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Arthropod Structure & Development 39 (2010) 340e349

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Arthropod Structure & Development

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Cardiovascular system of the Majidae (Crustacea: Decapoda)

Iain J. McGawa,b,*, Jonathon H. Stillman c,d

aOcean Sciences Centre, 1 Marine Lab Road, Memorial University, St John’s, NL A1C 5S7, CanadabBamfield Marine Sciences Centre, Bamfield, British Columbia, V0R 1B0, CanadacRomberg-Tiburon Center and Department of Biology, 3152 Paradise Drive, San Francisco State University, Tiburon, CA 94920, USAd Integrative Biology, 3060 Valley Life Sciences, University of California, Berkeley, CA 94720, USA

a r t i c l e i n f o

Article history:Received 25 January 2010Accepted 17 May 2010

Keywords:CrustaceaCirculatoryArteryHeartCardiovascularSpider crabMajidaeSinus

* Corresponding author at: Ocean Sciences Centre,University, St John’s, NL A1C 5S7, Canada. Tel.: þ1 703220.

E-mail address: ijmcgaw@mun.ca (I.J. McGaw).

1467-8039/$ e see front matter Crown Copyright � 2doi:10.1016/j.asd.2010.05.003

a b s t r a c t

The cardiovascular system of majid crabs was mapped using corrosion casting techniques. The generalform of the circulatory system was comparable to that of other malacostracan crustaceans, but withdistinct differences between several arterial systems. The anterior aorta exited from the anterior surfaceof the heart supplying hemolymph to the antennae, eyestalks, gastric muscles and brain. This artery wasmore complex compared with other decapods. The anterolateral arteries exited from the anterior dorsalsurface of the heart and supplied hemolymph to the hypodermis, stomach, antennal gland andmandibular muscles. The hepatic arteries were larger and more complex compared with other decapodfamilies, branching profusely within the hepatopancreas and gonads. The small posterior aorta exitedfrom the posterior-ventral surface of the heart. Standard sex-specific differences in this artery wereobserved. Exiting from the ventral surface of the heart, the sternal artery supplied each pereiopod ina segmental arrangement. The sternal artery arrangement was different to other brachyuran crabs,possibly a symplesiomorphy with segmented ancestors. In accordance with anatomical descriptions ofblue crabs and Cancer crabs it would also seem appropriate to classify the circulatory system of theMajidae as one that is “incompletely closed”.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

During the past two decades there has been a renewed interestin the physiological and, to a lesser degree, the anatomical aspectsof the decapod crustacean circulatory system (reviewed inMcMahon and Burnett, 1990; Wilkens, 1999; McMahon, 2001;McGaw and Reiber, 2002). Historically, the open cardiovascularsystem of decapod crustaceans conjured up ideas of a sluggishsystem, where hemolymph collected in spaces or sinuses andwhere there was very little control over regional perfusion(Maynard, 1960). Based upon an increased understanding of thephysiological control mechanisms and vascular complexity,a number of recent articles have suggested that the circulatorysystem is “incompletely closed” rather than open, witha complexity that rivals that of advanced invertebrates such as thecephalopods and some of the rudimentary vertebrates (McGaw andReiber, 2002; McGaw, 2005; Reiber and McGaw, 2009).

1 Marine Lab Road, Memorial9 737 3272; fax: þ1 709 737

010 Published by Elsevier Ltd. All

The generalized form of the decapod crustacean cardiovascularsystem consists of a single muscular ventricle suspended ina primer chamber, the pericardial sinus (Maynard, 1960). Hemo-lymph (blood) is pumped out into seven arteries (5 arterialsystems). Five arteries deliver hemolymph anteriorly. A singleartery exits on the posterior surface of the heart, while the largestvessel, the sternal artery, exits the heart ventrally and divides tosupply the limbs and mouthparts (McLaughlin, 1983). All arteriesbranch into arterioles which in turn divide to form either endo-thelial lined capillaries or a series of lacunae lined with a thinintima where nutrient, gas and waste exchange occurs (Wilkenset al., 1997). In part, it is the lack of a complete venous systemthat defines the circulatory system as open (Maynard, 1960).Instead themajority of hemolymph is returned to the heart througha series of sinuses. Recently it has been shown that the sinuses aredistinct structures rather than open spaces and their lacunae formprecise networks around organs (Farrelly and Greenaway, 2005;McGaw, 2005), so much so that the only real difference betweenthese and capillaries is the lack of an endothelial layer. Thedistinction between capillary and sinus therefore becomes muchless apparent (Haeckel, 1857; Johnson, 1980).

