Critical Review

The Adaptation of Polar Fishes to Climatic Changes: Structure,Function and Phylogeny of Haemoglobin

Cinzia Verde, Daniela Giordano and Guido di PriscoInstitute of Protein Biochemistry, CNR, Via Pietro Castellino 111, Naples, Italy

Summary

In the Antarctic, fishes of dominant suborder Notothenioideihave evolved in a unique thermal scenario. Phylogeneticallyrelated taxa of the suborder live in a wide range of latitudes, inAntarctic, sub-Antarctic and temperate oceans. Consequently,they offer a remarkable opportunity to study the physiologicaland biochemical characters gained and, conversely, lost duringtheir evolutionary history. The evolutionary perspective hasalso been pursued by comparative studies of some features ofthe heme protein devoted to O2 transport in fish living in theother polar region, the Arctic. The two polar regions differ byage and isolation. Fish living in each habitat have undergone re-gional constraints and fit into different evolutionary histories.The aim of this contribution is to survey the current knowledgeof molecular structure, functional features, phylogeny andadaptations of the haemoglobins of fish thriving in the Antarc-tic, sub-Antarctic and Arctic regions (with some excursions inthe temperate latitudes), in search of insights into the conver-gent processes evolved in response to cooling. Current climatechange may disturb adaptation, calling for strategies aimed atneutralising threats to biodiversity. � 2007 IUBMB

IUBMB Life, 60(1): 29–40, 2008

Keywords Polar fishes; adaptation to climatic changes; haemoglobin;

structure; function; phylogeny.

Abbreviations Hb, haemoglobin; AFGPs, antifreeze glycoproteins;

ACC, Antarctic circumpolar current; APF, Antarctic

Polar Front.

INTRODUCTION

Evolution is the major unifying principle of biology, and evi-

dence of evolutionary processes pervades all levels of bio-

logical organisation from molecules to ecosystems (1).

Evolutionary adaptation to temperature is a multivariate

problem, with complexity at the community, organismal, cellu-

lar and molecular levels. Concerning molecules and cells, we

have an increasing understanding of thermal adaptation in a

small number of proteins and enzymes, but many gaps remain

in our knowledge about the evolutionary adaptation in cellular

processes. A classical example of how temperature has selec-

tively influenced evolution in one of the polar environments is

the Antarctic teleost family Channichthyidae (icefish), whose

genome has lost the genes encoding haemoglobin (Hb) (2–5).

The evolution of antifreeze glycoproteins (AFGPs) in Antarctic

and Arctic fish is a classical example of adaptation developed

independently at both poles. To avoid death by freezing, fish in

these environments have evolved antifreeze compounds secreted

into their blood at high concentrations (6).

Much of our knowledge of the effect of the environment on

vertebrate evolution has arisen from the study of fishes, which

share most physiological mechanisms with humans. Their bodies

are in direct contact with water, and the close physical and physi-

ological relationship with the aquatic environment makes them

sensitive sentinels of environmental challenge and offers impor-

tant advantages for defining the organism-environment interface

and responses to temperature adaptation. The biochemistry of O2

transport in polar fish offers a wealth of important indications on

physiological adaptations of the organism to the habitat climate.

Hence, the studies on Hbs of polar fish are helping to improve

the understanding of the extremely complex structure-function

relationship in this ancient and versatile protein.

Hb has evolved structural and functional diversities to adapt

and modify its features under selective pressure of all types, but

both the predominantly helical structure and a large number of

amino acid residues are well conserved. The primary role of Hb

that of carrying O2 to vertebrate tissues is probably the origin

of its adaptation to widely different environmental conditions.

Its specialised function imposes severe structural constraints to

the molecule, endowed with remarkable functional flexibility,

considering that only a small fraction of the residues of the

polypeptide chains undergo replacement during evolution.

Address correspondence to: Guido di Prisco, Institute of Protein

Biochemistry, CNR, Via Pietro Castellino 111, I-80131 Naples, Italy.

E-mail: g.diprisco@ibp.cnr.it

Received 9 October 2007; accepted 9 October 2007

ISSN 1521-6543 print/ISSN 1521-6551 online

DOI: 10.1002/iub.1

IUBMB Life, 60(1): 29–40, January 2008

The main theme of this review is a summary of the current

knowledge on molecular structure, biological function and phy-

logeny of Hbs of fish species living in both polar habitats but

having different evolutionary histories. In benthic, non-migra-

tory, cold-adapted fishes, the stability of thermal conditions may

have generated no or few variations in selective pressures on

globin sequences through evolutionary time, so that the sequen-

ces retain the species phylogenetic ‘signal’. In pelagic, migra-

tory, cold-adapted or temperate fishes, variations in selective

pressures on globin sequences caused by variations in tempera-

ture accompanying the dynamic life style may have disrupted

the phylogenetic ‘signal’ in phenetic trees. Our aim is to take

advantage of the interplay of phylogeny with structure/function

relationships and of the integration of molecular, functional and

ecological data, as necessary tools to clarify evolution in polar

habitats. To date, this multidisciplinary approach has not been

sufficiently exploited. New information will help to understand

the evolution of the O2-transport proteins and the interrelations

between different phylogenies and environmental conditions.

For the sake of the reader, we have chosen a palaeohistory

and palaeogeography background. It is a useful tool because of

the many differences characterising the two polar environments.

THE ANTARCTIC

Since the late Precambrian, 590 million years ago (mya),

and during 400 million years (my), together with South Amer-

ica, Africa, India, Australia and New Zealand, Antarctica had

witnessed temperate and even subtropical climatic scenarios.

Antarctica was part of Gondwana, a supercontinent, which in

turn was part of Pangaea and began to break up c. 135 mya.

The separation of the fragments took place during the early

Tertiary.

The continental drift took Antarctica near the current geo-

graphic position at the beginning of the Cenozoic, c. 65 mya.

Progressive cooling began. The formation of vast areas of sea

ice occurred since the end of the Eocene (40 mya), and the ear-

liest cold-climate marine faunas are thought to date back to lat-

est Eocene–Oligocene (35 mya). Conditions on land fluctuated

greatly between cold and warm between 65 mya and 2 mya,

and terrestrial faunas and floras changed accordingly; habitats

supporting terrestrial faunas and floras have been continuously

available for lengths of time ranging from several million to

only a few thousand years. The previous terrestrial biota has

probably been eliminated by environmental change, and the

present biota is very recent. Extensive and thick ice sheets

began to form periodically every 1–3 my only 14 mya, after the

middle Miocene.

Because of the reduction of thermal exchanges with northern

latitudes, cooling continued until it reached the low tempera-

tures characterising the extremely cold and arid desert that Ant-

arctica is today. The sea became colder and colder and covered

with ice during each Austral winter.

