|A large chiton sometimes seen on Singapore's shores.|
Not all chitons have eyes though.
Eyes Made of Rock Really Can See, Study Says
Mollusks' mineral lenses can distinguish shapes, not just light.
Ker Than National Geographic News 14 Apr 11;
When it comes to hard stares and stony gazes, no animal can match the chiton, a small mollusk with eyes made of rock crystal. Now a new study shows just what these strange eyes are capable of.
Scientists had long known that chitons have hundreds of beadlike structures resembling eyes on the backs of their shells. The lenses "are like big, clear pieces of rock," said study leader Dan Speiser, a marine biologist at the University of California, Santa Barbara.
What's been unclear, however, is if the creatures could actually see using these organs or whether the eyes were good only for sensing changes in light intensity.
"It's been known for over a hundred years that these eyes exist, but no one's really tested what sort of vision they provide," Speiser said.
His latest research—conducted while he was a graduate student at Duke University in North Carolina—revealed that the sea creatures' eyes are the first known to be made of the mineral aragonite, the same material chitons use to make their shells.
What's more, these stony eyes likely have unique advantages over the squishy eyeballs of other animals.
Mollusks in Lockdown
To test the chiton's vision, Speiser and his team collected Indian fuzzy chitons (Acanthopleura granulate) from the Caribbean.
When left alone, a chiton will lift part of its oval-shaped body to breathe. But when threatened, the animal will clamp down tightly on the seafloor to protect its soft underbelly.
In the lab, the scientists placed individual animals on a stone slab beneath a white screen, which could change colors. Once the chitons seemed relaxed, the team either placed a black disk directly above the mollusks or changed the color of the background screen from white to gray.
The black disk was designed to simulate a suddenly appearing predator, while the dimming screen mimicked subtle changes in natural light that chitons might experience in the wild—for example, when a cloud passes in front of the sun.
In the experiment, the chitons went into lockdown mode when shown the black disk, but the animals remained at ease when the screen dimmed. This suggests the chiton's eyes are able to distinguish shapes, a prerequisite for true vision.
"The eyes allow the chitons to see objects—not with much detail—but they can distinguish between approaching objects and just decreases in light," Speiser said.
Speiser estimates chiton vision is about a thousand times coarser than human vision, and it's likely they see only in black-and-white.
"Even compared to other animals with small eyes, chitons don't see particularly well," Speiser said.
Rock Eyes Better for Tidal Creatures
Chitons' rock eyes do appear to have some specific advantages. For one thing, the hard aragonite is extremely resilient, an important trait for chitons, which are constantly being pummeled by waves in their natural habitats, shallow tidal pools.
"If their eyes were made of protein"—which is the case for humans and most other animals—"they would get worn right away," Speiser said.
For another thing, the experiments suggest aragonite allows the chitons to see equally well in air or underwater, something that's probably useful as tides ebb around the mollusks.
"Behaviorally, the chitons react the same" in both mediums, Speiser said.
That's probably because aragonite has two refractive indices, the extent to which a particular material focuses incoming light. With an aragonite eye, one index creates an image on the eye in water while the other works in air.
Meanwhile, a few mysteries remain about chiton eyes. For instance, it's still not known why only some chiton species have eyes, or how the creatures are able to use the same material to make both their eyes and their shells.
"It's going to be interesting to see how they're shaping these lenses,” Speiser said. "How do they make them the right size and shape and keep them translucent? They're exerting some very fine control."
The chiton-eyes research will be detailed in the April 26 issue of the journal Current Biology.
Eyes of rock let chitons see predators
University of California - Santa Barbara EurekAlert 14 Apr 11;
(Santa Barbara, Calif.) –– Using eyes made of a calcium carbonate crystal, a simple mollusk may have evolved enough vision to spot potential predators, scientists say.
Daniel Speiser, a postdoctoral fellow in the Department of Ecology Evolution and Marine Biology at UC Santa Barbara, studied mollusks that he collected in the Florida Keys. His research of their vision, performed during his graduate studies at Duke University, resulted in a study published today by Current Biology.
The three-inch-long mollusks, called chitons, have hundreds of eye-like structures with lenses made of aragonite, a type of rock. It's the first time scientists have found an animal that makes eye lenses from aragonite and not the rock's close cousin, calcite.
Scientists discovered the chiton's unique eyes decades ago. But it wasn't clear whether chitons used these eyes to see objects overhead, or simply to sense changes in light. "Turns out they can see objects, though probably not well," said Speiser.
"It's surprising how these creatures make their eyes from rocks," said Sönke Johnsen, associate professor of biology at Duke. Most animals make their eyes from cells with proteins and chitin. "But it seems like an easy way to evolve eyes, by using what you've already got," he said. Chitons also make their shells from aragonite.
Johnsen and Speiser studied West Indian fuzzy chitons, or Acanthopleura granulata, which have flat shells made of eight separate plates. Hundreds of tiny lenses on the surface of the plates cover clusters of light-sensitive cells beneath.
To test the creature's vision, Speiser placed individual chitons on a slate slab. When left undisturbed, they lift part of their armored, oval-shaped body to breathe. At this point, Speiser showed them either a black disk ranging from .35 centimeters to 10 centimeters in diameter or a corresponding gray slide that blocked the same amount of light. The disk or slide appeared 20 centimeters above the chitons.
When shown the gray screens, the chitons did not respond. But they clamped down when shown a black disk three centimeters or larger in diameter. That would be 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, Johnsen said.
Because the chitons responded to the larger disks and not the gray slides, they seem to be seeing the disk and not simply responding to a change in light, said University of Sussex biologist Michael Land, an expert on animal vision who was not involved in the research. It's not yet clear if they respond only to the removal of light by the disk as opposed to added light.
Land also said it's not likely that the chitons' eyes were part of the evolutionary route to human eyes.
Chitons are an ancient, primitive species that first appeared on Earth more than 500 million years ago. But the oldest chitons with eyes only began to appear in the fossil record in the last 25 million years, making their eyes among the most recent to evolve in animals. Speiser said chitons probably evolved to have eyes with lenses so they could see their predators and defend themselves against being eaten.
Speiser and his colleagues also tested whether the chitons' eyes work in both air and water, since some species spend time in both. The experiments made a strong case for the chiton lens being able to focus light differently, depending on whether the animal is above or below water, Land said.
He added that chiton eyes are still an anomaly in the evolution of vision. The retinas are structurally similar to snail and slug retinas. But snail and slug retinas respond to the appearance of light, while chiton retinas may only respond to the removal of light, a difference that might be worth another look, Land said.