Why baby flatfish grow into the wonkiest animals in the world

Noriyuki Otani / Alamy Stock Photo

By Alice Klein

The mystery of how a Japanese flatfish pushes both eyes to one side of its body has finally been unravelled.

When Japanese flounder (Paralichthys olivaceus) are newly hatched, they have eyes on opposite sides of their heads just like most other fish.

But after three weeks, one eye migrates over the top of the skull to the other side.  The skin on the side of the body with two eyes then turns sand-coloured, while the other side stays pale.

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At this point, the fish tips sideways and swims flat against the ground. The side facing upwards has two eyes to look for prey and sea-floor colored skin for camouflage in what is some of the most asymmetric body shape of all vertebrates.

Although some invertebrates like sponges can be asymmetric, almost all vertebrates have the same left and right sides to promote efficient locomotion.

Skin activation

How this unusual configuration arises has long perplexed scientists. To look for clues, Manfred Schartl at the University of Würzburg in Germany, and his colleagues identified the genes that distinguish Japanese flounder from other fish.

From this, they discovered that Japanese flounder have light-sensitive proteins in their skin called opsins. These are normally found in retinal cells in eyes, where they convert light to electrochemical signals.

But in the flounder skin, they activate retinoic acid, which controls skin colour, and thyroid hormones, which influence body shape.

Schartl’s team also revealed that in the late larval stage, the fish swims upright but with a slight sideways tilt, so that sunlight hits it unevenly. This activates opsins to a greater extent on one side, leading to the eye and skin colour asymmetry that eventually allows the fish to flip sideways.

The process probably evolved from a random mutation that was naturally selected due to its advantage at the bottom of the ocean, Schartl says.

There are two main bonuses associated with being able to lie flat and blend into the sea floor, says Culum Brown at Macquarie University in Sydney, Australia. “One is that you can ambush prey and the other is that you can hide from predators,” he says. “Switch the position of one eye, change your pigmentation a wee bit, and you’re sorted.”

Journal reference: Nature Genetics, DOI: 10.1038/ng.3732