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Flatfish Against Evolution

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07.05.2007

A specimen of the starry flounder (Platichthys stellatus) has been recently caught in the Japanese Sea, its uncommon appearance drew attention of the specialists of the Pacific Scientific Research Fisheries Center and Far-Eastern State University. The fish differed from its congeners by uncommon colouring and body shape.

Even a person far from ichthyology is aware of peculiarities of flatfish. A flatfish larva newly hatched from spawn possesses a two-sided symmetry, but as it develops all of it “moves” to one side, and an adult fish that descended to the soil is similar to a rug. Both eyes and both pectoral fins are on its upper side, and the down side turns into a real reverse side: the scales are less developed there and the coloring is pale and monochromatic.

However, the caught starry flounder was equally brightly and variegatedly colored on both sides. The right, “lower” eye as if incompletely crawled over to the upper side and came to a standstill on the cinciput. As a result, one eye of the fish looks up and the other – to the left. The flounder’s snout is longer, which facilitates movement in the water column. The upper part of the fish head and the front part of the body are of “wrong” shape, there are other changes in the body proportions. Judging by all these indications, the caught flounder is somewhat more similar to a larva than a grown up specimen should be.

From time to time, researchers find flounders of different species, whose lower side of the body is fully or partially colored. Such phenomenon is an indisputable atavism, a “reminiscence” of the times when flounder ancestors were swimming in the water column with their back upwards, but not flopped down on their side at the very bottom. The above mentioned changes in the shape of the body are also an indisputable atavism from the same times. All these changes are often found in complex with representatives of different flounder species. Probably in case of such fish with uprushes to symmetry in the course of evolution, not only one gene is released for some reason, but a whole complex of genes is, which are responsible for atavistic indications manifestation. It is not improbable that these genes were primary conjunct.

The flatfishes (Pleuronectiformes) are a relatively young group of bony fishes (Teleostomi) from the evolutionary point of view. Apparently, the youth accounts for rather frequent anomalies in appearance. Along with atavistic changes in the body structure and coloring, reversal of the body side is rather often observed with flatfish, that is they “lie down” on the wrong side. Usually all representative of the same species exist with a definite side down. The starry flounder, for example, is directed down with the right side but sometimes the researchers note mass reversal. Sometimes, the frequency of reversal reaches 30-50% at the Pacific coast of North America. One more peculiarity of the starry flounder – is its ability to easily form bigeners. There is a lot of originality in its ecology: it inhabits the ocean but it also comes to rivers in search of food. Evidently, this species is specialized much less than other flatfishes (Pleuronectiformes) species.

Nadezda Markina | Source: alphagalileo
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