Fish that live on the sea bottom benefit by being flat and hugging the contours.There are two very different types of flatfish and they have evolved in very separate ways. The skates and rays, relatives of sharks, have become flat in what might be called the obvious way. Their bodies have grown out sideways to form great “wings”. They look as though they have been flattened but have remained symmetrical and “the right way up”. Conversely, fish such as plaice, sole, and halibut have become flat in a different way. They are bony fish which have a marked tendency to be flattened in a vertical direction; they are much “taller” than they are wide. They use their whole, vertically flattened bodies as swimming surfaces, which undulate through the water as they move. Therefore, when their ancestors migrated to the seabed, they lay on one side rather than on their bellies. However, this raised the problem that one eye was always looking down into the sand and was effectively useless. In evolution this problem was solved by the lower eye “moving” around to the other side.We see this process of moving around enacted in the development of every young bony flatfish. It starts life swimming near the surface, and is symmetrical and vertically flattened, but then the skull starts to grow in a strange asymmetrical twisted fashion, so that one eye, for instance the left, moves over the top of the head to finish on the other side. The young fish settles on the sea bottom, with both its eyes looking upwards, an odd Picasso-like vision. Incidentally, some species of flatfish settle on the right side, others on the left, and others on either side.
đang được dịch, vui lòng đợi..
