. Other major barrierreefs include the Florida Reef Tract andbarrier reefs associated with the islands ofNew Caledonia, New Guinea, and Fiji inthe Pacific. There are many other smallerbarrier reefs, especially in the Pacific.Like the Great Barrier Reef, these usuallyare not single reefs but complex systemsof smaller reefs.AtollsAn atoll is a ring of reef, and often islandsor sand cays, surrounding a centrallagoon (Figs. 14.21 and 14.22). The vastmajority of atolls occur in the Indo-WestPacific region, that is, the tropical Indianand western Pacific oceans. Atolls arerare in the Caribbean and the rest of thetropical Atlantic Ocean. Unlike fringingand barrier reefs, atolls can be found farfrom land, rising up from depths of thousandsof meters or more. With practicallyno land around, there is no river-bornesilt and very little freshwater runoff.Bathed in pure blue ocean water, atollsdisplay spectacular coral growth andbreathtaking water clarity. They are adiver’s dream.Atoll StructureAtolls range in size from small rings lessthan a mile across to systems well over30 km (20 mi) in diameter. The twolargest atolls are Suvadiva, in the MaldiveIslands in the Indian Ocean, and Kwajalein,one of the Marshall Islands in thecentral Pacific. These atolls cover areas ofmore than 1,200 km2 (700 mi2). Atollsmay include a dozen or more islands andbe home to thousands of people.An atoll’s reef flat is much like thereef flat on a fringing or barrier reef: aflat, shallow area. Unlike those of barrierreefs, however, the atoll’s reef flat is rarelymore than a kilometer or so wide. Thefore-reef and back-reef slopes can now bethought of as outer and inner slopes, respectively,since they extend all the wayaround the ring-shaped atoll.Chapter 14 Coral Reefs 309FIGURE 14.20 The Great Barrier Reef is a complicated system of thousands of smallreefs, sand cays, and lagoons. Shown here are four reefs that are part of the Great Barrier ReefMarine Park. Photograph courtesy of the Great Barrier Reef Marine Park Authority.FIGURE 14.21 Fulanga atoll, in the South Pacific nation of Fiji.

Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003The reef crest of an atoll is stronglyinfluenced by wind and waves. Becausemost atolls lie in the zone of the tradewinds, the wind usually comes from aconsistent direction. Consequently, thewind affects various parts of the atoll indifferent ways. Encrusting coralline algae,which can endure the constant poundingof the waves, build a distinct algal ridgeon the reef crest of the windward side ofthe atoll, the side that faces the prevailingwind. On the few Caribbean atolls, thecoralline algae may be replaced by especiallywave-resistant corals. The leeward,or sheltered, side of the atoll has no algalridge. Spur-and-groove formations, too,develop only on the windward side.The fore-reef, or outer, slope isnearly vertical, though there is usually aseries of ledges and overhangs. The reefwall continues down to great depths. Thewater may be hundreds, even thousands,of meters deep just a stone’s throw fromthe reef.The lagoon, on the other hand, is relativelyshallow, usually about 60 m (195 ft)deep. The bottom of a lagoon is very uneven,with many depressions and coral pinnacles.Some pinnacles rise almost to thesurface, where they may form “mini-atolls,”small rings of coral within the lagoon.•Atolls are rings of reef, with steep outer slopes,that enclose a shallow lagoon.

•How Atolls GrowWhen atolls were discovered, scientistswere at a loss to explain them. It wasknown that corals can grow only in shallowwater, yet atolls grow right in the middle ofthe ocean, out of very deep water. Therefore,the atoll could not have grown upfrom the ocean floor. If the reefs grew onsome kind of shallow structure that was alreadythere, like a seamount, why is thereno sign of it? The islands on atolls are simplesand cays that have been built by theaccumulation of reef sediments and wouldnot exist without the reef. They are productsof the reef and could not provide the

original substrate for reef growth. Finally,why do atolls always form rings?The puzzle of atoll formation wassolved by Charles Darwin in the midnineteenthcentury. Darwin is most famous,of course, for proposing the theoryof evolution by natural selection, but histheory of atoll formation is also an importantcontribution to science.Darwin reasoned that atolls could beexplained by reef growth on a subsidingisland. The atoll gets its start when adeep-sea volcano erupts to build a volcanicisland. Corals soon colonize theshores of the new island, and a fringingreef develops (Fig. 14.23a). As with mostfringing reefs, coral growth is most vigorousat the outer edge of the reef. Theinner reef is strongly affected by sedimentand runoff from the island.310 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyReef flatSand cayWindFore-reef(outer)slopeAlgalridgeBack-reef(inner) slopeBack-reef(inner) slopePinnacleFore-reef(outer)slopeLagoonSpur andgroove

FIGURE 14.22 Typical structure of an atoll.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003Surprisingly, Darwin’s explanation ofhow atolls are created was pretty muchignored for a century while scientists proposedvarious other hypotheses. None ofthese alternative hypotheses held up. Finallyscientists found conclusive evidencethat Darwin was right. Unlike other hypothesesfor atoll formation, Darwin’shypothesis predicted that, below thethick calcium carbonate cap formed bythe reef, there should be volcanic rock—the original island. In the 1950s theUnited States Geological Survey drilledseveral deep holes on Enewetak atoll in