Despite these recent advances, our understanding of thecomparative anatomy of the decapod crustacean circulatory system

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is limited to a few families (reviewed in McGaw, 2005; Wirkner,2009). Articles on the gross anatomy suggest that there aredistinct differences between portunid, cancrid and lithodid crabs(McGaw and Reiber, 2002; McGaw, 2005; McGaw and Duff, 2008).Within the Decapoda the familyMajidae includes a diverse group ofdecapods with pear-shaped carapaces and elongated legs andchelae. These are the spider crabs and include the world’s largestarthropod, the Japanese spider crab, Macrocheira kaempferi. Theaim of the present study was to examine the anatomy of a variety ofcrabs from the family Majidae and to compare and contrast thesefindings with the gross anatomy of the other decapod families.

2. Material and methods

Kelp crabs, Pugettia producta, spider crabs, Libinia emarginata,tanner crabs Chionocetes bardii, and moss crabs, Loxorhynchuscrispatuswere collected by trapping or purchased from commercialsuppliers. They were maintained in running aerated seawater(32 � 1l and 14 �C � 1 �C) prior to preparation. The procedureemployed to map the circulatory system has been used previouslywith other crustacean families (McGaw and Duff, 2008). A smallhole was drilled into the carapace directly above the pericardialsinus and a polyethylene catheter (PE205) was implanted and heldin place with dental wax and cyanoacrylate glue. A syringe pumpwas used to infuse Batson’s Monomer resin No. 17 (Polysciences

Fig. 1. Photograph of the internal anatomy of the kelp crab Pugettia producta. a) The single cthe cardiac and pyloric sacs occupies most of the anterior region. The midgut (not shown) arhindgut that loops up and over the heart. The gonads are large, filling the anterior and lateralhouse 8 pairs of phyllobranchiate gills, with the gill bailer evident on the dorsal surface. btopancreas which extends backwards to the first abdominal segment. The subesophageal gveins which return hemolymph to the heart are situated under the thoracic epimera.

Inc., Warrington, PA) thinned with methacrylate acid methyl ester(4:1) (Greenaway and Farrelly, 1984) into the pericardial sinus. Thecrabs were kept immersed in seawater while infusion volumes ofapproximately 25% of the wet mass of the animal were injected at5 ml/min. The tip of several of the pereiopods were cut to allowhemolymph to escape; infusion pressures were maintained at orbelow 15 mm Hg (within physiological range) to prevent arterialdamage and inflation of the sinuses caused by over filling andexcess pressure. Following injection of the Batson’s Monomer, thecrabs were placed in a refrigerator at 5 �C for 24 h to allow the resinto cure. The soft tissue was macerated over 5 days in a saturatedpotassium hydroxide solution. The remaining carapace andchitinous tissues were then dried before being dissolved byimmersing the crab in concentrated hydrochloric acid (1 M) for48 h. This procedure left a corrosion cast of the circulatory systemwith all tissues removed. The hemolymph capacity of each arterialsystem was estimated by weighing each of the vessels and calcu-lating their capacity as a percentage of the entire system.

The main arterial systems were photographed with a digitalcamera. Line diagrams of each arterial systemweremade by editingdigital photographs of an arterial system and by comparison amongseveral casts; most of the fine capillary-like vessels were removedto better illustrate the course of the main vessels. Fine-scale detailsof specific areas of the circulatory system were examined usinga scanning electronmicroscope. The casts were sputter-coatedwith

hambered heart (H) is situated in the center of the body. The large foregut consisting ofises from the pyloric sac and runs along the ventral surface eventually giving rise to theregions of the carapace, extending backwards under the heart. The branchial chambers) Removal of the heart, foregut and gills (right branchial chamber) exposes the hepa-anglion is evident in the anterior region just behind the rostrum. The branchiocardiac

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gold and examined with a JEOL scanning electronmicroscope (JSM-5600). The images were adjusted and labeled in Adobe Photoshop7.The internal anatomy of the four species was very similar. There-fore, the results are described in general terms, unless otherwisestated.