The isolation of Antarctica was completed 25–22 mya, in the

transition between Oligocene and Miocene, when the opening

of the Drake Passage separated Tierra del Fuego from the Ant-

arctic Peninsula 23.5–32.5 mya (7). This event also led to the

development of the Antarctic Circumpolar Current (ACC, par-

tially responsible for cooling of Antarctic waters) and of the

Antarctic Polar Front (APF). The APF, at the northern boundary

of the ACC, well north of the Antarctic coast, is a well-defined,

roughly circular oceanic system running between 508 S and 608S. Along the APF, the surface layers of the Antarctic waters,

moving north, sink beneath the less cold and less dense sub-

Antarctic waters; the ocean temperature is 1–2 8C during winter

and hardly reaches 4–5 8C (but only in the 50-m layer at the

surface) in the summer. Just north of the APF (where the sea

is always turbulent), the water temperature has an abrupt rise of

3 8C. This rise seems modest, but in the isolation and adaptive

evolution of ecosystems is indeed a factor of the utmost impor-

tance.

Over geological time, environmental conditions and available

habitats in Antarctica have changed dramatically. In the recent

years, biological research in Antarctica has envisaged coordina-

tion of existing work and stimulation of new studies on evolu-

tion, also in view of the increasing evidence of climate change.

We now have a framework to improve our understanding of the

evolutionary history and biology of the unique Antarctic biota

and to integrate this with the knowledge of the climatic and tec-

tonic context within which evolution occurs. Much information

is becoming available on the evolution of fish, crustaceans,

squid, macrobenthos, phytoplankton, algae, fungi, lichens,

mosses, bacteria and protozoa.

The perciform suborder Notothenioidei, mostly confined

within Antarctic and sub-Antarctic waters, is the dominant com-

ponent of the southern ocean icthyofauna. The ancestral noto-

thenioid stock probably arose as a sluggish, bottom-dwelling

teleost species that evolved some 40–60 mya in the shelf waters

(temperate at that time) of the Antarctic continent. Over the

past 40 my, the Antarctic shelf has undergone tectonic and

oceanographic events leading to progressive change in composi-

tion of the ichthyofauna. With the local extinction of most of

the temperate Tertiary fish fauna as the southern ocean cooled,

the suborder experienced extensive radiation, dating from the

late Eocene, c. 24 mya (8), which enabled it to exploit the

diverse habitats provided by a progressively cooling marine

environment. Eastman and McCune (9) have indicated notothe-

nioids as one of the few examples of marine species flock, due

to the geographic, thermal and hydrologic isolation of the Ant-

arctic shelf.

Parts of the work on the fish suborder Notothenioidei will be

herewith reviewed. This single group of fishes (to date the best

known in the whole world) replaced the cosmopolitan, diverse

ichthyofauna of the Eocene. Bovichtidae, Pseudaphritidae, Ele-

ginopidae, Nototheniidae, Harpagiferidae, Artedidraconidae,

Bathydraconidae and Channichthyidae are the families of the

suborder (Fig. 1).

30 VERDE ET AL.

Notothenioids are red-blooded except Channichthyidae (2),

the only known adult vertebrates whose colourless blood is

devoid of Hb. An excellent book (10) provides extensive back-

ground information on the evolution of Antarctic fish.

As water temperatures decreased and ice appeared, the high-

Antarctic notothenioids acquired AFGPs. However, not all noto-

thenioids are high-Antarctic. All Bovichtidae (except one spe-

cies), monotypic Pseudaphritidae and Eleginopidae, as well as

some species of Nototheniidae, inhabit waters north of the APF;

some inhabit either sub-Antarctic or temperate waters. Most of

these have never acquired AFGPs. Within Notothenioidei, most

cladistic analyses identify Bovichtidae as the phyletically basal

family. Notothenioidei offer opportunities for the identification

of the biochemical characters or the physiological traits respon-

sible for thermal adaptation.

THE ARCTIC

Although high latitudes and cold climates are common to

both the Antarctic and the Arctic, in many respects, the two

regions are more dissimilar than similar. Because of the APF,

the climatic features of the Antarctic waters are more extreme

and constant than those of the Arctic. In the Arctic, isolation is

less severe, and the range of temperature variations is wider.

The colonised terrestrial portions are extensive; they are directly

linked to temperate areas, greatly facilitating adaptation and

redistribution of terrestrial organisms on one hand and, on the

other hand, producing wide and complex terrestrial feedback to

the climate, which adds to those originating from ocean and

atmosphere circulation. In addition, the anthropogenic impact

on the environment is greater.

The main differences between the Arctic and the Antarctic

regions are the older age and longer isolation of the latter (11).

Geography, oceanography and biology of species inhabiting the

polar regions have often been compared (12) to outline the dif-

ferences between the two polar ecosystems. The exchange of

Atlantic and Arctic waters through the passage between Green-

land and the Svalbard Islands was not possible until 27 mya

(11). The Arctic region was in a high-latitude position by the

early Tertiary, but the climate remained temperate with water

temperatures of 10–15 8C. Arctic land masses reached their

present positions and temperatures dropped below freezing dur-

ing the Miocene, about 10–15 mya. Permanent ice cover has

been present only since c. 0.7 mya (11).

ANTARCTIC/ARCTIC COMPARISON

At different times during the Cenozoic, tectonic and oceano-

graphic events played a key role in delimiting the two polar

ecosystems and in influencing the evolution of their fauna. Fau-

nal composition and diversity are strongly linked to geological

history. The Antarctic has been isolated and cold longer than

the Arctic, with ice-sheet development preceding that in the

Arctic by at least 10 my.

Figure 1. Cladogram of relationships within the perciform suborder Notothenioidei pruned to the level of family. The original cla-

dogram is a strict consensus of four trees resulting from maximum-parsimony analysis of the complete gene 16S rRNA dataset.

AFGPs characteristic of the Antarctic clade are mapped.

31HEMOGLOBIN IN POLAR FISH

The modern polar faunas are therefore different in age,

endemism, taxonomy, zoogeographic distinctiveness and range

of physiological tolerance to various environmental parameters.

Both the Antarctic and the Arctic possess cryopelagic taxa liv-

ing at the interface of sea ice and water. Freshwater habitats

exist in the Arctic, but are more limited in the Antarctic, with

consequent limitations to species diversification. There are

adaptive radiations of fish, isopods and amphipods in the Ant-

arctic but not in the Arctic. The existence of latitudinal gra-

dients in species diversity needs to be investigated and critically

examined in a wide variety of organisms.

Differences in the respective ichthyofaunas do exist. The

Antarctic fauna includes 322 fish species grouped in 50 families

(13), whereas the Arctic fauna has 416 species of 96 families.

