Systematics of Polar Fishes

2 SYSTEMATICS OF POLAR FISHES PETER RASK MØLLER JØRGEN G. NIELSEN M. ERIC ANDERSON

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Introduetion A. Definition B. Historical View II. Biogeography A. Distribution Patterns B. Origin of Polar Fishes III. A Classification of Fishes Occurring in Arctic and Antarctic Regions, with an Annotated List of Fish Families and Notes on Revisions and Phylogenetic Hypotheses

I. INTRODUCTION Description and ordering of taxa form the basic fundament of all biological disciplines, including physiology. The systematics of polar fishes is an active research fieid: New species are still being described from both Arctic and Antarctic areas, because of taxonomic revisions and new collec­ tioI;lS from continuing fisheries and scientific expeditions. The phylogenetic relationships of families, genera, and species are now being studied more intensively than ever, mainly because of the development of new molecular methods, in which the sequences of various DNA nucleotides are used. Phylogenetic analyses based on morphological characters are still equally relevant, but because they are usually based on time-consuming examination of large samples, the molecular-based phylogenies have become more com­ mon. Ideally, both morphological and molecular data should be employed in the reconstructions of evolution. In the family accounts below, we mention as many phylogenetic studies as possible, because an understanding of phylogenetic relationships is often essential for the interpretation of results in comparative physiological studies. Mapping of physiological "characters" 25

The Physiology of Polar Fishes: Volume 22 FISH PHYSIOLOGY

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2005 Elsevier Ine. All rights reserved

DOl: 10.1016/S 1546-5098(04)22002-7

26

PETER RASK MØLLER ET AL.

on phylogenetic trees provides an excellent overVlew of how the physiological characters evolved (e.g., did the antifreeze proteins evolve more than once in codfishes?). From Figure 2.1, it is evident that it happened either two independent times (in the Gadus-Arctogadus c1ade [with a secondary loss in Theragra] and in the Eleginus-Microgadus c1ade) or only once in the Gadus-Microgadus c1ade with secondary losses in Theragra, Melanogrammus-Merlangius, and Pollachius c1ades. The first­ mentioned hypothesis is the most parsimonious, because it involves only three evolutionary steps in contrast to four in the last mentioned. Some precautions to the above example should be taken because six species (M. proximus, P. pollachius, T. chalcogramma, T. minutes, T. luscus, and G. argenteus) have not been examined for antifreeze proteins (C. Cheng, personal communication, April 2003). The mutual benefit of the two disciplines is seen when physiological characteristics in conflict with phylo­ genies inspire evolutionary biologists to reexamine the phylogenetic recon­ struction of a group. Many families have, however, not been studied phylogenetically, and the available work should be viewed as a first stage in a far from finished process.

Gadus morhua

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Theragra cha/cogramma

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Gadus macrocepha/usIG.ogac Boreogadus saida I

Arctogadus g/acia/is Me/anogrammus aeglefinus

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Merlangius mer/angus Pollachius virens Pollachius pollachius

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E/eginus navaga

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Microgadus proximus

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Microgadus tomcod

I I

I I

Trisopterus minutus Trisopterus esmarkii Trisopterus /uscus Micromesistius poutassou Gadicu/us argenteus

Fig. 2.1. Presence of antifreeze proteins (in bold) mapped on a cytochrome b-based phylo­ genetic reconstruction of all recent codfishes, Gadinae, except Eleginus gracilis, Micromesistius australis, and Theragrafinnmarchica. (Modified from Møller et al., 2002.)

2. SYSTEMATICS OF POLAR FISHES

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A. Definition In this chapter, we define polar fishes from a biogeographical point of view, employing areas with major shifts in species composition as borders for the Arctic and Antarctic areas. These are often defined by oceanic frontal systems and topographical structures such as submerged ridges. Sub-Arctic and sub-Antarctic areas are not included because these have a degree of endemism and are often visited by summer migrants from temperate areas. In the North Atlantic, we follow Andriashev and Chernova (1995), with a few exceptions (Figure 2.2a). In the west, the border goes from Belle Isle Strait (c. 52 °N) up the coast of Canada (depths-