Slime-oozing fish sheds light on eye evolution

Source: ANU

A primitive fish that oozes reams of slime when it is unsettled could help explain how the human eye evolved, according to a team led by a vision expert from ANU.

The scientists compared the eyes of the eel-like hagfish and its lamprey cousin to show that the eye gradually evolved over millions of years - something that had even Charles Darwin stumped.

"Darwin knew that his theory of natural selection would have difficulty in explaining the existence of an organ as specialised as the eye - unless a series of gradual changes could be proved," Professor Trevor Lamb, head of the ARC Centre of Excellence in Vision Science at ANU, said.

"We think we've found the 'missing link' that shows how the eyes of vertebrates (including humans) came to be."

Lamb said hagfish had diverged from our own evolutionary line somewhere around 530 million years ago.

"Hagfish are jawless and ugly animals that continue to inhabit the oceans at great depth, and are renowned for the revolting 'slime' they exude when they're disturbed," he said.

"They behave as if blind, though they have a primitive eye-like structure beneath a clear patch on either side of the head. It was previously thought that this hagfish 'eye' had degenerated from a lamprey-like precursor."

Working with Professor Shaun Collin from the University of Queensland and Professor Ed Pugh from the University of Pennsylvania, Lamb discovered that the hagfish 'eye' has all the signs of being an evolutionary missing link.

They argue that hagfish did not degenerate from lamprey-like ancestors, but are instead the remnants of an earlier sister group.

"The hagfish 'eye' has no lens, no muscles to aim it, the simplest of retinas, and primitive photoreceptors," Lamb said.

"In the relatively brief interval of about 30 million years, between when we think hagfish split off and when lampreys split off, the vertebrate eye appears to have evolved most of its modern characteristics.

"These features can be watched as modern lampreys develop. The larval lamprey is blind and possesses an 'eye' similar to that of the hagfish. But as it goes through metamorphosis that simple eye undergoes an amazing developmental sequence until it looks just like the eye of any jawed fish.

"Modern lamprey adults have eyes that are very similar to our own, and so it's clear that the last common ancestor that we share with them, which lived some 500 million years ago, already had a camera-style vertebrate eye with virtually all the hallmarks of our own.

"Yet an ancestor of hagfish and lampreys, that lived just 30 million years earlier, had a far less advanced eye. We're keen to study the hagfish further to test these ideas."

The researchers' findings are published in the latest edition of Nature Reviews Neuroscience.