In Antarctica, endemism of the benthic fauna is 88% and rises

to 97% when only the suborder Notothenioidei is considered. In

comparison, endemism in the Arctic is 25% for marine fish

(11). The Arctic fauna has no endemic higher taxonomic cate-

gory equivalent to the Antarctic notothenioids, and there has

been no comparable adaptive radiation of any fish group.

There are advantages in using organisms from both poles in

evolutionary studies. Some fish families (for example, Zoarcidae

and Liparidae) are represented in both polar oceans. On the

other hand, hypotheses of adaptation to a common environmen-

tal parameter are afforded higher certainty when the trait can be

documented in phyletically unrelated taxa from both habitats.

Because the two polar faunas include such taxa, the compara-

tive approach permits examination of convergent and parallel

evolutionary trends at levels ranging from molecules to organ-

isms. Pertinent examples include the convergent evolution in

which genes coding the identical AFGPs of northern cods and

of southern notothenioids have different evolutionary histories,

and the recent studies showing that Arctic fish globins diverge

from those of Antarctic notothenioids (see below).

Palaeontologists are beginning to assemble a long history of

temperate-, cool/temperate- and cold-climate marine biotas that

have evolved in a mid- to high-latitude setting. Polar biotas

have lower taxonomic diversity, possibly resulting from differ-

ence in evolutionary rates, with higher rates in the tropics and

lower rates towards the poles. For the study of evolution, it is

important to assess the age of polar habitats and species through

palaeobiological data (using palaeontological information to

compare latitudinal diversity gradients in fossil record and liv-

ing biota in the southern and northern polar regions) and to

exploit the advantages of molecular and biochemical approaches

for physiological and genetic analysis of adaptive evolution in

cold environments.

The differences in the two polar environments are differently

reflected in global climate changes. Accordingly, studies on

their ecosystems are likely to provide answers to different ques-

tions, often complementary to one another. In summary, the

Arctic is the connection between the more extreme, simpler

oceanic Antarctic system and the more complex temperate and

tropical systems.

SOME RECENT RESULTS ON ADAPTATIONAND EVOLUTION

To survive in the cold, polar fishes have evolved suitable

physiological and biochemical mechanisms. During the progres-

sive geographic isolation initiated 65 mya, the physiology of

Antarctic fishes became gradually adapted to the cooling of the

habitat, and they now lead a comfortable life only in the cold.

If we observe their behaviour in a tank where a temperature

gradient has been established, we shall see that, after careful ex-

ploration, they will settle in the coldest region, because it is

where their metabolism works at best. In fact, if the water tem-

perature is raised of only a few centigrades, they will die. Since

a long time, the coastal Antarctic waters, where in turn survival

of temperate fish would be impossible because they would

freeze, are constantly at 21.87 8C, the equilibrium temperature

of sea water and ice.

The capacity to adapt is variable. Stenothermal organisms

may not tolerate temperature changes of 2–3 8C. Because of the

larger seasonal temperature variations in Arctic waters, local

fish have adapted to survive over a much wider thermal range.

Antifreeze Compounds in Antarctic and Arctic Fishes

The biosynthesis of AFGPs, which allows polar fish to sur-

vive at sub-zero temperatures, is one of the most intriguing evo-

lutionary adaptations and meets the criteria for a ‘key innova-

tion’ (14). Most of our knowledge on the molecular and physio-

logical bases of this unique adaptive strategy comes from the

over 30-years-long studies of the team of Arthur DeVries [for

review, see (15)].

The proteins of the family of AFGPs have a most unusual

amino-acid sequence, made of the repeating glycotripeptide

(-alanyl-alanyl-threonyl-)n in which Thr is bound to a disaccha-

ride, which binds water molecules, preventing them to form ice

crystals. The AFGP gene evolved from a functionally unrelated

pancreatic trypsinogen-like serine-protease gene, through a mo-

lecular mechanism by which the ancestral gene provided the

front and tail of the emerging AFGP gene (6). The finding in

the notothenioid genome of (i) a chimeric AFGP-protease gene

intermediate, (ii) a protease gene still bearing the incipient cod-

ing element, and (iii) independent AFGP genes reveals a fasci-

nating case of ‘evolution in action’.

By using a molecular clock rate (0.5–0.9% my) estimated

for mitochondrial DNA of salmon and the amount of sequence

divergence between trypsinogen of an Antarctic fish (Dissosti-

chus mawsoni) and AFGP genes in their homologous segments,

it was deduced that the conversion of the ancestral gene to the

first AFGP gene occurred 5–14 mya. This value agrees well

with the generally accepted time frame (10–14 mya), in which

the Antarctic water reached the present conditions. The notothe-

nioid families, which occupy all niches throughout the water

column, emerged during rapid phyletic diversification 7–14

mya, therefore radiation into cold habitats must follow the

emergence of the antifreeze protective function.

32 VERDE ET AL.

Analysis of AFGP in the polar cod (Boreogadus saida, fam-

ily Gadidae) showed that the genome of this Arctic species

(phylogenetically unrelated to notothenioids, which belongs to

different superorder and order) contains genes, which encode

nearly identical proteins. This would suggest a common ances-

try. On the contrary, the genes of the two fish groups are not

homologous, hence have not followed the same evolutionary

pathway. Assuming an endogenous, yet unknown genetic origin,

the cod AFGP genes have evolved from a different, certainly

not trypsinogen-like, genomic locus. An example of the conver-

gent evolution has thus been discovered (16).

In the Arctic, the biosynthesis of AFGPs occurs only in the

coldest winter months, whereas Antarctic fishes must rely on

this evolutionary adaptation all year long.

The O2-transport System of Polar Fish

The Hbs of High-Antarctic Notothenioidei. During the cooling

of the environment, the evolution of notothenioids has led to

unique specialisations, including modification of haematological

features. Notothenioids differ from temperate and tropical spe-

cies in having fewer erythrocytes and reduce Hb concentration

and multiplicity in the blood. There are very few erythrocyte-

like cells and no Hb in Channichthyidae, the most derived fam-

ily. These modifications counterbalance the potentially negative

physiological effects (i.e., higher demand of energy needed for

circulation) due to the increase in blood viscosity produced by

the subzero seawater temperature. At the same time, low tem-

peratures reduce the overall metabolic demand for O2, while

increasing its solubility in the plasma, so that more O2 can be

carried in physical solution, and less needs to be bound to Hb.