3. Results

3.1. General anatomy

Majid crabs possess a pear-shaped carapace that is usuallylonger than it is wide; the legs are longer compared with otherdecapod crustaceans. The large single chambered ventricle (H) islocated in the centre of the body (Fig. 1a). Hemolymph returning tothe pericardial sinus from the gills enters the heart via the pairedostia. The foregut consists of a large cardiac sac with a smallerpyloric chamber situated posteriorly to the cardiac sac. The midgutexits from pyloric chamber and dips under the heart, but subse-quently loops up and over the dorsal surface of the heart and intothe abdomen to become the hindgut. Removal of the pericardialsinus, heart and foregut (Fig. 1b) exposes the underlying area withthe gonads and hepatopancreas extending anteriorly and posteri-orly, occupying most of the ventral surface of the central cephalo-thorax. The subesophageal ganglion (brain) is situated in the frontalregion, alongwith themuscles and internal skeletal elements of theantennae. The branchial chambers are large and house 8 pairs ofphyllobranchiate gills, with the gill bailer visible on the surface ofthe gills (Fig. 1a,b).

The general form of the circulatory system of majid crabs issimilar to other decapods, consisting of a single chamberedventricle that delivers hemolymph into 7 arterial systems (Fig. 2).Five of these arteries exit from the anterior surface of the heart(Fig. 2). Three percent of the hemolymph is contained within the

Fig. 2. Corrosion cast drawing of a kelp crab, Pugettia producta dorsal view showing the heheart these are the anterior aorta (AA) and the paired anterolateral arteries (ALA) and hepaticartery gives rise to the pereiopod arteries (P2eP5), and the cheliped arteries (P1).

anterior aorta. The paired anterolateral arteries hold approxi-mately 11% of the hemolymph, while 27% is delivered throughthe paired hepatic arteries. Approximately 3e4% of the hemo-lymph is contained within the posterior aorta, the variation involume being accounted for by the larger abdomen of the femalecrab (Fig. 6). The majority of the hemolymph (56%) is carried inthe sternal arterial complex. However, when considering theentire hemolymph volume, the heart, gills and sinuses act asa reservoir, holding over 70% of the hemolymph at any one time(Fig. 2). The gross anatomy of each arterial system is discussed inturn.

3.2. Anterior aorta

The anterior aorta (AA) exits from the medial dorsal anterioraspect of the heart (Fig. 3a,b). It continues anteriorly over thegonads and the foregut, and dips ventrally towards the rostrum onthe anterior apex of the cephalothorax. Fine vessels issue along thelength of the main artery supplying tissues in the immediatevicinity. Toward the end of the main artery small vessels arise oneither side and trifurcate into anteriorly, laterally and ventrallydirected courses, characterized by a dense network of vessels.A second pair issue slightly anterior to the first branches and splitsinto laterally and ventrally directed sub-branches. Collectivelythese vessels perfuse the large cardiac stomach muscles that linkinto the gastric mill apparatus and contract the gastric mill in theforegut region of the animal.

Terminally the main anterior aorta splits into an upper andlower branch. The upper branch continues anteriorly, and bifur-cates to supply the frontal spine of the rostrum and muscles ofthe antennae. Three vessels arise from the lower branch. Thepaired optic arteries (OA) exit at approximately 180� relative toone another and terminate as a series of blind ending arterioles

art (H), gills (G) and large branchiostegal sinus. Three arteries exit anteriorly from thearteries. The dorsal sinus (DS) is visible in the anterior region of the animal. The sternal

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Fig. 4. Line diagram of the a) Left anterolateral artery (ALA) of Pugettia producta which serves as a source for several other vessels, the cardiac stomach artery (CSA), mandibularartery (MA) and antennal gland artery (AGA) arise on the inner surface of the anterolateral artery. An outer branch splits to form the lateral gonadal artery (LGA) and the hypodermalartery (HYA), which arises on the outer surface and supplies hemolymph to the hypodermis of the branchial chamber. b) SEM (magnification of �230) of the antennal glandshowing the complexity.

Fig. 3. Line diagram of the anterior aorta complex of Pugettia producta produced from corrosion casts. a) Dorsal view and b) Lateral view. AA ¼ anterior aorta, CA ¼ cerebral artery,OA ¼ optic arteries. c) SEM of eye arteries at magnification of �25.

