A recent study co-authored by Daniel Speiser, a postdoctoral fellow at UCSB’s Dept. of Ecology Evolution and Marine Biology, provides strong evidence that the chiton, a simple mollusk, has developed eyes essentially made of rock in order to see its predators.
Chitons evolved their advantageous eyes using the same substance as their shells, aragonite, making them durable enough to withstand the violent, rocky life of an intertidal animal, according to Speiser.
“It is really interesting how resourceful and creative animals are with building eyes — they can use so many different materials. Most eyes are made with proteins, like ours, but these animals use minerals,” Speiser said. “It broadens the scope of what an eye can be.”According to Speiser, these largely immobile animals likely evolved their eyes in order to defend themselves against predators.
“They need to see things from all directions because they cannot really turn around — they make a snail look fast. When they see a predator, their auto-response is just to clamp down on the rocks.”
Although scientists have known for decades that chitons have hundreds of tiny eyes embedded in their shells, it is only recently that scientists like Speiser are inquiring as to how and why this interesting trait evolved.
“No one had thought to ask ‘How do these eyes work?’” Speiser said. “I was excited that no one had worked on this for a while. I have something interesting here and I am not competing with a ton of others.”
Their protective clamping instinct was used to test the capacity of chiton vision; researchers found that chitons clamped in reaction to black disks blocking light, while they remained at ease when grey slides blocked identical amounts of light.
This suggests chiton vision has the important ability to distinguish between threatening changes in light due to approaching predators and unthreatening occurrences like clouds passing over the sun, according to Speiser.
While the study estimates chiton vision cannot be particularly impressive — it is about 100 times lower than the resolution of human sight — it is still exciting that these animals have been spending the last 25 million or so years evolving eyes using simply the shell on their back.
“A single aragonite crystal is transparent and can occur naturally. So the lenses are probably made of single crystals, while the rest of the shell is made of a bunch of crystals,” Speiser said. “What is really crazy is how they shape the lenses. They have to be shaped just right in order to work. Somehow they are able to have really fine control over it — it is pretty wild.”
According to Speiser, discovering exactly how these mollusks are making their rock lenses is going to be an interesting but difficult task.
Speiser came to UCSB to conduct comparative behavioral experiments with other eye-less species found off the campus’ coast. He also plans to collect more samples in Mexico this summer.
The researchers’ next step, however, will be in the direction of a more genetic understanding of the chiton’s rock eyes.
“We are going to end up sequencing genes that have to do with lens growth,” Speiser said. “The first step will be trying to find the genes that are involved.”
According to Speiser, one of the most exciting discoveries they have made thus far is that chiton eyes work equally well in both air and water, unlike most animals’ eyes.
While research in the area is still pending, the characteristics of aragonite lenses — such as their durability and agency in both air and water — may be useful for materials scientists to study.