the Marshall Islands. These cores revealedexactly what Darwin predicted:volcanic rock far beneath the calcium carbonateof the reef. The thickness of thecarbonate cap is impressive. The volcanicisland that underlies Enewetak is coveredby more than 1,400 m (4,600 ft) of calciumcarbonate!Scientists now believe almost unanimouslythat Darwin’s hypothesis of atollformation is correct. There are, of course,a few details to be added to the picture, inparticular the effects of changes in sea level(see “Climate and Changes in Sea Level,”p. 35). When sea level is low, atolls may beleft above the surface. The corals die andthe reef is eroded by the wind and rain. Ifsea level rises rapidly, the atoll may bedrowned, unable to grow in deep water. Ineither case, corals recolonize the atollwhen the sea level returns to “normal”.THE ECOLOGYOF CORAL REEFSCoral reefs may be impressive to geologists,but to the biologist they are simplyawesome. They are easily the richest andmost complex of all marine ecosystems.Literally thousands of species may live ona reef. How do all these different specieslive? How do they affect each other?What is their role in the reef ecosystem?These and countless other questions fascinatecoral reef biologists.Our ability to answer the questions,however, is surprisingly limited. This ispartly because reefs are so complicated.Just keeping track of all the different organismsis hard enough; the task of figuringout what they all do is mind-boggling.Furthermore, until the last quarter of acentury or so most marine biologists livedand worked in the temperate regions ofthe Northern Hemisphere, far from theChapter 14 Coral Reefs 311The trade winds, the steadiest windson earth, blow from latitudes of about30° toward the Equator.Chapter 3, p. 53; Figure 3.17Substrate The bottom or other surfaceupon or in which an organism lives.Subsidence The slow sinking into themantle of a part of the earth’s crust thatcontains a landmass.Chapter 11, p. 236IslandFringing reef

Barrier reefLagoonCalcium carbonateOriginal islandAtoll

FIGURE 14.23 (a) An atoll beginsas a fringing reef around a volcanic island.(b) As the island slowly sinks, the reef flatgets wider and deeper and eventually becomesa lagoon. At this stage the fringing reef hasbecome a barrier reef. (c) Eventually theisland sinks altogether, leaving only a ring ofliving growing reef—an atoll.(a)(b)(c)Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003nearest reef. As a result, relatively few biologistsstudied reefs. Tremendousprogress has been made in recent years,but there is still much to learn. The restof this chapter summarizes what isknown about the ecology of coral reefsand points out some of the importantquestions that remain.The Trophic Structureof Coral ReefsThe tropical waters where coral reefs arefound are usually poor in nutrients (see“Patterns of Production,” p. 343) andtherefore have very little phytoplanktonor primary production. In these barrenwaters, coral reefs are oases of abundantlife. How can such rich communitiesgrow when the surrounding sea is sounproductive?A good part of the answer lies in themutualistic relationship between coralsand their zooxanthellae. We have alreadylearned what the zooxanthellae do for thecoral: They provide food and help makethe calcium carbonate skeleton. In returnthe zooxanthellae get not only a place tolive, but also a steady supply of nutrientssuch as nitrogen and phosphorus. Mostof the coral’s nitrogen and phosphoruswaste products are not released into thewater. Instead, they are taken up andused as nutrients by the zooxanthellae.Using sunlight, the zooxanthellae incorporatethe nutrients into organic compounds,which are passed on to the coral.When the coral breaks down the organicmatter, the nutrients are released and the

whole process begins again (Fig. 14.24).The nutrients are recycled, used over andover, so that far fewer nutrients areneeded than would otherwise be the case.Nutrient recycling is thought to be one ofthe main reasons that reefs can grow innutrient-poor tropical waters.Nutrient recycling occurs not justbetween corals and their zooxanthellae,but among all the members of the coralreef community. Sponges, sea squirts,giant clams, and other reef invertebrateshave symbiotic algae and recycle nutrientsjust as corals do. Recycling also takesplace outside the bodies of reef organisms.When fishes graze on seaweeds, forexample, they excrete nitrogen, phosphorus,and other nutrients as waste. Thesenutrients are quickly taken up by otheralgae. Many corals provide shelter toschools of small fish (see Fig. 8.20). Thefish leave the coral at night to feed andreturn during the day. The waste productsof the fish can be an importantsource of nutrients and help the coralgrow faster. In this way nutrients are cycledfrom whatever the fish feed on tothe coral. Nutrients pass through thecommunity again and again in this cycleof feeding and excretion.Coral reef communities use nutrientsvery efficiently as a result of recycling.The recycling is not perfect, however,and some nutrients are lost, carried awayby the currents. Thus, the reef still needsa continual supply of new nutrients. Recyclingalone is not enough to account forthe high productivity of reefs.The reef is able to provide some ofits own nutrients. Coral reefs are nowknown to have among the highest rates ofnitrogen fixation of any natural community.The main nitrogen fixers arecyanobacteria, especially a free-living onecalled Calothrix and another group thatlives symbiotically in sponges. There isevidence that corals, too, have symbiontsthat can fix nitrogen, providing nutrientsfor the zooxanthellae. Just what the symbiontsare is not known. All these nitrogenfixers provide a substantial source ofnitrogen nutrients. Nitrogen, therefore,probably does not limit coral reef communities,though not all reef biologistsagree about this.Ocean currents bring in additional

nitrogen and, more importantly, phosphorusand other nutrients that are notproduced on the reef. Corals, bacteria,algae, and other organisms can absorbnutrients directly from the water. Eventhough the water is low in nutrients, ifenough water washes over the reef thenutrients add up. In business terms, thisis a low-margin, high-volume proposition.More importantly, the water carrieszooplankton, a rich source of nutrients.When zooplankton are captured by the“wall of mouths,” the nutrients in thezooplankton are passed on to the reefcommunity. In fact, many biologiststhink that corals eat zooplankton not somuch to feed themselves as to get nutrientsfor their zooxanthellae.•Coral reefs are very productive even though thesurrounding ocean water lacks nutrients becausenutrients are recycled extensively, nitrogenis fixed on the reef, and the zooplanktonand nutrients that occur in the water are usedefficiently.