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within the eyestalk (Fig. 3c). The medial branch continues itscourse ventrally and an elongated area is evident, this is the corfrontale, within a short distance it splits into another threevessels. A set of anteriorly directed paired arteries supply the

Fig. 5. a) Line diagram of the right hepatic artery of Chionocetes bardii (HA) in dorsal view. Th2 posteriorly directed branches b) A ventral view of the right hepatic artery with small arteriSEM (�25) of the fine vessels of the hepatopancreas.

antennules and musculature thereof. The final medial branch islarge, this is the cerebral artery (CA), it curves posteriorly to forma series vessels that supply hemolymph to the subesophagealganglion of the animal.

e fine arterioles have been removed to more clearly show the 2 anteriorly directed andoles intact to show the density and complexity, a portion is highlighted and shown in c)

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3.3. Anterolateral arteries and associated branches

The paired anterolateral arteries (ALA) originate from theanterior surface of the heart at an angle of approximately 40�

relative to the anterior aorta (Fig. 4a). A series of fine vessels arisefrom the anterolateral artery and its associated branches resultingin considerable inter-digitations with the hepatic arteries. Themainartery continues its course towards the head region and bifurcates.The outer branch is the mandibular artery (MA) supplying themuscles of the mandibles through two sub-branches. The innerbranch is the antennal gland artery (AGA). The antennal gland isvisible as a dense mass situated at the end of the vessel. Scanningelectron micrographs (�230) reveals the structure of the antennalgland as a series of closed-loop vessels of approximately 20e30 mmin diameter (Fig. 4b).

The main anterolateral artery serves as a source for a number ofother vessels (Fig. 4a). About two-thirds along the length of themainvessel an outer branch extends and bifurcates almost immediately.The upper branch is the lateral gonadal artery supplying hemo-lymph to the anterior surface of the gonads, hypodermis and thebranchiostegal sinus. The course of the lower branch is ventral andsuperficial terminating in the branchial chamber. This arterysupplies hemolymph to the hypodermis of the branchial chamber.Arising on themedial surface of themain artery the cardiac stomach

Fig. 6. a) Lateral view of the posterior aorta (PA) complex of a male Chionocetes bardiiand b) Lateral view of a female Loxorhynchus crispatus posterior aorta complex.PLA ¼ posterior lateral arteries, PA ¼ main posterior aorta, IAA ¼ inferior abdominalartery.

artery (CSA) dips ventrally and anteriorly, passing through part ofthe hepatopancreas and supplying hemolymph to the pyloric regionof the foregut. A second smaller branch arises anteriorly supplyingthe lateral surface of the foregut (Fig. 4a).

3.4. Hepatic arteries

The paired hepatic arteries (HA) arise from the lateral surface ofthe heart directly below the anterolateral arteries (Fig. 5a). Almostimmediately these vessels dip directly ventrally to form 4 sub-branches. The two anteriorly directed branches supply the hepa-topancreas and ventral surface of the foregut and extend under theedge of the branchial chamber connecting with the branchiostegalsinus. The two posterior branches are larger and extend to theposterior margin of the carapace. These vessels supply the hepa-topancreas, midgut and the gonads and make extensive inter-connections with branches of the pereiopod arteries. An hepaticartery with intact fine vessels shows the very dense network ofcapillary-like vessels emanating from each branch (Fig. 5b). Thesevessels terminate as hepatic arterioles and are approximately100 mm in diameter (Fig. 5c). They supply blood to the hemal spacesin the hepatopancreas.

3.5. Posterior aorta and inferior abdominal artery

The posterior aorta issues from the posterior-ventral surface ofthe heart. A lateral view of the male and female crab vessels illus-trates the complexity (Fig. 6). After exiting the heart the vesselbifurcates, forming the left and right posterior lateral arteries (PLA).These vessels issue at 180� relative to one another before turningagain through 90� and descending anteriorly through the abdomenwhere they join with the inferior abdominal artery. The mainposterior aorta (PA) has an anteriorly directed course along themidline of the abdomen. Smaller branches issuing along its lengthsupply the musculature of the abdomen. In the female crab finevessels supply hemolymph to each of the pleopods (Fig. 6b).Terminally the posterior aorta bifurcates in the telson of theabdomen. Issuing from the main sternal artery, the inferiorabdominal artery joins with the posterior lateral arteries. There-after the inferior abdominal artery runs parallel to the mainposterior aorta, bifurcating around the hindgut, supplying hemo-lymph to this structure (Fig. 6a).

