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My Biology teacher asked us to write 3 texts, explaining the evolution of the Upside Down Catfish Synodontis nigriventris, each one according to Lamarckism, Darwinism and Neodawrinism theories. She asked the same thing for other species but I'm having trouble with this one, since the other ones are simple to explain (like giraffes' necks or flamingo's legs).

She gave us a sheet in which there is some information: They swim upside down, and their colors are inverted, because their backs are light and their bellies are dark (unlike the general fish). Their predators usually swim below them and it is harder for predators to find fish if their belly is light, because of the sun rays.

Well, all of this suggests that this fish swims upside down so that he can avoid predators, but how could he do this according to Lamarckism and Darwinism? Also, how does he have inverted colors, and did he get them because of evolution? I believe that the things which cause the evolution are the predators and the environment, but which characteristic is the first one and which is the one who appears because of the evolution? Also, both of them could be a product of evolution, right?

So how can I really explain this according to the theories of evolution?

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Well, I haven't managed to find many studies that explores this but I suspect that their coloring was a result of their upside down swimming rather than the other way around. They evolved from benthic (bottom-living) creatures that ate off the ground and would have had mouthparts evolved for that. So in order to feed at the surface they would have to turn themselves around. The areas that they lived in may have changed to have poorer benthic food quality making surface feeding more advantageous. They also do this for aquatic surface respiration in hypoxic (low oxygen) waters.

Check out "Chapman, L. J., Kaufman, L. and Chapman, C. A. (1994). Why swim upside down? A comparative study of two mochokid catfishes. Copeia" for more information.

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