The Evolutionary History of Icefish

drakeopenFigure 3. Diagrammatic illustration of the opening of the Drake Passage and the establishment of the Antarctic Circumpolar Current (Image taken from: http://seehint.com/catalog/2013%5C2013_07/drakeopen.jpg).

Icefish only occur in oceanic waters on the Antarctic continental shelf as a result of the physical barriers imposed by the tectonic and geological history of Antarctica. Up until around 38 million years ago (Ma), Antarctica was part of a land mass along with Australia and South America (Gondwana). Separation lead to a gradual reduction in sea temperatures as the Antarctic land mass moved in a southerly direction at a rate of around 7cm per year (Orsi et al., 1995). The final separation from South America between 37 and 34 Ma led to the opening of the Drake Passage and allowed for the establishment of the Antarctic Circumpolar Current (ACC), which is driven by strong westerly winds, and surrounds the Antarctic land mass (Orsi et al., 1995). Antarctica has consequently been surrounded by continental shelf waters for about 12 Ma (Clarke and Crame, 1989). The great distances between the Antarctic land mass and those surrounding it, the abyssal plains of depths greater than 4000m, and the thermal barrier presented to lower latitudes by the ACC meant that Notothenioidei evolved in relative oceanographic isolation from other fish families (Mintenbeck et al., 2012).

 

The video above illustrates the separation of the Antarctic, Indian and Australian plates which began 165 Million years ago. Please visit sciencedirect.com/science/article/pii/S1342937X1300213X for more information. Video taken from: http://vimeo.com/68311221.

The stable conditions found around the Antarctic Continental Shelf resulted in extensive radiation of the Perciform suborder Notothenioidei (Eastman, 1995). At present, there are 8 families, 43 genera and 122 species in the suborder; 6 of these families are found south of the Antarctic Polar Front (Eastman and Eakin, 2000). Around 35% of Antarctic fish belong to the suborder Notothenioidei (Montgomery and Clements, 2000). The modern species present in the suborder are thought to have evolved from a small benthic ancestor, between 40-60 Ma, which lacked a swim bladder (Eastman, 1995). The stable conditions in the last 15 million years and a lack of competing taxa (clupeids are a noticeable absentee) have led to large scale adaptive radiation (Eastman, 1995). In addition, after the geological separation of Antarctica from the other land masses in Gondwana, many of the species that existed at the time became extinct, which resulted in the vacation of a large number of niches that Notothenioids exploited (Mintenbeck et al., 2012). This has resulted in a modern fish fauna with endemism at around 88% in the Southern Ocean on the Antarctic continental shelf (Andriashev, 1987). The high level of endemism is a consequence of the adaptive radiation from isolation at sub-zero temperatures (Mintenbeck et al., 2012).

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