3.6. Sternal artery and associated branches

The sternal artery (STA) is the largest artery in the circulatorysystem and originates on the ventral surface of the heart close tothe posterior aorta (Fig. 7a). The course of the sternal artery isslightly to the left before descending ventrally and anteriorly belowthe heart. The five pereiopod arteries originate from the sternalartery in a simple segmental fashion. The pereiopod arteries followvery similar plans: The densities of the capillaries are greatestwithin the muscles of the thoracic sterna. Each artery extends intothe tip of the dactylopodite and capillary-like vessels branchextensively along the entire length; some of these finer vesselswere lost during the maceration process (Fig. 2). The chelipedvessels (P1) exit laterally, ending as a complex network of vesselssupplying the posterior extensor and anterior flexormuscles withinthe propodite (Fig. 2). The main vessel finally bifurcates at thedactylopodite with each vessel extending to the tips of the chelae.Branchial arteries arise on the dorsal surface of pereiopod arteries(P2eP5) and curve up and under the thoracic epimera supplyinghemolymph to the gill tissues (Fig. 7a).

A single vessel, the ventral thoracic artery issues anteriorly fromthemain sternal artery (Fig. 7b). This artery and its branches supply

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Fig. 7. The sternal artery complex of Libinia emarginata: a) The main sternal artery (STA) gives rise to the five paired pereiopod arteries (P1eP5). The ventral thoracic artery (VTA)exits from the anterior surface of the main artery. The branchial arteries (BA) originate on the dorsal surface of the pereiopod arteries (P2eP4) and supply hemolymph to tissues ofthe gills. b) Ventral view of the ventral thoracic artery complex (VTA) which splits to supply hemolymph to the mouthparts of the crab. Sub-branches supply the exopodite (ex2,3)and the endopodite (en2,3) of the second and third maxillipeds (M2, M3), the musculature of the scaphognathites (scp) and the mandibles (mn). See also text.

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the mouthparts of the crab. The first paired arteries (M3), exit atabout 45� from the main vessel and supply the 3rd maxillipeds.Each vessel splits at the end, the outer branch supplies the exo-podite (ex3); the inner branch is large and curves upwards tosupply the endopodite (en3). The second pair of vessels (M2) ariseat a similar angle, anteriorly to the first pair. They too bifurcate, theouter branch supplies the exopodite (ex2) and the inner branchsupplies the endopodite (en2) of the second maxilliped. The finalpaired arteries (M1) are larger in diameter than the first pair andform a series of smaller vessels that supply the musculature of thescaphognathites (scp). On the current preparation a single branchexits from the left scaphognathite artery, although on some otherspecimens it exits on the right side. Its course is anterior, before itsplitting in two and bifurcating again shortly afterwards, the outerbranch supplies the first maxillipeds (mx1), while the inner branchsupplies the mandibles (mn).

3.7. Sinuses and gill vessels

Hemolymph collects in sinuses before flowing through the gillsand back to the heart. In the majid crabs most of the sinuses appearas distinct structures between or covering the vessels. The generalpathway of hemolymph through the sinuses is such that hemo-lymph from the eye sinuses and hepatic sinuses drains into a dorsalsinus (DS) which lies between the edges of the stomach, hepato-pancreas and gonads and can be seen as a distinct structure (Fig. 2).Hemolymph from these sinuses eventually drains into the ventralthoracic sinus which lies below the heart. The ventral thoracic sinusradiates out between the muscles of the thoracic sterna as thebranchial sinuses where they join with the infrabranchial sinus.

The infrabranchial sinus (IBS) runs along the ventral edge of thegill, forming distinct ring-like structures around the bases of each ofthe pereiopods (Fig. 8a). From here the course of the infrabranchial

sinus is directly upwards forming the afferent branchial veins(ABV). Paired gill lamellae arise on each side of the afferent bran-chial vein (8b). Scanning electron micrographs of the gill bar showthe marginal canal (mc) running along the anterior edge and thelacunae on the surface of the lamellae (Fig. 8c). After passingthrough the lamellae hemolymph collects in the paired efferentbranchial veins (EBV). The efferent branchial veins drain into thebranchiocardiac veins (BCV), which are located under the thoracicepimera (Fig. 8b). Four pairs of branchiocardiac veins fuse to forma sinus that runs along the ventral edge of the heart, and returnshemolymph at the posterior end of the pericardial sinus (Fig. 8d).From here hemolymph drains into the heart through the ostia(Fig. 1a).