The vast majority of notothenioid species have a single Hb

(Hb 1), a minor component (Hb C) in traces, and sometimes

another (Hb 2); Hb C and Hb 2 have one of the globins in com-

mon with Hb 1 (17). In many species belonging to all seven

red-blooded families, major and minor Hbs have been structur-

ally and functionally characterised. The amino-acid sequences

and the similar functional features of major and minor Hbs in a

given species led us to conclude that minor Hbs are vestigial

(or perhaps larval) remnants, devoid of physiological impor-

tance at least in the adult phase (18). Hb 2 and Hb C are pres-

ent in much higher amounts in juveniles (di Prisco, unpub-

lished). The hypothesis that they are dominant components in

the larval stage offers an explanation to the high mutual

sequence identity of the minor components and, conversely, to

the low similarity with major Hbs (Table 1).

Table 1 is an overview of sequence identities in teleost glo-

bins. In notothenioids, the identity among the major components

(Hb 1) is very high (right-hand grey triangles). The minor com-

ponents Hb 2 and Hb C usually have the b- and a-chain in

common, respectively, with Hb 1. The chains, which are not in

common, have low identity with Hbs of temperate non-notothe-

nioids (as expected), but also with Hb 1 of all notothenioids.

On the other hand, the identity with one another is high (left-

hand grey triangles).

Indeed, the two Hb types would be subjected to very differ-

ent evolutionary pressures. The temporary needs that larvae

must face to survive may correspond to lower evolutionary

pressure on the minor components, whose biosynthesis will be

gradually curtailed in the juvenile and adult phases; in contrast,

the primary structure of the major component Hb 1 may reflect

the much more refined adjustments of the O2-transport system

developed by adult fish.

Multiplicity of Hb components in fish usually reflects the

need to respond to variable environmental features or different

habitats. A single Hb present in lower amounts than in temper-

ate fish can be regarded as the consequence of a less critical

role of the O2 carriers in the vast majority of Antarctic notothe-

nioids, possibly in relation to their sluggish style of life, slower

metabolism, as well as to the peculiarity of the environment

(high stability and constancy of physico-chemical conditions).

Only one notothenioid family has completely abolished Hb

as O2 carrier. One of the most unusual adaptations of verte-

brates was reported by Ruud (2). The colourless blood of an

‘icefish’ was devoid of Hb, a feature found to be shared by the

16 icefish species of the family Channichthyidae. Their blood

has a very small particulate fraction (leukocytes and very few

erythrocyte-like cells). The loss of Hb in icefishes is accompa-

nied by the loss of myoglobin (19).

The icefishes maintain normal metabolic function by deliver-

ing O2 physically dissolved in the blood to tissues. Without

doubt Channichthyidae are not disadvantaged by their lack of

Hb. Reduction of the hematocrit to near zero appears selectively

advantageous because it diminishes the energetic cost associated

with circulation of a highly viscous, corpuscular blood fluid.

Why have Channichthyidae (the notothenioid crown group)

alone taken such a radical course, leaving the other families

with only partial reductions in Hb? The physiological role of

Hb in O2 transport in temperate and tropical fish is undisputed;

however, does Hb remain essential for O2 transport in the Ant-

arctic red-blooded families, or is it a redundant vestigial relict

which may be redundant under stress-free conditions? The an-

swer was found by reversibly ‘poisoning’ Hb of the red-blooded

nototheniid Trematomus bernacchii with carbon monoxide (20).

CO is lethal for most vertebrates (including temperate fish)

whose life depends on O2, because it binds to Hb with much

higher affinity than O2, thus replacing this ligand and prevent-

ing transport of the ultimate respiratory electron acceptor. We

monitored the block of O2 binding, in two experiments in which

fishes bearing a cannula in the caudal vein were used. In the

first one (which took less than 3 min), approximately half of the

total volume of the fish blood was removed, saturated with car-

bon monoxide by gentle bubbling and reinjected into the caudal

vein. Aliquots of blood were taken periodically and analysed

spectrophotometrically for carbomonoxy-Hb. Ten min after

reinjecting the blood, the latter derivative accounted for 56% of

the total Hb. Carbomonoxide-Hb became reconverted slowly

33HEMOGLOBIN IN POLAR FISH

Table 1

Selection of sequence identities (%). Abbreviations (Ccl, etc) from left to right, correspond to species from

bottom to top. For full species names, see (17). High-Antarctic in black, non-Antarctic underlined,

temperate in grey and Arctic in grey background

a-Chains

Species Ccl Ca Ss Aa Aa Tt Om Gm A min A min Pa Tn Gg Nc Ga Ao Pu Gg Na Nc Pa TnHaemoglobin A C I 2 1 2.3 3 2 2 2 1.2 1 1 1 1,2 1,CT. bernacchii Hb1,C 59 64 54 54 52 76 54 59 66 74 67 62 64 66 90 87 91 94 96 89 91 97

T. newnesi Hb1,C 58 62 63 53 52 76 53 60 64 72 66 63 64 66 91 85 76 92 93 87 88

P. antarcticum Hb1,2 59 66 58 56 54 77 59 57 68 75 69 64 66 66 84 88 80 91 93 86

N. coriiceps Hb1 55 61 51 50 51 71 52 56 62 68 64 61 61 62 81 83 71 86 90

N. angustata Hb1 59 66 57 54 53 76 56 59 67 75 69 64 65 66 88 90 77 95

G. gibberifrons Hb1 59 63 55 53 52 74 55 61 69 74 71 65 67 68 89 86 78

P. urvillii Hb1,2 57 63 58 54 57 76 59 57 66 76 64 63 64 65 75 74

A. orianae 61 66 58 56 53 73 59 58 66 73 69 64 65 66 81

G. acuticeps 57 61 54 52 52 74 54 59 64 71 68 65 64 66

N. coriiceps Hb2 56 65 61 56 61 66 61 66 80 68 92 93 95

G. gibberifrons Hb2 56 65 62 57 61 65 61 66 80 68 92 95

T. newnesi Hb2 55 64 63 57 61 64 62 66 57 66 90

P. antarcticum Hb3 60 68 64 57 59 66 64 67 79 69

A. minor Hb2,3 61 69 62 57 56 73 61 58 72

A. minor Hb1 59 66 69 57 64 68 67 65

G. morhua Hb2 59 64 58 52 55 61 57

O. mykiss HbI 64 67 91 56 68 61

T. thynnus 61 65 59 54 54

A. anguilla HbC 54 64 69 54

A. anguilla HbA 61 66 57

S. salar 62 67

C. auratus 76

C. clarkiib-ChainsSpecies Ca Aa Aa Tt Om Bs Gm A min A min Tb Tn Pu Pa Gg Ga Ao Pu Gg Na Nc Pa TnHaemoglobin A C I 1,2 2,3 3 1,2 C C 2 2 2 1 1 1.2 1.2 1.3 1.2T. bernacchii Hb1 63 55 60 67 56 60 71 76 75 69 69 69 71 68 82 80 81 95 91 88 90 93

