Dawn of the Picasso Fish

Sometimes a species is so complex, so marvelous, or simply so weird that it's hard to imagine how it could have possibly evolved by natural selection. Among the weirdest is the flounder. Not many animals would be at home in a world made by Picasso, but the flounder would fit right in. It belongs to a group of fish called flatfish, or pleuronectiforms, that all spend their adult lives hugging the sea floor, where they ambush smaller fish. Flatfish are teleosts, a huge group of fish species that include more conventional creatures like trout and goldfish. While they have a lot of teleost anatomy, flatfishes also have some bizarre adaptations for their life at ninety degrees. All vertebrates, ourselves included, use hair cells in the inner ear to keep ourselves balanced. In most flatfish species, the hairs have rotated so that swimming sideways feels normal to them. Many flatfish can camouflage the upward-facing side of their body. The underside is pale, and in many species the fin is tiny. And then, of course, there are the eyes. On a typical teleost like a goldfish, the eyes face out from either side of its head. On a flounder, both eyes sit on one side, gazing upwards. It takes time for this Picasso-esque anatomy to emerge: flatfish are born with eyes in the normal position, but as they grow, one eye moves across its head to join its partner. To accommodate this migrant, the bones of the flatfish head twist and turn to make room. When Charles Darwin published the Origin of Species in 1859, a number of critics took him on, but none more seriously than a British zoologist named St. George Jackson Mivart. In 1871, Mivart published a full-scale challenge to evolution by natural selection, called On the Genesis of Species. In one of his attacks, Mivart wielded the flatfish: