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The origin of flatfishes

Atlantic Halibut
Credit: Biodôme de Montréal (Dominique Capelle)
Atlantic Halibut
The origin of flatfishes

A nod to Picasso

Born flat? Not at all! That strange appearance of flatfishes on the seabed isn’t there at birth. For example, the halibut larva looks like the larvae of other fish, described in science as “symmetric pelagic” – meaning it swims freely in the water column, belly downward, and it has an eye on each side of the head. The juvenile stage, when the young halibut reaches about ten centimeters in length, is when a remarkable metamorphosis takes place. It adopts sideways swimming and a benthic lifestyle (meaning on the ocean bottom), accompanied by a slow migration of the left eye towards the right one. That transformation will be completed in adulthood: the two eyes will end up on the same side of the fish’s head, that being the right. Known as “ocular,” that side is pigmented and corresponds to the top of the halibut’s body. Expert in camouflage, thanks to their coloration, they avoid predators and take their prey by surprise. The other flank, the left one (the “blind” side, which it rests on), is not pigmented. A number of flatfish species are present in the waters of the St. Lawrence, and the ocular side can vary depending on the family. When we add to that a mouth that opens horizontally and bulging, independent eyes, these exceptional fish are not all that foreign to the art of Picasso in his surrealist phase!

Species in this group that will be presented when the Biodôme reopens

The Atlantic halibut (Hippoglossus hippoglossus) is the world’s largest flatfish, capable of reaching two meters in length and a weight of 300 kilograms. A second representative of the same family, the winter flounder (Pseudopleuronectes americanus), is less than 50 centimeters long and weighs a little over one kilogram.

An end to the flatfish evolution puzzle

A study published in 2012, involving fossils dating from the Eocene (more than 45 million years ago) by paleobiologist Matt Friedman of Oxford University, provided an update on the gradual evolution of flatfishes. Thanks to the detailed description of the extinct species known as Heteronectes, meaning “different swimmer,” the researcher discovered that among the first members of this group, the adult fish’s “migrating” eye had not yet moved over from the opposite side of the skull. One fossil, with the characteristic flat shape, presents a striking cranial asymmetry: the left socket has shifted towards the top of the skull, but doesn’t reach the dorsal midline. This fantastic discovery shows the perfect intermediate stage between symmetrical fish and flatfishes. Something that can only fill human beings with wonder in their quest for meaning!

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