T. newnesi Hb1,2 59 55 57 65 53 57 70 72 75 67 67 67 68 66 80 78 77 89 84 84 83

P. antarcticum Hb1,3 61 56 62 67 56 62 71 74 72 69 69 69 71 69 79 76 77 91 87 85

N. coriiceps Hb1,2 58 54 60 66 53 58 69 75 78 70 70 69 71 68 80 82 78 89 93

N. angustata Hb1,2 60 54 60 65 55 60 71 76 78 70 70 70 72 69 80 83 80 93

G. gibberifrons Hb1 62 55 60 67 55 60 71 77 76 71 71 71 73 69 82 81 78

P. urvillii Hb1 64 56 60 69 59 64 71 71 77 69 68 68 69 67 74 73

A. orianae 58 53 58 65 52 60 69 71 74 67 67 67 70 68 77

G. acuticeps 58 54 58 63 54 61 65 70 71 66 67 65 68 65

G. gibberifrons Hb2 63 59 58 62 55 65 73 82 67 91 89 87 90

P. antarcticum Hb2 65 58 60 65 57 65 71 84 68 91 91 84

P. urvillii Hb2 60 56 60 63 52 64 71 84 68 88 86

T. newnesi HbC 60 56 60 60 56 61 68 81 66 95

T. bernacchii HbC 61 56 58 62 54 62 70 82 75

A. minor Hb1,2 63 56 59 71 58 60 71 73

A. minor Hb3 63 57 58 65 58 65 72

G. morhua Hb2,3 63 58 58 69 54 62

B. saida Hb1,2 64 59 61 58 59

O. mykiss HbI 63 53 66 58

T. thynnus 61 54 60

A. anguilla HbC 64 56

A. anguilla HbA 71

C. auratus

34 VERDE ET AL.

but totally to oxyHb within 2 days. In the second protocol,

water of a sealed aquarium was equilibrated with 7% carbon

monoxide in air. The aquarium contained cannulated specimens

of T. bernacchii, as well as control specimens of a Hb-less ice-

fish. After 5 hours, over 95% of T. bernacchii Hb was con-

verted to the carbomonoxy derivative, and hence was unable to

carry O2. Normal aeration and water circulation were then

established, and analysis of blood showed that carbon monoxide

had been completely removed from Hb within the next 48 h.

Death would have inevitably occurred to any temperate fish in

either of the two experiments. However, during both regimes,

fish showed no signs of distress, even during enforced exercise.

The survival of red-blooded T. bernacchii, in spite of the func-

tional incapacitation of Hb, leaves little doubt that, in the cold,

stable environment of the Antarctic seas, routine O2 delivery is

still possible also in the absence of functional Hb. Similar to

icefishes, red-blooded Antarctic fish can carry routinely needed

O2 dissolved in the blood. The southern ocean provides an envi-

ronment in which vertebrate species may survive without O2-

binding proteins.

The adaptations that reduce O2 demand and enhance O2

delivery include modest suppression of metabolic rates, large

gills, scaleless and highly vascularised skin, large capillary di-

ameter, large increase in cardiac output and blood volume. The

development of such compensatory physiological and circula-

tory adaptations argues that loss of Hb and erythrocytes was

probably maladaptive under conditions of physiological stress.

The most plausible evolutionary scenario is that the phyloge-

netic trend to reduced hematocrits and Hb synthesis in notothe-

nioids developed concurrently with adaptations of their respira-

tory and circulatory systems, leading ultimately to the acorpus-

cular, Hb-less condition of the icefishes.

The globin-gene organisation in red-blooded notothenioids

has been characterised, the status of globin genes in channich-

thyid genomes has been studied and potential evolutionary

mechanisms leading to the Hb-less phenotype have been eval-

uated (5). The evolution of icefishes to the Hb-less phenotype

may have occurred by at least two mechanisms: (i) direct gene

deletion or (ii) a multi-step process involving transcriptional

inactivation of the adult and the embryonic/juvenile globin com-

plexes, followed by elimination of the non-functional genes

themselves. Channichthyids diverged from other notothenioids

c. 7–15 mya, but radiation of species within the icefish clade

appears to have been confined to the last 1 my (21). Our find-

ings on globin genes in Hb-less species, suggesting that the loss

of gene expression is a primitive character (established in the

ancestral icefish before diversification within the clade), open

promising pathways for evolutionary studies.

Icefish genomes retain small, inactive remnants of genomic

DNA sequences closely related to the adult a-globin genes of

their red-blooded notothenioid ancestors and contemporaries,

whereas the ancestral b-globin-gene sequences have been either

deleted or diverged beyond the limits of detection (3–5). South-

ern blots of genomic DNA from red- and white-blooded noto-

thenioids, probed with fragments of the genes flanking the ends

of the embryonic/juvenile complex, indicated that icefishes have

also lost embryonic/juvenile globin genes. The current hypothe-

sis is that the inability to express Hb arose from a single, large-

scale deletional event removing all globin genes with the excep-

tion of the 3’ end of adult a-globin, namely almost the entire

notothenioid globin-gene complex. The transcriptionally inactive

remnant, no longer under positive selection pressure for expres-

sion, subsequently experienced random mutational drift, with-

out, as yet, complete loss of sequence information. Therefore,

these a-globin genetic remnants should prove useful as tools for

the development of a molecular phylogeny of icefishes and for

calibration of a vertebrate mutational clock free of selective

constraints.

In red-blooded teleosts, juvenile and adult globin loci are

typically composed of tightly linked pairs of a- and b-globingenes. In a recent study, Near et al. (22) demonstrated that the

phylogenetically derived icefish species, Neopagetopsis ionah,

possesses a complete, but non-functional, adult ab-globin com-

plex. Maximum-likelihood ancestral state reconstruction sup-

ports a scenario of icefish globin-gene evolution involving a sin-

gle loss of the transcriptionally active adult globin cluster

before the diversification of the extant species in the clade. The

inactive ab-globin pseudogene complex of N. ionah may be

considered an intermediate ‘genomic fossil’ revealing key

mechanisms on the pathway to loss of Hb expression by all ice-

fish species (22).

In three red-blooded nototheniids with different life style, the

Hb system differs from that of all the sluggish benthic species,

which have a single major Hb. Active cryopelagic Trematomus

newnesi (23) and Pagothenia borchgrevinki (24), and pelagic,

sluggish but migratory Pleuragramma antarcticum (25) have

multiple and functionally distinct Hbs.

