Chitons Evolved Eyes Four Times in Two Different Ways

Small, armoured and easily overlooked, chitons possess eyes unlike those of any other animal.

Certain marine molluscs carry thousands of rounded, tiny eyes embedded within their segmented shells. Each has a lens formed from the mineral aragonite. Despite being minute and apparently simple, these sensory structures, known as ocelli, are believed to provide genuine vision, allowing chitons to distinguish shapes as well as light.

Chiton visual systems

Other chiton species have smaller ‘eyespots’ instead. These operate more like separate pixels, resembling the elements of an insect’s or mantis shrimp’s compound eye, and together create a visual sensor spread across the chiton’s shell.

A new study investigating the origins of these contrasting visual systems has uncovered remarkable evolutionary flexibility in these rock-living animals. Their ancestors rapidly developed eyes on four separate occasions, producing the two very different visual systems seen today.

While this pattern is not quite as recurrent as the sideways-walking body plan of crabs, which has evolved at least five times, it again demonstrates how evolution can generate multiple answers to fundamental challenges, such as using light to avoid becoming prey.

"We went in knowing there were two types of eyes, so we were not expecting four independent origins," says evolutionary biologist and lead author of the study, Rebecca Varney from the University of California Santa Barbara. "The fact that chitons evolved eyes four times, in two different ways, is pretty amazing to me."

Four independent origins of chiton eyes

To map this evolutionary history, the researchers compared fossils and analysed DNA samples from specimens held at the Santa Barbara Museum of Natural History, using the evidence to reconstruct the chiton evolutionary tree.

Their analysis found that each of the two visual systems evolved twice, and that these developments happened in rapid succession. Oddly, however, the groups that developed similar visual structures were not each other’s closest relatives; instead, they were distantly related lineages divided by millions of years.

Eyespots first appeared in one chiton group between 260 and 200 million years ago, during the Triassic period, when dinosaurs were first emerging. This slightly preceded the earliest shell eyes, which another group developed in the Jurassic, roughly 200 to 150 million years ago.

Shell eyes then arose for a second time between 150 and 100 million years ago, in the Cretaceous, among Toniciinae and Acanthopleurinae chitons. This makes them the most recently evolved lensed eye known to us.

Finally, eyespots emerged again in another branch of the chiton evolutionary tree as recently as the Paleogene, about 75 to 25 million years ago.

Shell slits and visual complexity

Having established the timeline, Varney and her colleagues also wanted to understand the conditions that may have shaped this repeated evolution.

Chitons have openings in their shell plates that allow optic nerves to pass through. Species with fewer slits generally evolved fewer but more complex shell eyes. Conversely, chitons with a greater number of slits went on to develop larger numbers of simpler eyespots.

"Clarifying the role of [trait] history in shaping evolutionary outcomes is critical to our understanding of how and why characters can evolve in predictable ways," the researchers conclude.

How these structures transmit visual information to the chiton brain remains the subject of continuing research.

Another recent study has shown that, in at least one chiton species, the more elaborate shell eyes send visual information to a ring-shaped neural structure that circles the entire body. Optic nerves linked to this ring can then detect an object’s position according to which sections of the ring are activated.

The study was published in Science.