•The production and efficient use ofnutrients by coral reef communities resultin high primary productivity. This isreflected in the overall richness of the312 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyCoral polypCoralpolypCO2 Seawater+ O2Organicmatter+ O2OrganicmatterH2OCO2 + H2O+ nutrientsCO2 + H2O+ nutrientsSunlightZooxanthellaeDiffusionDiffusion DiffusionPhotosynthesisRespiration

FIGURE 14.24 Carbon dioxide and nutrients are continually recycled between a coralpolyp and its zooxanthellae.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003

community. Scientists don’t really know,however, just how much primary productionthere is on coral reefs, or even whichparticular organisms are the most importantproducers. There is no doubt thatzooxanthellae are very important, but becausethey live inside corals, it is veryhard to measure exactly how much organicmatter they produce. For a time itwas thought that very few animals eatcoral, since there is so little live tissue ona coral colony. It was therefore believedthat, even though zooxanthellae producea lot of organic matter, most of it wasconsumed by the coral and not much waspassed on to the rest of the community.As biologists looked closer, however,they found more and more animals thateat corals or the mucus they shed(Fig. 14.25). Primary production by coralzooxanthellae therefore may be importantnot only to corals but to the communityat large. Exactly how muchproduction corals and their zooxanthellaecontribute is still unknown.Seaweeds are also important primaryproducers on the reef (Fig. 14.26), especiallythe small, fleshy or filamentous typesthat are called turf algae because theyoften grow in a short, thick turf on the reefflat. A great many fishes, sea urchins,snails, and other animals graze on theseseaweeds. The turf algae may performmore photosynthesis on the reef than thezooxanthellae, but biologists are not sure.•Zooxanthellae and turf algae are probably themost important primary producers on coralreefs.

•Cyanobacteria, some other bacteria,and coralline algae are also primary producerson coral reefs. They probably accountfor less primary production than dozooxanthellae and turf algae.Coral Reef CommunitiesWith so many species on coral reefs, theinteractions among them are exceedinglycomplex. What is known about these interactionsis fascinating; what remains tobe learned will be even more so.CompetitionSpace is at a premium on coral reefs, as itis in the rocky intertidal (see “The Battlefor Space,” p. 242). Corals, seaweeds, and

many others need a hard place on whichto anchor themselves. Corals and seaweedsneed not just space but space inthe sunlight. The reef is crowded, andmost of the available space is taken. As aresult the sessile organisms, those thatstay in one place, must compete for space.•Sessile coral reef organisms must competefor space. Corals and seaweeds compete for lightas well.

•Corals compete for the space theyneed in different ways. The fast-growingones tend to grow upward and thenbranch out, cutting their neighbors offfrom the light. Other corals take a moredirect approach and actually attack theirneighbors (Fig. 14.27). Some use theirmesenterial filaments for this. When theycontact another coral, they extrude thefilaments and digest away the tissue ofthe other coral. Still other corals developspecial long tentacles, called sweeper tentacles,that are loaded with nematocystsand sting neighboring colonies. Coralsdiffer in their aggressive abilities. Themost aggressive corals tend to be slowgrowing,massive types, whereas the lessaggressive forms are usually fast-growing,upright, and branching. Both strategieshave their advantages, and both kinds ofcorals thrive on the reef.•The two main ways in which corals competefor space are by overgrowing or directly attackingtheir neighbors.

•Corals compete for space and lightnot only with each other, but also withseaweeds and sessile invertebrates. Likecorals, encrusting algae have to produce acalcium carbonate skeleton and thereforegrow relatively slowly. They tend to befound in places where corals don’t do wellbecause of sedimentation, wave action, orpredation.Under the right conditions seaweeds,except for encrusting forms, can growmuch faster than either corals or encrustingalgae. Even with the nitrogen fixationand nutrient cycling that occur on reefs,seaweeds are probably somewhat nutrientlimitedmost of the time. Hence they grow

Chapter 14 Coral Reefs 313Triggerfishes ParrotfishesZooplankton (mucus)ButterflyfishesFlatwormsShrimps (mucus)SeaurchinsSea fans (mucus) Bacteria (mucus)Sea starsCrabs (mucus)SnailsPrimary Production The conversionof carbon from an inorganic form,carbon dioxide, into organic matter byautotrophs, that is, the production of food.Chapter 4, p. 73Nitrogen Fixation Conversion ofnitrogen gas (N2) into nitrogencompounds that can be used by primaryproducers as nutrients.Chapter 10, p. 228FIGURE 14.25 A number of animals eat coral directly, and many others feed on the mucusthat corals produce. The primary production of coral zooxanthellae is thus passed on to coral feeders,and then to the animals that eat them.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003fairly slowly. The reef also has an abundanceof hungry grazers that eat the seaweeds.The combination of nutrientlimitation and grazing keeps the seaweedsin check. If the nutrient levels increase orthe grazers are removed, seaweeds may rapidlytake over, overgrowing and chokingout corals and other organisms.Soft corals are also important competitorsfor space on reefs, and in someplaces they make up almost half the livingtissue (Fig. 14.28). Like most seaweeds,soft corals lack a calcium carbonate skeletonand are able to grow faster than hardcorals. Some soft corals contain sharp littlecalcium carbonate needles, or spicules,that discourage predation. Many of themalso contain various chemicals that aretoxic or taste bad to predators. Because ofthese defense mechanisms, only a fewspecialized predators eat soft corals. Thedefensive chemicals can also be releasedinto the water, where they kill hard coralsthat come too close (Fig. 14.27). Another314FIGURE 14.27 When differentspecies of coral come in contact, they attackeach other. The pink band separating thebrown (Porites lutea) and blue (Mycedium

elephantotus) corals is a dead zone where theblue coral has killed the brown one in theprocess of overgrowing it. The width of thepink band corresponds to the length of theblue coral’s tentacles. To the upper left of thebrown coral is a soft coral (Sarcophyton),which may be attacking the brown coral byreleasing poison. The brown coral seems to bestuck between a rock and a soft place!ProducersDetritusConsumersFrom othercommunities(estuaries, subtidal,especially mangrovesand seagrass beds)PlanktonHerbivoryPredationDetritus feeding or exportContribution to detritus poolPredatorsfishes, squids, snailsGrazersfishes, urchins, snails,chitonsDetritus feederssea cucumbers, worms,amphipods, soft coralsCoral and coral mucus feedersfishes, sea stars, crabs, etc.Plankton feedersfishes, sea fans, feather starsSeaweeds, coralline algae,photsynthetic bacteriaCorals / Zooxanthellae

