Daniel Speiser on chitons with eyes made of rock

Some small mollusks – a species of chiton – have evolved an unusual way to see the world, through hundreds of tiny eyes across their shells.

The shells of a small marine mollusk – the West Indian fuzzy chiton – contain a network of hundreds of tiny eyes that allow the chitons to see predators, according to a paper published in the April 14, 2011, issue of Current Biology.

Dan Speiser holding a mollusk collected at UCSB’s campus point. Commonly called mossy chiton, the animal is easy to find in California. This species has no eyes however. Image credit: George Foulsham, Office of Public Affairs, UCSB

EarthSky spoke with the paper’s lead author, Daniel Speiser, a post-doctoral fellow at the Department of Ecology Evolution and Marine Biology at the University of California at Santa Barbara. He explained that tiny calcium carbonate crystals connected to light-sensitive cells embedded in the shells of West Indian fuzzy chitons are more than just light detectors. The crystals form a network of eyes – an unusual rudimentary visual system. He said:

The earliest descriptions of chiton eyes are from over one hundred years ago and there have been a number of studies conducted over the last century that demonstrate that most chitons are light-sensitive. However, no one, until now, has tested whether they actually use their eyes to see or not. Now we know that chiton eyes do indeed provide spatial vision, which comes with a number of advantages.

One normally expects that eyes provide an animal with spatial vision, but this had never been tested in chitons. We knew that eyeless chitons were light-responsive, so it seemed probable that eyes provided some additional advantage to eyed chitons. We ran some behavioral experiments and saw that a chiton species with eyes has spatial vision, but that a species without eyes does not.

(Top left) A West Indian fuzzy chiton resting on limestone rocks near Tavernier, FL, USA. (Top right) A closer view of the chiton, where the anterior-most valve is to the right and the eyes appear as small black spots. (Bottom) Chiton eyes, with their translucent lenses and pigment layers clearly visible. The black arrows point to newer, less eroded eyes and white arrows to older, more eroded eyes. Image Credit: Daniel Speiser and Current Biology.

The West Indian fuzzy chiton, also known by its taxonomic name, Acanthopleura granulata, is a marine mollusk related to limpets, clams, and sea snails. They’re found on rocks in intertidal zones along coastlines from southern Florida and Mexico to Panama and the West Indies islands.

Chitons are regarded as primitive mollusks, with a body design that first appeared in the fossil record more than 500 million years ago. These oval-shaped creatures are found across the world, living among the rocks in intertidal habitats, except for a few species found in the ocean depths as far down as 20,000 feet (6,000 m). Chitons have an armor of shells made of eight segments, each connected to a strong muscular girdle that surrounds the creature’s body. This design gives them flexibility to move across rocky surfaces, and when dislodged, allows them to protectively curl up in a ball-like shape.

Two West Indian fuzzy chitons and two checkered nerites on a rock at Le Gosier, Guadeloupe, in the Lesser Antilles. Image credit: Hans Hillewaert via Wikimedia Commons.

Even though chitons have been around for over 500 million years, the development of eyes in some species occurred in just the past 25 million years, fairly recently in the evolutionary history of these animals.

Not all chiton species have eyes. Speiser told EarthSky,

Most chitons don’t have eyes as adults. Among the relatively few chiton species that do have eyes, we don’t really know how the eyes vary. There is certainly some variation (between species) in the size of the eyes, the total number of eyes, and how the eyes are arranged. This is something we’re going to look at closely in the near future.

The West Indian fuzzy chiton is one of the eyed chiton species – there are hundreds of tiny eye-like structures across their shells. Each eye has a clear crystal lens made of aragonite, a form of calcium carbonate. Speiser described their structure:

The lenses are made of the same material as the shell and they are embedded directly into it, so I’d guess that lens production is fundamentally similar to shell production. However, we don’t know how chitons make sure that their lenses are shaped just right. The lenses are quite translucent when the animals are alive and, unlike lenses made of protein, they stay clear even after animals have been fixed, preserved, etc. They’re only about 50-100 microns [0.002-0.004 inches] wide. That made them tricky to work with!

Chitons continually add new eyes to the margins of their shells (or “valves”) as they grow. They need to do this in part because older eyes get eroded away (chitons live on rocks in intertidal zones and are exposed to waves).

These images show an intact chiton lens (left) and a decalcified, sectioned eye (middle). The illustration (right) shows eye morphology, where pl = pigment layer, rh = rhabdoms, and re = retina cells. The other structures are labeled in full. Image Credit: Daniel Speiser and Current Biology.