Another major sinus, the branchiostegal sinus, is located in thebranchial chambers (Fig. 9a). A series of vessels and complex inter-digitating lacunae of approximately 100 mm diameter (Fig. 9b)comprise the branchiostegal circulation. Hemolymph from thebranchiostegal lacunae empty into a large branchiostegal vein thatruns along the ventral edge of the branchial chamber and subse-quently splitting into several smaller vessels. These veins returnhemolymph into the pericardial sinus at its posterior lateral edge(Fig. 9a).

4. Discussion

The general form of the circulatory system of the majid crabs,with seven arteries originating from a single chambered ventricle,was comparable to that of other malacostracan crustaceans(Pearson, 1908; McLaughlin, 1983; McGaw and Reiber, 2002). Theanatomical arrangement of three of the arterial complexes; theanterolateral arteries, posterior aorta and ventral thoracic arterywere similar to those described for blue crabs, cancrid crabs and toa certain degree the lopholithodid crabs (McGaw and Reiber, 2002;

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Fig. 8. The gills and associated vessels of Libinia emarginata: a) Lateral view of the right gills showing the branchial sinuses (BRS) connecting with the infrabranchial sinus (IBS),which then becomes the afferent branchial vein (ABV) that delivers hemolymph to the gill lamellae. b) Lateral view of a gill bar, with one side of gill lamellae (GL) removed. Arrowsshow direction of hemolymph from the infrabranchial sinus (IBS) into the afferent branchial veins (ABV) through the lamellae (GL) where it is oxygenated and returned back into theheart via the efferent branchial veins (EBV) and branchiocardiac vein (BCV) c) SEM (�25) showing arrangement of the gill lamellae. Hemolymph exiting the afferent branchial vein(ABV) along the marginal canal (MC) and through the network of lacunae that can be seen on the surface of the lamellae (GL). d) Ventral view of the gills and veins showing theconnection with the branchiocardiac veins. Two sets of gill bars connect into each branchiocardiac vein (BCV). These veins then fuse and run along the ventral edge of the heart andempty via a single vein at the lateral and posterior aspect of the pericardial sinus. The arrows show the pathway of hemolymph from the branchiocardiac veins (BCV) and into thepericardial sinus (PS).

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McGaw, 2005; McGaw and Duff, 2008), suggesting these vessels areconserved across most of the decapods. There were however,notable anatomical differences in several other arterial systems.

The anterior aorta of the Majidae was well developed comparedwith other brachyuran and anomuran crabs (Pike, 1947; McGawand Reiber, 2002; McGaw, 2005; McGaw and Duff, 2008). Smallvessels originating from the terminal end of the main anterior aortasupplied the antennules and antennae and the muscles thereof.These vessels were not evident on the corrosion casts of otherbrachyurans where the antennae and antennules tend to receivetheir blood supply from sub-branches of the anterolateral arteries(McGaw and Reiber, 2002; McGaw, 2005). However, it is unclear ifthese small branches of the anterior aorta are absent in the otherfamilies, or whether these delicate vessels were simply lost duringthe corrosion casting process (Hossler and Douglas, 2001). In majidcrabs the muscles of the gastric mill apparatus were closely asso-ciated with the anterior aorta, so much so that they were suppliedby a complex series of sub-branches from the anterior aorta. Incontrast, the gastric muscles of cancrid and portunid crabs aresupplied by the cardiac stomach artery, which arises from the

anterolateral arteries. As with the arrangement of the antennalvessels, the pear-shaped cephalothorax of the Majidae means thatthe foregut is shifted anteriorly, beyond the reach of branches of theanterolateral arteries.

In portunid, cancrid and lopholithodid crabs the cor frontale isevident as a raised area at the end of the anterior aorta. Thisstructure is thought to act as an auxiliary heart, aiding hemolymphflow into the brain region (Steinacker, 1978; Davidson and Taylor,1995). The cor frontale of majid crabs was not as defined as in theportunid and cancrid crabs. Dissection of live specimens suggestedthat the structurewasmore elongated than the cor frontale of otherbrachyurans. Its location immediately anterior to the cerebralartery backs-up the theory that it aids hemolymph flow into thesubesophageal ganglion (Steinacker, 1978).