FIGURE 14.26 A generalized coral reef food web. Coral reefs are extremely diverse, and most components include many organisms in additionto those listed here.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003competitive advantage enjoyed by somesoft corals is that they are not completelysessile. Though they stay in one placemost of the time, they can move aboutslowly. This helps them invade and occupyavailable space on the reef.•Soft corals are important competitors for spaceon reefs. They grow rapidly, are resistant topredators, and can occasionally move about.

•With all these competitive weaponsat their disposal, why don’t soft coralstake over? Not much is known about howsoft corals compete with reef-buildingcorals and other reef organisms or whatdetermines the winner. Many soft coralsappear to have shorter lives than reefbuilding

corals although some may live fordecades. They are also more easily tornaway by storm waves. They have symbioticzooxanthellae like reef-buildingcorals, but are much less efficient photosynthesizers.They also seem to dependon very favorable physical conditions. It isnot known how these factors interact todetermine when and where soft coralscompete successfully for space on the reef.Like soft corals, sponges often havespicules and nasty chemicals that protectthem from predators. They can be importantusers of space on reefs, much more soin the Caribbean than in the Pacific andIndian oceans. Part of the reason for thisseems to be that there are far fewer speciesof coral in the Caribbean than in theIndo-West Pacific. This is a result of geologicalhistory. During the most recent seriesof ice ages, the sea surface was cooler.Corals survived in the heart of the Indo-West Pacific region, around Indonesiaand New Guinea, but many coral speciesbecame extinct in other parts of the ocean.When the ice age ended, corals spread outagain across the Pacific, recolonizing areaswhere they had died out. The Caribbean,however, was not recolonized because theIsthmus of Panamá blocked their dispersal.It is thought that the Caribbean containsonly those coral species thatmanaged to survive the ice ages there.Coral reef fishes are another groupin which competition may be important.Along with corals, fishes are probablythe most conspicuous and abundant animalson the reef (Fig. 14.29). Many ofChapter 14 Coral Reefs 315FIGURE 14.28 Soft corals can form dense patches. This photo is from New Guinea.FIGURE 14.29 Diving on a coral reef is like taking a swim in a tropical fish aquarium—brightly colored fishes seem to be everywhere. The competitive relationships among the variousspecies of fishes are poorly understood.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003these fishes share similar diets; for example,many species eat coral, many grazeon algae, and many are carnivores. Differentspecies of fishes of the same feedingtype at least potentially compete witheach other.There is much debate about howcompetition affects coral reef fishes. One

hypothesis is that competition is relativelyunimportant, and that the abundances ofindividual species are determined mainlyby how many of their larvae settle outfrom the plankton. With favorable currentsand other conditions, many larvaewill be available to settle out and thespecies becomes abundant. If the supplyof larvae is low, for example if currentscarry the larvae away from the reef, thespecies becomes rare. This is called a presettlementhypothesis because it holdsthat the nature of reef fish communities isdetermined by the availability of differentspecies’ larvae before the larvae settle.An alternative, post-settlement, hypothesisis that for most species there areplenty of larvae available to settle out onthe reef. The species that survive and becomeabundant are those that competesuccessfully for space, food, and other resourcesafter the larvae settle, that is, as juvenilesand adults. This hypothesis holdsthat so many different fish species can liveon the reef because each does something alittle different from other species, therebyavoiding competition and the risk of competitiveexclusion. In other words, eachspecies has its own particular ecologicalniche. According to this view, the structureof the fish community on a reef is determinedby the range of resourcesavailable on that reef, and the fish communitiesof reefs vary because differentreefs offer different resources.It is still not clear whether the supplyof larvae for settlement or post-settlementcompetition is more important in structuringreef fish communities. Their relativeimportance probably varies fromspecies to species and from reef to reef,and other factors such as predation andnatural disturbances may also be significantfactors.•There are two schools of thought about whatcontrols the structure of reef fish communities.One holds that reef fish abundances are determinedby how many larvae are available tosettle out from the plankton. The other assertsthat there is an ample supply of larvae for mostspecies, and that reef fish communities arestructured by competition among juveniles andadults after the larvae settle out.

