Antarctic Silverfish
a fish that has overcome environmental and evolutionary problems.
The ancestors of the Antarctic Silverfish left them with no haemoglobin and no swim-bladder, but they have solved these two problems and now float in the freezing waters
The Antarctic Silverfish (Pleuragramma antarcticum) - link with images - is a one of the Ice Fish, not an insect like the silverfish! It is found all the way round the earth in the Antarctic, where it swims, or rather ‘floats’, in mid-waters feeding on small crustaceans. The water here is very cold and Silverfish have anti-freeze in their blood and body-fluids. They are also unusual in other ways because, like all the other Ice Fish, they have no haemoglobin in their blood and have lost their swim-bladder.
While all the other Ice Fish seem to prefer spending their lives near the bottom conserving energy, the Silverfish inhabits the mid-water. Because they have no haemoglobin they find it difficult to get enough oxygen to their muscles, so active swimming is out for them. They have no swim-bladder either and maintain ‘neutral buoyancy’ by having very light bones and sacs of very light fat in their tissues. They simply ‘float’ around. Most mid-level fish are very active swimmers, and the Silverfish has not become a ‘floater’ through ‘choice’.
When fish were busy evolving all round the world there was one habitat that presented particular problems – the freezing waters of Antarctica, but since there was a very rich food supply here (many copepods and the famous ‘krill’) this habit was well worth exploiting. Evolving an anti-freeze is obviously a very difficult trick, and it seems likely that only one species evolved it. This one was the ancestral Ice Fish and all the others evolved from it – they form what is known as a ‘Species flock’.
For some reason that I fail to comprehend this ‘common ancestor’ of all the Ice Fish lost its haemoglobin at the same time. (Maybe there is a biochemical explanation and some-one can tell me about it?). There is a lot of oxygen in very cold water and while Ice Fish can obviously survive, they must remain inactive most of the time. They simply lurk on the bottom snuffling up food. They lost the need for a swim-bladder, and their lack of haemoglobin makes them almost transparent - you can’t get much better camouflage than lying on the floor, invisible!
But there was all that juicy food above them. How to get up into mid-waters without being able to summon up the energy to swim and having no swim-bladder? This problem was solved by the Silverfish with his fatty sacs and neutral buoyancy. But why silver and not transparent like the rest of the Ice Fish? – simple, it’s better camouflage when you leave the bottom.
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