When the eyes do not stay in place: the peculiar case of sole larvae

I bet you know most fishes have larval stages belonging to the plankton (actually, to the ichthyoplankton). This is not surprising, considering many marine organisms pass part of their live in the plankton (we call this group meroplankton). What is indeed surprising is the peculiar behavior of some fish larvae. An extreme and very characteristic case is the development of the sole, and that of many other flat fish. This fish usually rests in the ocean’s bottom, and because of that, it developed a physiological adaptation: having both eyes on the same side of the face. By doing so, it can be aware in 3D of all what is on top of it. Well, I guess it would be also rather inconvenient having an eye facing the bottom and full of sand all the time. Its larval stages, however, have one eye on each side of the face. This is because they are planktonic and need a 3D view of what it is in front and on the sides. During larval development, one eye migrates from one side of the face to the other one. When both eyes are on the same side of the face, the fish, even if still tiny, adopt a benthic behavior. In the picture below, you can see the eye migration progress along the different larval stages. Cool, isn’t?

Larval developement of several species of flat fish showing the migration of the eye

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