Using
eyes made of a calcium carbonate crystal, a simple mollusk may have evolved
enough vision to spot potential predators, scientists say.
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.
"It's
surprising how these creatures make their eyes from rocks," said Duke biologist
Sonke Johnsen. 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 former Duke Ph.D. student Daniel 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.
Scientists discovered the 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, who recently became a post-doctoral
fellow at the University of California, Santa Barbara. The results of the new
chiton study appear in the April 26 Current Biology.
To test the creature's vision, Speiser placed individual chitons on a slate slab. When left undisturbed, they
would lift part of their armored, oval-shaped body to breathe. At this point,
Speiser would show 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 3 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. But 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 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 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.
Citation: Speiser, et. al. A Chiton Uses Aragonite Lenses to Form Images. Current Biology. 10.1016/j.cub.2011.03.033.