In concordance with other decapods the sternal artery was thelargest artery, but it carried proportionally less hemolymph than inother brachyurans; 50e60% as opposed to 70e80% (McGaw et al.,1994; McGaw and Reiber, 2002; McGaw, 2005). In the majidcrabs the large hepatic arteries formed close inter-digitations withthe pereiopod arteries, this suggests that in addition to receiving

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Fig. 9. a) The branchiostegal sinus (BSS) of Pugettia producta forms a fine lattice work of lacunae on the dorsal surface of the branchial chambers. Hemolymph is returned to theheart via the branchiostegal veins (BSV) which join into the anterior aspect of the heart. An area of the branchiostegal sinus is highlighted and shown in b) SEM of branchiostegalsinus at a magnification of �18.

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hemolymph via the sternal artery, the muscles of the thoracicsterna also receive a co-supply of hemolymph from the hepaticarteries. This probably accounts for larger size of the hepaticarteries compared with other brachyuran crabs and the propor-tional smaller capacity of the sternal artery and associated vessels.Certainly the hepatic arteries in majids and even the portunid crabsare substantially larger and more significant that depicted in someof the highly cited works (McLaughlin, 1983).

Furthermore the sternal artery complex of majid crabs differedfrom the portunid and cancrid crabs in that the pereiopod arteriesarose from the main artery in a segmental fashion. The Majidaeare ancestral to the Cancridae and Portunidae (Spears et al., 1992);within the decapods the plesiomorphic state of the pereiopodartery arrangement is segmental and the pattern found in Cancercrabs and blue crabs is probably the more derived state (Wirkner,2009; Wirkner and Richter, 2010). This segmental arrangement ofthe majids is likely to offer more resistance to hemolymph flowinto each pereiopod vessel (Wilkens et al., 1997) than theanatomical arrangement of portunid crabs where all thepereiopod arteries arise from a common point on the main sternalartery (McGaw and Reiber, 2002; McGaw, 2005). Indeed theportunid crabs are faster moving animals than the spider crabs(Hill, 1978; Gonzalez-Gurrianran and Freire, 1994) which exhibitvery low flow rates through the pereiopod vessels (StillmanUnpub. Obs.). Nevertheless, hemolymph perfusion rates areunlikely to be solely dependent on the anatomical arrangement.Although there is no evidence of smooth muscle or valves thatcontrol hemolymph flow to individual pereiopod arteries, flowrates may be influenced by changes in down-stream resistanceresulting from skeletal muscle contractions (Wilkens, 1997;Wilkens et al., 1997) or hormone release (Wilkens and Taylor,2003). Despite the advances made in measurement of regionalblood flow in crustaceans during the past 15 years, the parti-tioning of hemolymph flow among individual pereiopods isunknown and therefore the actual hemodynamics remain spec-ulative at this point.

In part, it is the presence of sinuses, rather than a completeseries of veins which defines the crustacean circulatory system asopen. Even quite recently the sinuses and their associated lacunaewere reported to be poorly defined irregular spaces that were hard

to study (Martin and Hose, 1992). Nevertheless, over a century anda half ago Haeckel (1857) proposed that no unbounded lacunaeexist in the crustacean system. The major sinuses are bordered byfibrous connective tissue and the lacunae by basal lamina directlyon the organ which they bathe (Johnson, 1980). The distinctionbetween sinus and capillary then becomes less clear-cut, suggest-ing a more organized structure (Reiber and McGaw, 2009). Recentwork substantiates Haeckel’s proposal, and shows that manysinuses are discrete channels with their form being determined bythe boundaries of specific organs or muscles (McGaw, 2005; Reiberand McGaw, 2009). This also appears to be the case for majid crabs;most of the sinuses appeared as distinct structures or fine layers oflacunae, however, some were lost during tissue maceration andsubsequent handling of the casts due to their delicate form. The factthat all areas of the arterial system were perfused in the presentstudy, including the return channels, the gills and branchiostegalsinuses, suggests that casts were complete. Therefore, the repre-sentation of the sinuses is probably accurate.