The Hb system of T. newnesi (23) comprises Hb C (20–25%

of the total), Hb 1 (70–75%; it has the a-chain in common with

Hb C) and Hb 2 (5%). This is the only species having two

major Hbs; in fact, Hb C is far from being present in traces,

thus not being a vestigial remnant. Only Hb C displays pH and

organophosphate regulation. This Hb system can ensure O2

binding at the gills (via Hb 1) and controlled delivery to tissues

(via Hb C) also when active behaviour may produce acidosis.

Even if Hb C has not been subjected to selection, its expression

can nonetheless be activated as a consequence of needs arising

from the fish life style, and high levels of Hb C, conceivably

redundant in other notothenioids (which have only traces of Hb

C, but count on Hb 1 and Hb 2), compensate for the lack of

regulation of Hb 1 and Hb 2 by protons and other effectors.

P. antarcticum has three major Hbs (Hb 1, Hb 2 and Hb 3),

the highest multiplicity within notothenioids (25). The Hbs dis-

play strong pH regulation, but show important differences in

thermodynamic behaviour, indicating high specialisation to fit

the unusual pelagic life style. Rather than the response to face

acidosis, this Hb system may have developed, as the main

adaptive feature, the response to the need to optimise O2 load-

35HEMOGLOBIN IN POLAR FISH

ing and unloading during seasonal migrations through water

masses, which can have different and fluctuating temperatures.

Hb 2 and Hb 3 have the a- and b-chain, respectively, in com-

mon with Hb 1. On the basis of their relative amounts, none of

the three Hbs of P. antarcticum can be considered as an evolu-

tionary remnant devoid of physiological relevance unlike the

minor components found in the benthic notothenioids, even

though the sequence data reveal high phylogenetic distance

between the globins of Hb 1 and those of Hb 2 and Hb 3 that

are not in common (see below: The molecular evolution of po-

lar fish Hbs). The expression of multiple genes remains high in

P. antarcticum and T. newnesi also in the adult stage, in closer

similarity with juveniles (di Prisco, unpublished), suggesting

refined mechanisms of regulation within the gene family.

P. borchgrevinki (24) has a higher Hb concentration than

other Antarctic notothenioids, possibly correlated with the rela-

tively active mode of life of this fish. It has five Hbs (Hb C, Hb

0, Hb 1, Hb 2 and Hb 3) with five globins. Only Hb 1 is a

major component, accounting for 70–80% of the total. Hb 1

and Hb 2 are functionally indistinguishable and display similar

proton and organophosphate regulation. The other Hbs display

different levels of regulation. The oxygenation temperatures are

lower than those of temperate Hbs. Temperature variations may

have a different effect on the functional properties of each Hb,

and chloride and phosphates may play an important role in the

conformational change between oxy and deoxy structures.

Studies on the O2 transport in all Antarctic species investi-

gated have shown that the O2 affinity of Hb (a property which

controls both the binding of O2 at the gill exchange surface and

its release to the tissues) is quite low (17). This feature is pre-

sumably related to the high concentrations of dissolved O2 in

the cold Antarctic waters.

The Hbs of Non-Antarctic Notothenioidei

A wealth of knowledge is available on the O2-transport sys-

tem of Antarctic fish, and information on the structure and func-

tion of Hb of non-Antarctic notothenioids is also steadily grow-

ing. The comparison of the biochemical and physiological adap-

tations between cold-adapted and non-cold-adapted species is a

powerful tool to understand whether (and to what extent)

extreme environments require specific adaptations or simply

select for phenotypically different life styles. The structure,

function and molecular phylogeny of Hb in non-Antarctic noto-

thenioids (Verde et al., unpublished) will be discussed here.

Species inhabiting waters north of the high Antarctic include

most Bovichtidae, some Nototheniidae and monotypic Pseu-

daphritidae and Eleginopidae. The origin of these non-Antarctic

notothenioids is not completely understood. Because AFGP

genes apparently evolved only once, prior to the radiation of

Antarctic notothenioids, the presence of AFGP genes in extant

temperate notothenioids can be used to infer the Antarctic evo-

lutionary origin (26).

Bovichtidae and Pseudaphritidae are the most primitive noto-

thenioid families, with 11 of 12 species living in temperate

waters. Of the 11 bovichtids, 9 are of the genus Bovichtus;

2 species, Cottoperca gobio and Halaphritis platycephala, are

monotypic. C. gobio is confined to the waters of the Magellanic

Province (southern South America, Burdwood Bank and Falk-

land Islands); H. platycephala is endemic to the inshore waters

of Tasmania. With the exception of Bovichtus elongatus from

waters of the northern Antarctic Peninsula, the eight other spe-

cies of the genus are found outside the Antarctic region. B.

diacanthus from Tristan da Cunha (378 S) lives near the north-

ern limit for notothenioids (10).

Some species belonging to Bovichtidae and monotypic Pseu-

daphritidae and Eleginopidae occupy a phylogenetically basal

position with respect to Antarctic Notothenioidei, providing evi-

dence that vicariance associated with the break-up of Gondwana

has been an important factor in notothenioid diversification

(27). Because of their geographical origin, these species are

especially important for calibrating the evolution molecular

clock in Antarctica.

Unlike most Antarctic notothenioids but similar to many

other acanthomorph teleosts, adult C. gobio, thriving in sub-

Antarctic waters just north of the Polar Front, has two major

Hbs sharing the b-chain. In the two Hbs, O2 binding is strongly

modulated by heterotropic effectors, with marked Bohr and

Root effects, and high O2 affinity compared to Antarctic noto-

thenioid Hbs. Higher multiplicity and O2 affinity have also been

observed in B. diacanthus, one of the most northern notothe-

nioids. Although the presence of multiple Hbs in the blood of

non-Antarctic notothenioids may be considered a plesiomorphic

condition for many perciform fishes, the more complex O2-

transport system in C. gobio and B. diacanthus may have been

maintained by positive selection to deal with the large tempera-

ture changes in waters north of the Polar Front.

Similar to many high-Antarctic notothenioids, the haemoly-

sate of Pseudaphritis urvillii (family Pseudaphritidae) (28) and

Eleginops maclovinus (family Eleginopidae) has a single major

Hb (Hb 1) and a minor component (Hb 2). The low amount of

Hb 2 can be considered a synapomorphic character linking

P. urvillii and E. maclovinus to most of the other notothenioids.

E. maclovinus is the sister-taxon of non-bovichtid and non-pseu-

daphritid notothenioids. This is consistent with the observation

that Eleginops is a sub-Antarctic species devoid of AFGPs.