How did Speiser and his team find evidence that West Indian fuzzy chitons had a sense of vision, as opposed to just detecting changes in light intensity? In a press release dated April 14, 2011, they described the experiments conducted on chitons collected from the Florida Keys.

The chitons were placed on a slate slab. Left undisturbed, each would feel safe enough to lift part of its body to breathe. From about 20 centimeters (about 8 inches) above, Speiser then proceeded to show them either a black disk, with a diameter ranging from 0.35 centimeters to 10 centimeters (0.14 to 3.94 inches), or a corresponding gray slide that blocked the same amount of light. The goal was to find out whether the chitons responded to a shape, in the form of the black disks, or a decrease in light caused by the grey slides.

Chitons in that experiment did not respond to the changes in light caused by the gray screens. But when shown black disks starting at 3 centimeters and getting larger, they protectively clamped down. In the press release, Sönke Johnsen, professor of biology at Duke University and paper co-author, described it as “the equivalent of humans looking in the sky and seeing a disk the diameter of 20 moons, making human vision about a thousand times sharper than chiton vision.” EarthSky asked Speiser, in terms of resolution, did it mean that the chiton’s visual system is able to resolve what would look to human eyes like a 20-moon diameter disk in the sky and not be able to discern any smaller detail?  He said,

Yes, you have that right. Chiton eyes are low-resolution. They really don’t pick up many details from their visual environment. This is why we suspect that the eyes are primarily used to detect predators. Their eyes could still be used for other purposes though, we just don’t have any evidence, one way or another, on that.

Our behavioral results (that chitons have a visual resolution of 9-12 degrees) can be explained by each eye forming an individual image, so we are interpreting the eyes as small camera eyes. We don’t have any evidence that each eye is acting a like a facet of a large compound eye, but we can’t entirely rule out this possibility either. We also don’t know whether chitons are combining input from all their different eyes into a single reconstruction of their visual environment. This would be quite a feat for an animal that is, by and large, brainless.

How did the chiton eyes compare to the visual system of other mollusks?

The lenses are very different – other mollusks make their lenses out of proteins, not calcium carbonate. Beyond that, we don’t know. The photoreceptors in chiton eyes could be similar to those in other mollusk eyes, or they could be quite different. That’s actually what I’m working on right now.

He also commented about the types of photoreceptor cells in the chitons.

Some of these cells (like those in octopus or insect eyes), send a signal when light levels increase. Others, like those in vertebrate eyes, send a signal when light levels decrease. We, of course, can tell when either situation occurs, but that has to do with cells downstream of the photoreceptors (rods and cones) themselves. From the behavioral trials I conducted, I can say that I only saw chitons respond to decreases in light, which suggests that chiton retinas may be rather different than the retinas in snail, slug, or cephalopod eyes. It’s hard to say much more at this point.

This drawing shows the way light would bend and focus in the chiton’s eye depending on whether the animal was above or below water. Image Credit: Daniel Speiser and Current Biology.

Speiser explained more about how the individual eyes work together to form the animal’s visual system.

The field of view is about 75 degrees for each eye. However, eyes are distributed all across the shell plates, so a chiton can see in all directions. This is useful because chitons are slow-moving animals – they’re not going to be able to turn around to see what might be sneaking up from behind.

We really don’t know if chitons have any sense of distance – all we know is that they can spot the sudden appearance of an object (and that they can distinguish between the sudden appearance of an object and uniform decrease in light levels).

The biggest surprise, Speiser said, was learning that chiton eyes work equally well in air and in water.

It makes a lot of sense for an intertidal animal to have eyes that work in both mediums, but the different refractive indices of air and water make this a rather difficult engineering problem. We’re excited about the possibility that chitons might be using the birefringence of their lenses to form two separate images: one that falls on the retina in air and one that falls on the retina in water. If we’re right, chitons take advantage of an intrinsic property of aragonite to gain a multi-functional eye. We have a ways to go before we prove anything, but the evidence we’ve gathered so far supports this intriguing possibility.

The West Indian fuzzy chiton has evolved a rather novel way to detect predators, with hundreds of tiny eyes across their shells. Each eye has a lens made of calcium carbonate crystals that focuses light onto light-sensitive cells. Together, the eyes create a visual system that allows the animals to detect predators.

West Indian fuzzy chiton in the Bahamas. Image Credit: Anne DuPont

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