•Predation on Corals

As in other communities, predation andgrazing are important in structuring coralreef communities. A variety of animalseat corals, but instead of killing the coraland eating it entirely, most coral predatorseat individual polyps or bite offpieces here and there (Fig. 14.30). Thecoral colony as a whole survives and cangrow back the portion that was eaten. Inthis respect, coral predation is similar tograzing by herbivores.Coral predation can strongly affectboth the number and the type of coralsthat live on a reef, as well as how fast thereef as a whole grows. In Kane‘ohe Bay,for example, a butterflyfish (Chaetodonunimaculatus) slows the growth of a particularcoral (Montipora verrucosa) that itlikes to eat. When the coral is protectedfrom the fish by a cage, it grows muchfaster. If it were not for the fish, this fastgrowingcoral would probably dominateother corals in the bay. Coral-eatingsnails (Coralliophila, Drupella) have similareffects on some reefs.The Crown-of-Thorns Sea StarAnother example of the effect of coralpredators is the case of the crownof-thorns sea star (Acanthaster planci;Fig. 14.31). The crown-of-thorns feedsby pushing its stomach out through themouth, covering all or part of the coral316 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyFIGURE 14.30 The chevron butterflyfish (Chaetodon trifascialis) is one of many reef animalsthat feed on corals without killing the entire coral colony. Its mouth is adapted for nipping offindividual coral polyps (also see Fig. 8.28).FIGURE 14.31 The crown-of-thornssea star (Acanthaster planci) is an important andcontroversial coral predator.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003colony with the stomach, and digestingaway the live coral tissue. The crown-ofthornshas distinct preferences for certaintypes of coral and avoids others—somecorals must taste bad. Other corals harborsymbiotic crabs (see the photo on p. 117),shrimps, and fishes that discourage thesea stars by pinching and biting theirtube feet.The crown-of-thorns has had amajor impact on some reefs. Beginning in

the late 1950s, people began to noticelarge numbers, sometimes thousands,of the sea star on reefs scattered acrossthe Pacific (Fig. 14.32). The sea stars inthese large aggregations move in a massacross the reef, consuming almost everycoral in their path. Reefs recover in 10 to15 years, though it may take longer forslow-growing coral species to grow back.The first response to the problemwas panic. Coral reefs are valuable resources:They support fisheries, tourism,and recreation. Reefs also protect manycoastlines from wave erosion. With thecrown-of-thorns apparently threateningreefs, people decided to take action andcontrol the sea star. The first attemptbackfired. With limited knowledge of theanimal’s biology, some people cut the seastars into pieces and dumped them backin the sea. Because sea stars can regenerate,the pieces grew into new sea stars!More sophisticated methods, such as poisoningthe sea stars, were tried, but thesemethods were time consuming and expensive,did not work that well, andsometimes did more harm than good.Fortunately, the outbreaks mysteriouslywent away by themselves. Did the seastars starve? Did they move away? Noone knows. Crown-of-thorns outbreaksare still appearing, and disappearing,without explanation.The cause of these outbreaks hasbeen the cause of considerable, sometimesemotional, controversy. At first itseemed obvious that they were unnaturaland that humans were to blame. After all,the plagues had never occurred before. Orhad they? Some biologists pointed outthat people began using scuba only ashort time before the sea star plagueswere noticed. Even if the plagues hadbeen occurring for a long time, therewere no scientists around to see them.Geologists found fossil evidence ofcrown-of-thorns outbreaks dating backthousands of years, and in some placesthere are old stories and other historicalevidence for past outbreaks. The outbreaks,then, may be a natural part of thereef ecosystem and have nothing to dowith humans. A leading hypothesis isthat in unusually wet years river runoffnaturally brings more nutrients into thesea than normal. According to this hypothesis,

the extra nutrients increase thegrowth of phytoplankton that are foodfor sea star larvae. It may also be that periodicpopulation explosions are a naturalpart of the sea star’s biology.The evidence for past outbreaks ofthe crown-of-thorns, however, is hotlydisputed. Some scientists are convincedthat the plagues are the result of somehuman activity that has altered the ecologicalbalance of reefs. Even if outbreaksdid occur in the past, they seem to behappening more often. One possibility isthat the plagues are indeed caused by nutrientsboosting the larvae’s phytoplanktonfood supply, but that the nutrientscome not from natural sources but fromfertilizers and sewage. Another hypothesisis that fishers have caught too manyfishes that eat young sea stars, allowingmore to survive to adulthood. Some biologistshave suggested that plagues occurbecause shell collectors have removed theChapter 14 Coral Reefs 317FIGURE 14.32 An outbreak of crown-of-thorns sea stars (Acanthaster planci) in Australia.The white branches of this colony of staghorn coral (Acropora) show the bare calcium carbonateskeleton left behind after being fed upon by several sea stars. Photograph courtesy of the GreatBarrier Reef Marine Park Authority.Competitive Exclusion The eliminationof one species by another as a result ofcompetition.Chapter 10, p. 218Ecological Niche The combinationof what a species eats, where it lives,how it behaves, and all the other aspectsof its lifestyle.Chapter 10, p. 219Tube Feet Water-filled tubes,possessed only by echinoderms, manyof which end in a sucker and can beextended and contracted to grip thingsand to move around.Chapter 7, p. 142Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003triton shell, a large snail that preys onadult crown-of-thorns. Others argue thateven without shell collectors the tritonshell has always been naturally rare andunlikely to control sea star populations.They also point out that crown-of-thornsoutbreaks continue to occur in areaswhere collecting triton shells has beenbanned for years.

Thus, the issue of what causescrown-of-thorns plagues remains unsolved.The question has practical implicationsfor the management andprotection of reefs. If the plagues arecaused by humans and threaten reefs,then we should probably do something tostop them. On the other hand, the“plagues” may be a natural, potentiallyimportant, part of the ecosystem. Wemight do more harm than good by interferingin a system that we don’t understand.The managers of the Great BarrierReef have taken a middle road. Theyhave developed an effective method tokill sea stars by injecting them with a poison.They use the poison, however, onlyif a crown-of-thorns outbreak is threateninga particularly valuable part of thereef, for example, a popular dive spot orthe site of scientific research. Otherwise,they allow the outbreak to run its course.•The crown-of-thorns sea star has undergonepopulation explosions on many Pacific reefs.There is still debate about what causes the outbreaksand what should be done about them.

