Bizarre sea beasts covered in eyes evolved their chicks four different times

With armor-like shells covering their eyes, chitons are truly strange little creatures. Now researchers have discovered even more strange things underlying the evolution of their visual systems. Not only do chitons have the most recently developed eyes with a lens, but their two types of eyes arose in four separate events.

Chitons are marine molluscs – and they are really strange. They are egg-shaped in shape, crawl slowly over rocks in the intertidal zone and are recognizable by their coats of mail that form an armor over their back. One species, with the unflattering but not unreasonable nickname “wandering meatloaf,” has teeth made of an iron-based mineral previously found only in rocks. And then there are the eyes. Chitons’ senses are not on their bodies, but on their shells.

“I don’t think there is any other animal that builds its eyes into its armor the way a chiton does,” co-author Daniel Speiser said in a statement.

The most common sensory organs, found in the outer layer of the shells of all chitons, are called aesthetes. Aesthetes are thought to form the basis of two different visual systems that have evolved in some of these curious critters. The first involves larger scale eyes, with lenses formed from the mineral aragonite that focus light, similar to human eyes. The second is smaller but more numerous “eyespots” that function as individual pixels, similar to the compound eyes of some insects.

But how did these peculiar peeping toms evolve? That’s what the team behind the new study wanted to know. “Is there something we can identify that is driving evolution in these different groups toward eyespots or shell eyes?” asked lead author Rebecca Varney.

Chiton visual systems

Aesthetes (green) are found on all chitons, while shell eyes (blue) and eyespots (red) have evolved only in a few groups. These structures are connected to the chiton’s nervous system via notches in the shell.

To investigate this, Varney and co-authors constructed a chiton family tree, and were surprised to find that the ancestors of chitons developed eyes on four different occasions.

“We went in knowing there were two types of eyes, so we weren’t expecting four independent origins,” Varney said. “The fact that chiton eyes developed four times, in two different ways, is quite astonishing to me.”

Even stranger, the groups that arrived at similar structures were not the ones most closely related. “It almost seemed too strange to be true,” noted senior author Todd Oakley.

The team found that shell eyes arose in some species of chiton between 250 and 200 million years ago, while in other species they arose during the Cretaceous period, about 150 to 100 million years ago, making them the most recent eye with a lens. In contrast, eyespots arose as early as 260 to 200 million years ago in one group of chitons, and only 75 to 25 million years ago in another group.

Evolution is quite adept at coming up with the same or similar solutions to life’s problems – crabs, for example, have evolved at least five times. To find out how this was the case for chitons, the team looked for clear differences between species.

They noticed that chitons with eyespots tended to have more slits at the edge of each shell segment, while those with shell eyes had much fewer. These slits facilitate the nerves that travel from the chiton’s shell to its body. The researchers therefore believe that the number of slits present when chitons’ visual systems developed predicts which type of visual system developed.

“Here we have a very clear demonstration in a natural system that evolution depends on what came before it, even when what came before seems completely unrelated,” Oakley said.

How many chitons can ‘see’ using these two visual systems is still a mystery, but is the focus of ongoing research. What we do know is that the evolution of chiton vision is as bizarre as they are.

The research has been published in Science.

Leave a Reply

Your email address will not be published. Required fields are marked *