Some hematological parameters (high hematocrit, erythrocyte

number and Hb content/cellular concentration) of Notothenia

angustata favour O2 transport in a temperate environment; but

Hb multiplicity and structural/functional features (29) closely

resemble those of Antarctic notothenioids. The amino-acid

sequence identity with cold-adapted N. coriiceps of the same

family is the highest ever found among notothenioids (see Table

1). Thus, N. angustata is a link between temperate and Antarc-

tic habitats. The two nototheniids diverged evolutionarily well

before the establishment of the APF, the barrier responsible for

isolation of the Antarctic marine organisms. If N. angustata had

36 VERDE ET AL.

migrated before cooling and had never been cold adapted, then

cooling would have been unable to exert any evolutionary pres-

sure in determining the Hb primary structure, and the strong

sequence similarity would merely reflect the common phyloge-

netic origin. However, at the end of the Miocene (5 mya) and

during the Pliocene, the APF moved northwards up to 398 S,

the latitude of northern New Zealand, facilitating fish migration.

Moreover, the genome of N. angustata contains antifreeze genes

(26), which indicate that—unlike P. urvillii—this fish was cold

adapted prior to its quite recent migration to temperate waters

(see below). Thus the sequence similarity may indeed be related

to cold adaptation.

In comparison with cold-adapted species, the Hbs of non-

Antarctic notothenioids often display higher multiplicity of

components, O2 affinity and variability in the non-functional

regions of the globin molecule. There may be selective advan-

tages to possess multiple Hbs. In fish having two Hb compo-

nents, only one being regulated by pH and organophosphates,

multiplicity offers a clear advantage. On the other hand, the

advantage to have multiple a- and b-genes coding for function-

ally equivalent Hbs is not clear. Multiple Hbs may allow higher

Hb concentration in the blood in response to environmental con-

ditions (30).

The Hbs of C. gobio, B. diacanthus, P. urvillii and E. maclo-

vinus display very high O2 affinity. The higher affinity (possibly

related to the habitat conditions) in Hbs of non-Antarctic noto-

thenioids (with the exception of N. angustata Hb) remains an

open question.

The Hbs of Arctic Fish

Most research at the molecular and ecological level in cold-

adapted habitats has so far been concentrated essentially on Ant-

arctic species. Only recently, the Arctic habitat is beginning to

receive attention. The urge to study the molecular mechanisms

underlying fish thermal adaptations and biodiversity in the high

Arctic becomes stronger as scientific efforts to understand cold

adaptation widen and shift to a more analytical phase. The Arctic

offers a remarkable opportunity for comparative studies on evolu-

tionary differences among cold-adapted species and on how

organisms from the polar habitats are affected by climate change.

As polar scientists, we are fully acknowledging the need to shed

light on the links between northern and southern regions, taking

into account the physiological and biological differences between

the two polar regions.

Arctic fish are characterised by higher biodiversity and,

unlike Antarctic notothenioids, have high Hb multiplicity. For

instance, the blood of the spotted wolffish Anarhichas minor, a

benthic, sedentary fish of the family Anarhichadidae (suborder

Zoarcoidei), contains three functionally distinct major Hbs. The

sequences of the three Hbs (31) show higher identity with Ant-

arctic than temperate fish species (see Table 1), which may

imply some degree of correlation with cold adaptation. On the

other hand, the suborder is closely related to Notothenioidei

(32), thus phylogenetic relatedness cannot be excluded.

In an attempt to link protein evolution, molecular adaptation

and polar environmental conditions, we studied the structure

and function of the Hbs of three species of the widely distrib-

uted family Gadidae, the Arctic cod Arctogadus glacialis, the

polar cod B. saida and the Atlantic cod Gadus morhua (33).

They all have high Hb multiplicity (three components display-

ing functional differences). Altogether, these species have a ge-

ographic distribution extending from high-Arctic to temperate

latitudes. A. glacialis is normally confined to polar seas (seldom

below 708 N) and is found in the ice-covered fjords of Green-

land. The other two species are cryopelagic and highly migra-

tory; B. saida is normally found above 608 N and widely dis-

tributed within the ice-covered zone, whereas G. morhua

reaches much lower latitudes. The temperature latitudinal gradi-

ent covered by these species of the same family may offer im-

portant advantages in studying specific molecular adaptations in

their Hb systems in response to local environment changes, and

further investigations are under way.

In comparison with notothenioids, which lost globin variabil-

ity probably because of environmental (temperature) stability,

the O2-transport system of Arctic species has plesiomorphic fea-

tures secondarily involved in cold adaptation and temperature

fluctuations (higher multiplicity and higher globin diversity).

Low O2 affinity and strength of the Root effect resemble the

values found in most Antarctic notothenioids (33). Phylogenetic

analysis (see below) shows how the constant physico-chemical

conditions of the Antarctic Ocean and the variations typical of

the Arctic Ocean are matched by amino-acid sequences.

THE MOLECULAR EVOLUTION OF POLAR FISH Hb

This part of the study implies using the primary structure of

Hbs as tool of choice to gain insight into the pathways of the

evolution history of a-and b-globins of notothenioids and also

as a basis for reconstructing the phylogenetic relationships

among polar species.

The Neighbour Joining trees for notothenioid globins (Fig. 2)

are in agreement with those obtained by morphological analysis

and sequence studies on mitochondrial RNA (8, 27, 34) and give

strong support to the monophyly of Antarctic notothenioids.

The globins of major and minor Antarctic fish Hbs cluster in

two separate, strongly supported groups, with the anodic and ca-

thodic globins of temperate fish Hbs forming the first diver-

gence lineage. The a- chain of non-Antarctic P. urvillii shared

by Hb 1 and Hb 2 branches off the clade of the major Antarctic

Hbs, and the same applies to the b-chain of Hb 1. The b-chainof P. urvillii Hb 2 places itself in a basal position with respect

to the clade of the Antarctic minor Hbs (28). The a-chain of C.

gobio Hb 1 also branches off the clade of the major Antarctic

Hbs, whereas the b-chain common to Hb 1 and Hb 2 is

included in the clade of the minor Antarctic Hbs (35).

37HEMOGLOBIN IN POLAR FISH

The obtained topology is in general agreement with the hy-

pothesis of four groups of globins, namely ‘Embryonic or Minor

Hb Group’, ‘Notothenioid Major Adult Hb Group’, ‘Anodic Adult

Hb Group’ and ‘Cathodic Adult Hb Group’ (36). The position of

the C. gobio and P. urvillii globins is in agreement with phyloge-

netic evidence from nuclear and mitochondrial genes, according

to which C. gobio was the first species to diverge from notothe-

nioids, before P. urvillii [reviewed in (27)].

All a-globins of the major notothenioid Hbs belong to

the ‘Notothenioid Major Adult Hb Group’, including those of

P. urvillii and C. gobio, which are the two notothenioid lineages

that diverged before the major radiation of Notothenioidei. All

b-globins are included in the same group, but with the excep-

tion of C. gobio b-globin. The position of this globin is intrigu-

ing. A possible explanation is that, in C. gobio, the peculiar

expression pattern of the b-globin gene is an example of neo-

teny resulting from the conservation of the embryonic expres-

sion pattern in the adult stage, followed by inactivation/deletion

of the gene encoding the adult globin form, an event similar to

that leading to the loss of Hb genes in icefish (35).

