Chitons see with eyes made of rock

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As a fish swims over the ocean floor, it’s being watched by hundreds of rocks. The rocks are actually the eyes of a chiton, an armoured relative of snails and other molluscs. Perhaps uniquely among living animals, it sees the world through lenses of limestone, and its eyes literally erode as it gets older.

Chitons are protected by a shell consisting of eight plates. The plates are dotted with hundreds of small eyes called ocelli. Each one contains a layer of pigment, a retina and a lens. People have known about the ocelli for years, but no one knew what they were made from or how much the chitons could actually see with them.

Daniel Speiser from the University of California, Santa Barbara has solved the mystery by studying the charmingly named West Indian fuzzy chiton. It all started with a surprising bath. Speiser had removed the lenses from a chiton and dipped them in a mildly acidic liquid, which was meant to clean them. Instead, it quickly dissolved them!

The vast majority of animal lenses are made of proteins, which should be unharmed by weak acid. The chiton lenses were clearly different. Speiser soon found that they are made of a mineral called aragonite. Aragonite is a version of calcium carbonate, or limestone, and it forms the shells of almost all molluscs, from oysters to snails to chitons. This means that the chiton’s eyes – or the lenses, at least – are made from the same substance as its armoured shell.

Chitons may be the only living animals with rocky eyes of this sort. Another group of extinct oceanic animals – the trilobites – had lenses made from calcite, another form of limestone. Some crustaceans have lenses that contain crystals of calcium carbonite, but these float in a sea of proteins; the chiton’s lens is a single solid block of mineral.

Brittlestars – relatives to starfish – come closest. They have small “microlenses” of calcite, but it’s not clear if they actually use these to see. Until now, the same was true of the chiton lenses, but Speiser has clearly shown that they can indeed detect objects.

They’re not very sharp though. Based on his measurements, Speiser calculated that each one has an “angular resolution” of around 9 to 12 degrees. That’s the angle that two objects would need to form with the eye, for the chiton to tell them apart. By comparison, humans have an angular resolution of 0.007 degrees. “The image produced by the chiton eye is therefore over a thousand times coarser than that produced by our eye.” says Speiser. “Imagine a computer monitor with only a thousandth of the pixels that you’e used to – that’s what switching from a human eye to a chiton eye would be like.”

To test this in real life, Speiser exploited the fact that chitons normally lift their flanks to breathe, by exposing the gills on their undersides. When they spot danger, they flatten their bodies and lower their armour. A passing shadow will do the trick, so Speiser flashed black circles of varying sizes over the animals. He found that they only hunkered down when the circles had an angular size of 9 degrees.

When he used grey screens that blocked out an equal amount of light, the chitons didn’t react, so they weren’t just responding to darker conditions. They had seen something that had spooked them. These animals can see objects, but not in any detail.

The chitons react in the same way in air and water, which suggests that their eyes work equally well in both environments. That’s not surprising – chitons live on the tide line so they need to be able to see in both air and water. The images that they get may be fuzzy, but that doesn’t particularly matter. These are not eyes that have evolved to resolve fine detail. They just need to be sensitive enough to spot passing shadows.

For now, Speiser doesn’t know if the chitons can combine the information from their hundreds of eyes to create a single, unified view of their surroundings. The alternative is that each eye acts as a primitive motion sensor that detects passing objects. “Having lots of eyes can be good thing if you can’t move or can only move slowly,” he says. “It would take a chiton at least a few minutes to turn around to see if something was sneaking up behind it, so it makes some sense that these animals have eyes that face in all directions.”

It’s also important to have lots of back-up eyes if yours can literally erode. Chitons live in the tidal zone and limestone, salt-water and waves don’t make a good combination. “Over time, wave action erodes chiton lenses away,” says Speiser. “Chitons seem to make up for this by adding new eyes as they grow.”

Even though the rocky eyes seem primitive, they are actually the most recent animal eyes to evolve. There are plenty of chiton fossils, but only those that are younger than 10 million years have eyes. That is puzzling in itself. “There’s nothing all that unique about the habitats where you find eyed chitons. In fact, there are many species that live in similar rocky, intertidal habitats that don’t have eyes,” says Spieser. “It could be that chiton eyes are just really unlikely to evolve, so it took a few hundred million years for it to happen. I don’t like this answer though.”

To get a better one, Speiser plans on comparing eyed and eyeless chitons to see if he can work out where and when the eyes evolved, and match that to some change in the environment or the emergence of a new predator. By comparing eyed and eyeless chitons, Speiser has already hit upon something interesting that could tell us about the evolution of eyes in other animals.

Eyeless chitons still have cells that are sensitive to light, but they lack the retinas, lenses or proper eyes of the fuzzy chiton. Speiser found that one eyeless species was actually more sensitive to light than the fuzzy chiton, and could detect smaller changes in brightness. However, it couldn’t resolve objects. It can tell how much light there is in its environment, but it can’t see any images.

This suggests that the evolution of proper eyes was a trade-off for chitons – they gained the ability to tell the difference between objects and shadows, but they lost some of their sensitivity to light. Speiser now wants to see if this trade-off applies to other animals. If so, it could explain why so many groups have eyes as well as light-sensitive cells on other parts of their body.

Reference: Speiser, Eernisse & Johnsen. 2011. A chiton uses aragonite lenses to form images. Current Biology, citation tbc