•GrazingGrazing on algae by herbivores is at leastas important in coral reef ecosystems as ispredation on corals. Many fishes, especiallysurgeonfishes (Acanthurus), parrotfishes(Scarus, Sparisoma; see Fig. 6.10a),and damselfishes (Pomacentrus, Dascyllus;see Fig. 8.20) graze intensively on reefs.Among the invertebrates, sea urchins(Diadema, Echinometra) are especially important.There are also many microherbivores,small invertebrates like snails,chitons, crustaceans, and polychaeteworms that eat algae.Many seaweeds grow rapidly andhave the potential to outcompete andovergrow corals. Under natural conditionsthey are kept in check by grazers,and to some extent by nutrient limitation.Caging experiments (see “Transplantation,Removal, and Caging Experiments,”p. 246) have been used todemonstrate the importance of grazers.For example, seaweeds are abundant onsand flats next to many Caribbean reefsbut relatively scarce on the reef itself. Totest the hypothesis that reef fishes were

responsible for this, biologists transplantedseaweeds from the sand flat tothe reef. If left unprotected, they weresoon eaten by fishes. When protected bycages, they grew even faster than on thesand flat! The seaweeds are perfectly capableof living on the reef, therefore, butare rare there because they get eaten.Caging experiments on the Great BarrierReef have had similar results.If grazers are removed, seaweeds canflourish and take over space from coralsand other organisms. In many parts ofthe Caribbean, for example, grazing reeffishes have become less common becauseof fishing. When this happened anotherimportant grazer, a sea urchin (Diademaantillarum; Fig. 14.33) became morecommon. The urchin apparently benefitedfrom the reduced competition. Foryears, the urchin seems to have picked upthe slack for the fishes and seaweed populationsremained more or less stable. In1983, however, a disease wiped out populationsof the urchin over much of theCaribbean. Seaweeds, released from grazingpressure, became much more abundanton many reefs, at the expense ofcorals. In Jamaica they took over manyreefs almost completely, and the formerlyrich coral reefs are now more seaweedbed than coral reef.Many reef scientists fear that suchchanges are becoming more and morecommon as fishing pressure on reefs escalatesbecause of increasing populationsand demand for tasty reef fish. There isoften a double whammy. Coastal developmentoften leads not only to more intensivefishing, but also to greater releaseof nutrients from sewage and agriculturalfertilizer. Thus, seaweed growth may beartificially enhanced by eutrophication atthe same time that the grazers that keepthe seaweeds under control are removed.There is increasing evidence that this is aglobal threat to coral reefs.•Grazers help prevent fast-growing seaweedsfrom overgrowing other sessile organisms onthe reef.

•In addition to controlling how manyseaweeds there are, grazers affect whichparticular types of algae live on the reef

and where. Coralline algae, for example,are abundant because the calcium carbonatein their tissues discourages grazers.Other seaweeds produce noxious chemicalsthat are poisonous or taste bad; theseseaweeds also tend to be abundant. Suchchemical defenses, however, appear to besomewhat less common on coral reefsthan in temperate communities. Thetasty seaweeds are most heavily grazedand thus tend to be rare. Even so, theygenerally grow rapidly and are an importantfood source.Damselfishes provide some interestingexamples of the effects of grazing onreefs. Many damselfishes graze on seaweedsinside territories that they vigorouslydefend, chasing away other fishesthat happen to venture inside. Many suchdamselfishes actually “farm” their territories.They weed out unpalatable algae,pulling them up and carrying them outsidethe territory. What is left in the territoryis a dense mat of tasty seaweeds,usually fine, filamentous types. Protectedby the damselfish, these algae grow veryrapidly and outcompete corals andcoralline algae. Outside the territory,parrotfishes and surgeonfishes gobbleup the algae, clearing space for other organisms.Thus, the community inside the318 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyFIGURE 14.33 The long-spinedblack sea urchin (Diadema antillarum) is oneof the most important grazers on Caribbeanreefs. Closely related species are found onreefs in other parts of the world.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003territory is very different from that outside.One interesting point is that cyanobacteria, which are nitrogen fixers, aremuch more common inside damselfishterritories than outside. Thus, damselfishesmay indirectly have an importantrole in the nutrient balance of the reef.Herbivores obviously affect coral reefalgae, but they may have indirect effectson animals as well. Many reef grazersscrape their food off the reef with somekind of specialized hard structure. Parrotfishes,for instance, get their name becausetheir fused teeth form a parrotlike

beak (see Fig. 8.13d). Sea urchins usetheir Aristotle’s lantern to scrape surfaces.If grazing is intense, as it often is,the surface of the reef flat may be scouredsmooth; the marks made by herbivoresare often visible. In the process of scrapingoff the seaweeds that are their food,herbivores incidentally remove settlinglarvae and other small animals. It may bevery difficult for new coral colonies to establishthemselves, for instance, unlessthe larva happens to settle in a crevice orother protected spot. If this kind of grazingpressure is intense enough it canerode the reef.Living TogetherAmong the vast number of species thatlive on coral reefs, many have evolvedspecial symbiotic relationships. There arefar too many cases of symbiosis on thereef to describe here. In fact, coral reefsprobably have more different symbioticrelationships than any habitat on earth.The few examples discussed here will giveyou some idea of how fascinating theserelationships are.•Symbiotic relationships are very important incoral reef communities. Coral reefs probablyhave more examples of symbiosis than anyother biological community.