The trees of Fig. 3 lack some of the basal non-Antarctic glo-

bins (P. urvillii and C. gobio), but include all the Arctic species,

thus allowing comparison of the respective evolutionary histor-

Figure 3. Phenetic trees of amino-acid sequences of globins.

Bootstrap proportions (percentage of 10,000 replicates) are

given at the nodes. (A) a-chains; (B) b-chains. Notothenioidsare in blue, zoarcoids in green, gadids in red and non-acantho-

morph fishes in black.

Figure 2. Phenetic trees of amino-acid sequences of globins.

Bootstrap values (percentage of 10,000 replicates) are given at

the nodes. (A) a-chains and (B) b-chains. Major and minor

notothenioids are in black and grey underlined respectively,

non-Antarctic in blue, Arctic in green and temperate in red.

38 VERDE ET AL.

ies. According to the previous results (37), globin paralogs

(e.g., gene copies originated by duplication in a given genome)

currently found in Antarctic fish diverged �250 mya, that is, at

the onset of the Mesozoic.

Unlike the Antarctic globins, the Arctic globins occupy sev-

eral positions in both trees, suggesting independent evolutionary

pressures. The globin sequences of the Arctic zoarcoid A. minor

follow the track of species history, as A. minor consistently

appears close to the notothenioid clades as predicted by teleos-

tean phylogenies (32). In contrast, Arctic gadiform sequences

occupy different positions in the two trees with regard to tem-

perate and Antarctic sequences. On one hand, gadiform Arctic

a-chains appear related to the notothenioid–zoarcoid group.

On the other hand, the b1-chains of A. glacialis, B. saida and

G. morhua are excluded from the b2-chains of the same species

and from major and minor Antarctic globins with very good

bootstrap proportion. The b2-chain of the three gadid species

and another b-chain (possibly belonging to a larval Hb, and

whose sequence has been deduced from DNA) of G. morhua

constitute a clade well separated from the subclades of major

and minor Antarctic globins.

The differences between the two polar regions, the wide lati-

tudinal range in which the three Gadidae are found, as well

as the active, pelagic and migratory life style of B. saida and

G. morhua, offer a possible explanation to the dispersal of the

gadid globins in the trees between the Antarctic and non-Ant-

arctic clades. The stability of the environment may allow the

‘phylogenetic signal’ to be maintained in the Antarctic sequen-

ces under selective pressure, whereas environmental variations

may tend to erase this ‘signal’ in the gadid sequences, in which

sequence similarity would not reflect species interrelationships

anymore. The inclusion of globin sequences of other non-polar

gadiforms and of zoarcoids will be the next step to better dis-

criminate the part of these relationships inherent to fish phylog-

eny from that due to cold adaptation.

CLIMATE CHANGE AND EVOLUTION

Although the influence of evolution extends to the most

extreme environments, Antarctica is not considered as one of

the notable evolutionary sites (e.g., the East African Great

Lakes, Lake Baikal and the Galapagos), perhaps because polar

research has so far emphasised aspects of extreme biology

rather than unifying principles of evolutionary biology. But

Theodosius Dobzhansky comments that ‘nothing in biology

makes sense except in the light of evolution’ (38). Evolution is

at work, and the examples given here show the growing efforts

of polar biologists to gain insights into evolutionary processes.

In the last decades, another compelling argument has

attained the foreground: climate change. There are many exam-

ples of links between long- and short-term global climate

change and evolution in the Antarctic. The continental shelf

offers striking examples of faunal change and radiation, with

fishes and some invertebrate groups among the best studied.

Antarctica has a great potential as a centre of evolution and

so has the Arctic. In tune with this growing wave of interest,

studies are increasingly searching links between evolutionary

processes and molecular, physiological and behavioural adapta-

tions by which organisms are adapted to survive, grow and

reproduce. The critical examination of polar ecosystems

exposed to climate change provides a major contribution to the

understanding of evolutionary processes of relevance to life on

Earth. Life in the polar regions has experienced cycles of global

change, driven by periodic glaciations. Recently, man-induced

global warming and increased UV radiation due to ozone deple-

tion have become driving factors. Regional-scale and short-term

climatic variations seem to be more frequent and intense in the

recent years, raising deep concerns not only in scientists but

also in governments. Will climate change result in relaxation of

selection pressure on genomes, or in tighter constraints and ulti-

mately extinction of species and populations? The latter alterna-

tive is far from being unlikely.

Climate change can affect every aspect of an organism’s

biology, from cellular physiology and biochemistry to food web

and habitat. All organisms, both terrestrial and marine, are sus-

ceptible to environmental changes, small or non-motile organ-

isms being particularly vulnerable; they must alter their physiol-

ogy/biochemistry to cope up with changes in enzyme activity

and DNA damage. Cold adaptation is an important part of a

refined physiological equilibrium, which remains unmodified in

the absence of disturbances due to climate change. When distur-

bances do occur, evaluating the response of cold-adapted organ-

isms will yield important indications of general trends. The po-

lar regions include the high Antarctic hosting stenothermal fish

species, and the sub-Antarctic and Arctic, largely populated by

eurythermal fish. Thus, the range of responses will be wide,

depending on different degrees of sensitivity to (for instance) a

small temperature increase (39).

There is not much knowledge on the capacity of the marine

fauna to respond to changes, but the role of the polar habitats

in earth climate, and in its changes, has awakened great interest

for the evolutionary biology of its organisms.

New information (e.g., choice of target species, long-term

data sets and concerted efforts from international multidiscipli-

nary programmes) will help us to identify the responses of vul-

nerable species and habitats to climate change. This preliminary

step is essential to establish efficient strategies aimed at neutral-

ising the increasing threats to biodiversity.

ACKNOWLEDGEMENTS

We thank the Guest Editors for inviting us to write this chapter.

This study is supported by the Italian National Programme for

Antarctic Research (PNRA). It is in the framework of the

SCAR programme Evolution and Biodiversity in the Antarctic

(EBA) and the 2003 and 2005 TUNU-I and TUNU-II cruises

near Greenland. We also thank the captain, crew and personnel

of Raytheon Polar Services aboard the RVIB Nathaniel

39HEMOGLOBIN IN POLAR FISH

B. Palmer for their excellent assistance during the ICEFISH

2004 cruise. The ICEFISH cruise was supported by National

Science Foundation grant OPP 01-32032 to H. William Detrich

(Northeastern University, Boston, USA).

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