As most probably know, when water freezes it expands and crystalizes. For this reason, and with so much water in their bodies, most fish cannot survive in temperatures below -0.8oC. Though Antarctic waters often drop to -1.9oC, some fish still thrive in these conditions. These are the Antarctic Icefish.
Icefish - of which there are 132 known species - have adapted to the cold by carrying a sort of anti-freeze protein in their systems that allows them to survive at significantly lower temperatures than others. For a long time it was thought that having this anti-freeze protein and little competition from other fish resulted in icefish diversity, but in this recent study by Near, Dornburg et al, it was found that the story may be significantly more complicated.
Generally, closely related species share very similar ways of life. With today’s icefish while they share a common ancestry, they do not share the same niche. Some are bottom dwellers, others live at mid-depths and others live closer to the surface. It was deduced that that this may be caused by the somewhat erratic Antarctic climate. Though it is always cool, over many millennia there has still been a great deal of temperature variation. At times ice has covered the entire Antarctic continental shelf and at others there have simply been scattered ice caps. This constant back and forth in temperature would result in a lack of evolutionary direction for any organism, as it has for the icefish. As groups of fish adapt to different water conditions, they find different niches.
Seeing as colder conditions were the catalyst for the icefish to evolve as they they have, it will be interesting to observe how the current warming trend and the slow advent of invasive species effects what has been termed as the icefish's “unparalleled radiation of fish”. While it seems obvious that their numbers will dwindle, further study will have to be conducted on if/how the fish manage to cope with the warming trend and whether any specific niches of icefish are hit especially hard, by the change.