I was fortunate to spend November cruising on the Southern Ocean, so because we highlight frozen oceans during December at the Florida Oceanographic Coastal Center, let’s talk icefish.
Curiosity about the fish that can survive beneath the ice in these harsh environments can get the best of someone traveling the Southern Ocean and as it turns out, the fish in this subfreezing environment are extraordinary. Besides, Santa Claus lives on a frozen ocean and we know he gets lots of cookies, definitely does not eat reindeer and cannot grow beans, so he must get protein into his diet from fish.
In the polar regions due to the unique structure of the water molecule, the salt water can dip as low as 28˚F before it freezes, below the freezing temperature of fresh water, because salt crystals interrupt the structured crystal formation of ice. What’s more, that temperature does not vary much more than 6˚F during the course of a year and it’s a good thing that it doesn’t, because the same adaptations that allow fish to survive at temperatures hovering around freezing, would kill them at higher temperatures.
Although both the Arctic and Southern Oceans have much in common, geographically and environmentally they differ dramatically and animal groups, including fish, are very different. The Antarctic is dominated by a group of fish referred to as Notothenioids.
They are best known for adaptations allowing for energy-limited waters with outrageously low temperatures and darkness that can persist for up to four months. They are opportunistic feeders and there is a small temperature variation, but winter feeding is markedly different than in the summer likely indicating adaptation to the diminished food supply of winter.
Their blood not only contains the salts that other fish have but a number of different antifreeze compounds, their most important function being to stop ice from growing across their skin. The most dangerous situation is for a fish to come in contact with forming ice crystals, as with anchor ice, because it can infect their bodies, bursting their cells. They also lack the structures in their kidneys that would systematically remove the small molecules of antifreeze by putting them out in urine.
One group that epitomizes the Notothenioids is the crocodile icefish. Some refer to them as “white-blooded” or “bloodless”. In fact, their blood contains no hemoglobin and their muscles contain no myglobin, making them look very pale. The lower the water temperature the more oxygen it can hold; these fish have adapted to this high oxygen environment by losing the ability to carry oxygen in their blood.
The evolutionary track that led to this is probably similar to what led cave fish to lose the need for eyes. Icefish also have a low metabolic rate, increased venation in their skin and fins enabling absorption of oxygen through their skin and an increase in heart size and blood volume.
According to FishBase the crocodile icefish is used as a food fish. The Marine Stewardship Council has certified this fishery as well-managed and sustainable. It is found in parts of the southern Atlantic Ocean and throughout the Southern Ocean.
So there are fish under the frozen oceans and I believe that Santa Claus does indeed fish. I believe, I really do…
Read more about Ellie’s Antarctic Adventure in Florida Oceanographic Society’s blog, “Undercurrents,” located at FloridaOcean.org.