How do cold-blooded species cope in cold water?

11 October 2009

Question

How do cold-blooded species cope in cold water? We all know that reptiles have to sun themselves to build up the energy or ability to sustain their activity and to get to the right temperature. And this is attributed to the fact that they’re cold-blooded. How is it then possible for other cold-blooded species like fish and invertebrates like octopus, squid and so on to sustain the high levels of activity that they do, that they can live in near freezing or actually freezing water?

Answer

The answer is to do with their metabolic rates and the fact that they can operate at those low temperatures. I actually want to go into detail a little bit on what you call at the end, the freezing species. This is the most interesting thing that was discovered back in the 1960s which is that there are fish in the Antarctic that create antifreeze and that's how they live in very cold temperatures. Because the crazy thing about the sea is that it doesn't actually freeze until -1.9 degrees centigrade where as normally water freezes at zero, we know that. But it's because of those salts and the various impurities in seawater that means that ocean temperatures can get extremely low indeed especially down there in the southern ocean. And so these Antarctic cod it was discovered that they have glycoproteins in their blood. That means that their tissues their fluids inside them don't freeze until -2 degrees centigrade, so they are safe in the sea. The glycoproteins work in a very clever way by actually attaching to the surface of small ice crystals by plugging gaps if you like between them. And that stops them from getting any bigger so the fish themselves don't actually freeze despite the fact that they are sub zero in temperaturesand that's really rather cool. But then there are other reasons why they don't actually manage to swim around all the time and they are affected to some extent by what temperature there is and what's going on in the environment. Because it was discovered last year that some fish in the Antarctic hibernate. The first fish that were shown to actually slow down, slow their heart rate, slow their movements when it's very cold and dark. And that could actually be because it's dark and they need to be able to see to be able to catch their food. So in fact what they do is say, "Okay fine we'll have a bit of rest while we can't find our food and wake up when it gets warmer and lighter."

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