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How do fish in the Southern Ocean keep from freezing?

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Craig Marshall

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Photo of Craig MarshallMarine and terrestrial organisms are likely to have been exposed to quite different environments since Antarctica separated from the rest of Gondwana. Marine systems are well mixed and allow easy movement of individuals from place to place. Despite this, notothenioid fish dominate the Antarctic fish fauna and form a species flock implying that some mechanism must exist to isolate individual populations. Marine invertebrates such as echinoderms and pcynogonids seem to be less diverse and more similar to species found in the adjoining landmasses. In contrast, land organisms inhabit ice-free refugia that comprise only a few percent of the area of the continent. Animals from these areas may show much stronger regional variation and a different pattern of speciation. In this programme, we will collect material from both marine and terrestrial animals to determine their phylogenetic histories.

The fish fauna of Antarctica is characterized by two features that set it apart from that of other seas. Many fewer fish species are found in Antarctic waters than would be expected by comparison with other oceans [1]. The fish fauna of Antarctica is dominated by one group, the Notothenioidei [2, 3]. Ninety six notothenioid fish species have been described of 213 species in total from Antarctic waters [4]. Why should there be so few species overall and why should one group dominate the fish fauna so?

The geological history of Antarctica provides the beginnings of some answers. Until approximately 15 million years ago, the waters around Antarctica were not permanently frozen and ice-free water was probably available during at least some of the year [5]. Since the development of circum-Antarctic currents and the cooling of the continent, the surrounding waters have contained ice during some or most of the annual cycle. Since fish are less salty than the sea in which they live, they are supercooled at the freezing point of sea water and thus prone to freeze when nucleation centres in the form of ice crystals are present [6-9]. Species of fish that were not tolerant of supercooling were probably rapidly lost once Antarctic waters had cooled and frequently contained ice crystals to depths of more than a few metres.

The appearance of antifreeze glycopeptides (AFGP) in notothenioid fish is probably the major factor explaining both the overall paucity of species and the dominance of notothenioid fish, which, with two other families the Zoarcidae and Liparidae, comprise 87% of current Antarctic fish species [4]. The presence of AFGP allowed these fish to survive and left them free to exploit now vacant ecological niches. It is this expansion of an otherwise nondescript species present in only small number into this new environment that has proved so interesting evolutionarily.

Despite the small number of Antarctic species, the hundred or so notothenioid species appear to have all appeared in the last 15 million years [10] with at least one other radiation within the last 5 million years [11]. Current species vary widely in lifestyle, include species of neutral buoyancy without a swim bladder [3], fish lacking haemoglobin and sometimes myoglobin [12, 13], and vary in size from a few cm to more than 2 m. How did such a large number of species appear in such a short time? What role did changes in climate and ice-sheet extent play in the notothenioid radiation?

Understanding how Antarctic notothenioid fish might have speciated is complicated by our very poor understanding of the lifecycle of most of these fish [1]. Many of these fish have a pelagic larval stage, which, when combined with the presence of circum-polar currents, might be expected to keep the populations well-mixed. Almost no data exist for Antarctic fish, although recent evidence suggest that there exist a number of populations of Dissostichus eleginoides, a sub-Antarctic relative of D. mawsoni found right around Antarctica [14, 15]. In contrast, D. mawsoni itself does not seem to show such population structure (P. Gaffney, personal communication).

We propose investigating the genetic structure of populations of notothenioid fish. Initial work would concentrate on such species as Trematomus bernacchii and Pagothenia borchgrevinki which are found right around Antarctica [1], and are relatively easy to catch. Previous work on allozymes in T. bernacchii suggested that cryptic species might exist [16]. However, the use of relatively insensitive DNA markers was unable to extend this analysis [17].

We have begun to develop a series of markers that will allow us to examine both intra-specific and inter-specific variation. These include the use of the mitochondrial markers 12S and 16S rRNA genes, cytochrome b and D-loop, as well as the18S nuclear rRNA genes [18], and intron sequences from lactate dehydrogenase and citrate synthase derived from our own work. Our work so far has concentrated on establishing markers based on samples of trematomid fish from Cape Evans and Cape Armitage. This season we plan to collect samples of the same species from Cape Roberts. However, Cape Roberts is relatively close to Ross Island, and extending our collection of data as far as Cape Hallett is an important part of developing a widespread survey.

The samples collected at Cape Hallett and from elsewhere will be important for investigating two distinct aspects of speciation within notothenioid fish. Measurement of intra-specific variation from site to site will give us some estimate of whether these sites are genetically isolated. A priori considerations including the presence of a long-lived pelagic larval phase in those notothenioid fish for which date exist [19] and the existence of both small and large-scale currents suggests that genetic isolation of notothenioid fish populations is unlikely. However, set against this consideration is the number of Antarctic notothenioid fish species that have arisen in the past 15 to 20 million years [10, 20, 21] indicating that speciation of this group must occur quite commonly. If the mechanisms that generated this number of notothenioid species are still active, there should be evidence of genetic differences between different sites. In particular we are interested in the possibility of cryptic species that have been postulated to exist within Antarctic notothenioid fish [22]

The collection of samples from many individuals of common trematomid species (Trematomus hansoni, T bernacchii, T. pennelli, T.nicolaii, T.newnesi) will also allow us to investigate interspecific differences. The existing molecular phylogenies of notothenioid fish are based mostly on mitochondrial ribosomal RNA genes (12S and 16S rRNA) [10, 11, 21, 23, 24] although other markers have been used [25]. Existing phylogenies do not resolve the appearance of the trematomid fish particularly well and we plan to use the markers developed in this work to improve these trees if possible.

Of particular interest to us are the recent data from the Cape Roberts drilling project [5]. It may be possible to correlate molecular dates from phyologenetic data to significant climatic changes derived from the geological record and thereby gain some insight into the possible role of climate change on speciation. For example, periods when ice shelves are extensive are likely to significantly reduce habitat. The retreat of such ice shelves, now thought to occur on a timescale of decades or centuries, may rapidly expose large areas of habitat. Recolonization of large and empty habitat may have a significant effect on the rate and pattern of speciation.

Preliminary work on this project began in 2001 with collection of fin clips from Cape Evans and Cape Armitage. This work will continue with collection of material at Cape Roberts in 2002. This proposal is to begin work at Cape Hallett in 2003 as part of the Latitudinal Gradients Project. It is possible, but unlikely that a second season at Cape Hallett would be desirable. Collaboration with the groups of John Macdonald from the University of Auckland and Bill Davidson of Canterbury University will be an important part of this work.

References

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