The pathway of hemolymph into the gills is such that oncehemolymph has bathed the tissues it collects in the ventral thoracicsinus below the heart and radiates outwards as the branchialsinuses to fuse with the infrabranchial sinus (Pearson, 1908). Theinfrabranchial sinus of P. producta was a defined ring-like vesselrather than an amorphous structure (Fig. 8a). Maynard (1960),states that veins are distinguished from sinuses by having a distinctform and functioning solely for the return of blood; with thisreasoning the infrabranchial sinus of kelp crabs could feasibly beclassified as a vein. Hemolymph entering the gills flows along themarginal canal and through a network of lacunae on the surface ofthe lamellae where it is oxygenated. In terrestrial crabs (Farrellyand Greenaway, 2005) and to a lesser degree the portunid andcancrid crabs, spacing nodules on the edges of the lamellae keepthem from collapsing upon one another when in air. In the majidcrabs (Libinia emarginata and P. producta) no spacing nodules wereevident, resulting in the collapsing or folding of the lamellae seen inthe current preparation (Fig. 8c). Both these species are essentiallysub-tidal and it is unknown if majid crabs such as those in the genusInachus, which are primarily inter-tidal species, possess thesenodules. Oxygenated hemolymph leaving the lamellae enters theefferent branchial veins, from here the hemolymph continues into

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the branchiocardiac veins, and back into the heart. Typically, thebranchiocardiac veins connect into the pericardial sinus at separatepoints (Pearson, 1908; Blatchford, 1971; McGaw and Reiber, 2002).Here in the Majidae the arrangement is slightly different, eachbranchiocardiac vein fuses to form a single vessel that runs alongthe edge of the heart and then joins the pericardial sinus at theposterior lateral edge (Fig. 8d). It is possible that this entry pointcould be the “poche pericardiale” described by Cuenot (1895);a pouch-like structure on the posterior corners of the pericardialsac, the function of which is unknown (Pearson, 1908; Pike, 1947).

The branchiostegal sinus appeared as dense network of lacunaeand associated veins covering the dorsal surface of the branchialchambers. Although the branchiostegal circulation can bypass thegills, its exact function and the partitioning of venous returnbetween the branchial and branchiostegal circulations is not fullyunderstood (Taylor and Greenaway, 1984; Greenaway and Farrelly,1990; Taylor and Taylor, 1992). The branchiostegal circulation iswell developed in terrestrial and amphibious crabs, in theseanimals it is used for aerial gas exchange (Taylor and Greenaway,1979; Greenaway and Farrelly, 1990; Greenaway et al., 1996).However the majid crab species investigated here are essentiallyconfined to the sub-tidal zone; nevertheless the branchiostegalnetwork was still well developed in these as well as in other deep-water crab species that are unlikely to be aerially exposed (McGawand Duff, 2008). The exact function of ventilatory reversals has longbeen surmised, but they are thought to aid ventilation of deadspaces in the branchial chamber (Hughes et al., 1969; Davidson andTaylor, 1995). There is nothing to suggest that additional oxygencouldn’t be extracted from the water in the branchial chambers viathe branchiostegal sinus. If not, there would be a substantial mix ofdeoxygenated and oxygenated blood returning to the heart, andmixing appears to be minimal (McMahon, 2001). Thus, the deter-mination of the exact role of the branchiostegal circulation repre-sents a pressing question for crustacean physiologists.

The present study complements and extends recent work on thegross anatomy of the cardiovascular system of the Malacostracancrustaceans, showing that it varies in morphology and complexityeven within the brachyuran infraorder (McGaw and Reiber, 2002;Farrelly and Greenaway, 2005; McGaw, 2005; Wirkner and Richter,2007a,b, 2010; McGaw and Duff, 2008). Although ancestral to theportunid and cancrid crabs (Spears et al., 1992), the circulatorysystem of theMajidaewas equally as complex, with areas such as theantennal gland forming closed-loop systems. Classifying it in-linewith other decapod crustaceans as an “incompletely closed” systemwould seem appropriate (Reiber and McGaw, 2009).

Acknowledgments

We would like to thank the Director and staff of the BamfieldMarine Sciences Centre for use of facilities. We also thank Dr.Christian Wirkner for helpful discussion. This work was supportedby a NSERC Discovery Grant to IJM.

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