•Chapter 14 Coral Reefs 319Chitons Molluscs whoseshells consist of eightoverlapping plates on theirupper, or dorsal, surface.Chapter 7, p. 133Aristotle’s Lantern A complicated setof calcium carbonate (CaCo3) teeth andassociated muscles that is found in seaurchins.Chapter 7, p. 144Ahhhhh! You’re relaxing on a warm tropical night after a deliciousfish dinner. The palm trees sway, a warm tropical breeze blows—and suddenly your bowels begin to churn. Before long your lips burnand tingle, and your hands and feet feel like pins and needles. Yougo into the kitchen for a drink, but the cool water feels warm. Thecool floor and the cold compress you put on your forehead are alsowarm to the touch: The sensations of hot and cold are reversed. Asyou stagger to the bathroom, your arms and legs are heavy andweak. Now feeling really sick, you mumble, “Must have been somethingI ate.”You’re right. What you have is a case of ciguatera—tropicalfish poisoning. Maybe it will comfort you to know that you’renot alone. Tens, perhaps hundreds of thousands, of people get

ciguatera every year. And relax, you probably won’t die, thoughyou might be a little sick (if you’re unlucky, very sick) for monthsor even years. Don’t count on a cure, though there are variousfolk remedies, and some treatments might help you feel a littlebetter. There are also reports that the intravenous administrationof an inexpensive sugar called mannitol can help, but this remainsunconfirmed.So what is ciguatera, anyway? The disease has been knownfor hundreds of years. The name comes from the Spanish wordfor a Caribbean snail: You can also get ciguatera from eatingmolluscs, and perhaps sea urchins, though fish dinners are themost common cause. It is probably most common in the toppredatory fishes on the reef, like jacks, barracudas, and groupers,but it can also be caused by eating herbivorous fishes like parrotfishesand surgeonfishes. A particular kind of fish may be perfectlysafe in most places, but poisonous in particular spots. Tomake matters even more confusing, ciguatera may disappearfrom one area and pop up in another. All of which makes thingsdifficult for lovers of fresh reef fish.Ciguatera is caused by toxic dinoflagellates that live on thereef. Herbivorous fishes eat the dinoflagellates, and the poison ispassed on to the predatory fishes that eat them. As blooms of thedinoflagellates arise and die out, ciguatera comes and goes in thearea. Predatory fishes range over larger areas and eat many fish,which may all contain small amounts of the toxin. Thus, they aremost likely to cause ciguatera. Herbivorous fishes probably carrythe toxin only when they feed in an area with a bloom.The big problem is trying to tell when a fish carries the poison.You could just avoid reef fish altogether, but if you are inthe tropics that means missing out on a lot of tasty meals. Oneway to tell if a fish is carrying ciguatera is to feed a bit to a cat:Cats are highly sensitive to ciguatera. If any mongooses arehandy, they’re good tasters too. Of course, this isn’t exactly kindto the animals, who usually die if the food is affected by ciguatera.There has been research on a number of chemical tests, butnone of them are yet in widespread use. Many of these tests areexpensive or complicated, and not really suitable for routinelytesting seafood. Another problem is that there are actually severaldifferent dinoflagellate toxins that can cause ciguatera, and itis difficult to test for all of them. We aren’t even sure we knowwhat they all are. Until a reliable test is developed, you’ll have togo hungry or take your chances.“MUST HAVE BEEN SOMETHING I ATE”Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003We have already seen how mutualismbetween corals and their zooxanthellaeis the essential feature of reefformation. Many other organisms alsohave photosynthetic symbionts. Seaanemones, snails, and giant clams (Tridacna)all harbor zooxanthellae. The“deal” between the two partners is the

same as in corals: The zooxanthellae getnutrients and a place to live, and the hostgets food. Giant clams grow so large(Fig. 14.34) because their zooxanthellaeprovide a constant food supply.There are primary producers otherthan zooxanthellae that live inside reef animals.As mentioned previously, somesponges have cyanobacteria that fix nitrogenin addition to performing photosynthesis.Certain sea squirts house aphotosynthetic bacterium called Prochloron.Prochloron is especially interesting becausesome scientists think it is similar to thesymbiotic organisms that eventually becamethe chloroplasts of plants (see “FromSnack to Servant: How Complex CellsArose,” p. 77).Another important example of mutualismon the reef is the relationship betweencorals and the crabs, shrimps, andfishes that help protect them from thecrown-of-thorns sea star and other predators.Most corals host a number of symbionts,especially crustaceans. Some ofthese are only casual symbionts and canlive as well off the coral as on it. Othersare much more specialized and are foundonly on their host coral. These are calledobligate symbionts. Some of the obligatesymbionts are parasites and harm thecoral, some are commensals and apparentlydo not affect the coral one way orthe other, and some are mutualists thatbenefit the coral. It is often hard to tellwhich is which, because for most symbiontsthe nature of the relationship betweencoral and symbiont is not wellunderstood. Those who study such organismsmust frequently revise their ideasas more information is obtained. Thecrabs that protect their coral from predators,for instance, were once thought tobe parasites.Anemonefishes (Amphiprion; Fig.14.35) have an interesting relationshipwith several kinds of sea anemone. Theanemones inhabited by anemonefisheshave a powerful sting and are capable ofkilling the fish. The fish have a protectivemucus, however, that keeps the anemonefrom stinging. It is not known whetherthe mucus is produced by the fish themselves,by the anemone, or by both.When anemonefishes are newly introducedto an anemone, they typically rub

against and nip the anemone’s tentacles.They may be coating themselves withthe anemone’s mucus. To understandwhy this is an advantage, you have to rememberthat anemones have no eyes andno brain, but lots of tentacles. If theanemone simply stung everything ittouched, it would end up stinging itselfevery time the tentacles bumped intoeach other. The anemone recognizes itselfby the “taste” of its own mucus.When one tentacle touches another, theanemone detects the mucus coating onthe tentacles and refrains from stinging.Anemonefishes may be taking advantageof this. On the other hand, there is evidencethat the fishes’ own mucus protectsthem against the anemone.Anemonefishes are protected by theanemone’s stinging tentacles and broodtheir eggs under the anemone. In at leastsome cases the relationship is mutuallybeneficial because the anemonefish driveaway other fishes that eat anemones.Anemonefishes kept in aquaria will sometimesfeed their host, but this behaviordoes not seem to be important in nature. Itis also possible that the fishes benefit theirhosts in ways that are not yet understood.320 Part Three Structure and Function of Marine Ecosystems www.mhhe.com/marinebiologyFIGURE 14.34 The giant clam(Tridacna gigas) does not deserve itsreputation as a killer: Stories of people gettingtrapped in them and drowning are myths.Symbiotic zooxanthellae give the uppersurface its blue and green colors, and providefood that allows the clam to get so big. Theclam also filter feeds. The hole visible on theclam’s fleshy upper surface is one of thesiphons through which the clam pumps waterfor feeding and to obtain oxygen. The giantclam does not occur in the Caribbean.FIGURE 14.35 An orange-finanemonefish (Amphiprion chrysopterus) and itssea anemone host.Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003321

i n t e r a c t ive ex p l o ra t i o n

Check out the Online Learning Center at www.mhhe.com/marinebiology and click on thecover of Marine Biology for interactive versions of the following activities.

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?^Do-It-Yourself SummaryA fill-in-the-blank summary is available in the Online LearningCenter, which allows you to review and check your understandingof this chapter’s subject material.Key TermsAll key terms from this chapter can be viewed by term, or bydefinition, when studied as flashcards in the Online LearningCenter.Critical Thinking1. What factors might account for the fact that the vastmajority of atolls occur in the Indian and Pacific oceans andthat atolls are rare in the Atlantic?2. Scientists predict that the ocean will get warmer and the sealevel will rise as a result of an intensified greenhouse effect(see “Living in a Greenhouse: Our Warming Earth,” p. 410).How might this affect coral reefs?3. There are only a few reefs off the northeast coast of Brazil(see map in Fig. 14.10), even though it lies in the tropics.How would you explain this?For Further ReadingSome of the recommended readings listed below may be availableonline. These are indicated by this symbol , and will containlive links when you visit this page in the Online Learning center.General InterestChadwick, D. H., 2001. Kingdom of coral: Australia’s GreatBarrier Reef. National Geographic, vol. 199, no. 1, pp. 30–57.A fabulous tour of the earth’s largest reef area.Doubilet, D., 1999. Coral Eden. National Geographic, vol. 95,no. 1, January, pp. 2–29. Superb photos of coral reefdiversity.Levine, J. S., 1993. Dusk and dawn are rush hours on the coralreef. Smithsonian, vol. 24, no. 7, October, pp. 104–115. Thechange from day to night brings a changing of the guard oncoral reefs.Marshall, J., 1998. Why are reef fish so colorful? ScientificAmerican Presents, vol. 9, no. 3, Fall 1998, pp. 54–57. Theoften gaudy colors of reef fish serve to attract mates, fend offrivals, and deter predators.Pain, S., 1997. Swimming for dear life. New Scientist, vol. 155,no. 2099, 13 September, pp. 28–32. The tiny larvae of coralreef fishes are champion swimmers. They have to be.

Ross, J. F., 1998. The miracle of the reef. Smithsonian,vol. 28, no. 11, February, pp. 86–96. Scientists study masscoral spawning on reefs in Florida.Sale, P. F., G. E. Forester and P. S. Levin, 1994. Reef fishmanagement. Research and Exploration, vol. 10, no. 2,Spring, pp. 224–235. Coral reef fisheries are an importantfood source in many countries, but many have beendecimated by overfishing. Unfortunately, management ofreef fisheries is not well understood.In DepthBirkeland, C. (Editor), 1997. Life and Death of Coral Reefs.Chapman & Hall, New York. 536 pp.Booth, D. J. and D. M. Brosnan, 1995. The role of recruitmentdynamics in rocky shore and coral reef fish communities.Advances in Marine Biology, vol. 26, pp. 309–385.Miller, M. W., 1998. Coral/seaweed competition and the controlof reef community structure within and between latitudes.Oceanography and Marine Biology: An Annual Review, vol. 36,pp. 65–69.Munday, P. L. and G. P. Jones, 1998. The implications of smallbody size among coral-reef fishes. Oceanography and MarineBiology: An Annual Review, vol. 36, pp. 373–411.Wilkinson, C. (Editor). Status of Coral Reefs of the World:2000, Australian Institute of Marine Science, Townsville.363 pp.See It in MotionVideo footage of the following can be found for this chapter onthe Online Learning Center:• Staghorn coral (Honduras)• Gray angelfish feeding (Belize)• Clownfish in anemone (Papua New Guinea)• Giant clam (Papua New Guinea)• Giant clam siphons (Papua New Guinea)• Saddleback butterflyfishes feeding on coral polyps(Solomon Islands)• Branching reef corals (Acropora) (Solomon Islands)• Bleached reef corals (Solomon Islands)• Soft coral (Palau)• Reef corals (Palau)• Retraction of soft coral polyps (Palau)• Blue tangs feeding on algae (Bonaire, West Indies)• Bigeye trevally (Solomon Islands)• Crown-of-thorns sea star (Fiji)Castro−Huber: MarineBiology, Fourth EditionIII. Structure and Functionof Marine Ecosystems14. Coral Reefs © The McGraw−HillCompanies, 2003• Coral spawning (Solomon Islands)• Octopus eating a clam (Palau)• Relative of reef-building corals, feeding on plankton (Belize)Marine Biology on the NetTo further investigate the material discussed in this chapter, visitthe Online Learning Center and explore selected web links to relatedtopics.

• Coral reefs• Class Anthozoa• Color bleaching• Mangroves• Community ecology• Competition• Parasitism, predation, and herbivory• Biodiversity• Food websQuiz YourselfTake the online quiz for this chapter to test your knowledge.