Starfish have eyes—one on the end of each of their arms—but what they do with them has been anyone's guess until now.
Starfish have historically been thought of as simple animals. Since their eyes are also relatively simple and because they lack a brain, it's been difficult to figure out how or even if they could see.
But new research suggests that the eyes of sea stars—the term scientists prefer, as the invertebrates aren't actually fish—can form rough images, preventing the animals from wandering too far from home.
"This [study] represents a significant breakthrough in our understanding of how sea stars perceive the world," wrote Christopher Mah, a researcher at the Smithsonian's National Museum of Natural History in Washington, D.C., in an email.
Scientists studied a starfish species found in the tropical Indian and Pacific Oceans, known as the blue sea star (Linckia laevigata), and published their findings online January 7 in the journal Proceedings of the Royal Society B.
Low Expectations for Complexity
Expectations for complexity in these animals have been low because historically, sea stars were viewed as "simple creatures without complicated behavior," said Mah, who was not involved in the study.
"In fact, behavior and body form have been shown to be remarkably complex [in sea stars]," he explained.
Scientists have known about sea star eyes for about 200 years, but aside from studying their structure, not much research has been done on them, said Anders Garm, a neurobiologist at the University of Copenhagen in Helsingor, Denmark.
Part of the reason is that it's been hard to get any physiological information out of the eyes until recently, thanks to advances in scientific equipment, he said.
Previous research suggested sea star eyes were sensitive to light, possibly giving the animal an idea of the location of dark and light spots in their dappled underwater world.
Home Sweet Home
Despite the confirmation of sea star sight, the animals won't be developing reputations for great vision any time soon.
"The image formed in the starfish eye is a very crude image," says study co-author Garm. "It only has about 200 pixels."
But it's enough to enable the blue sea star to recognize large, immovable structures, he said.
This species is tightly tied to coral reefs. If it wandered off to the sandy flats surrounding those reefs, it wouldn't be able to find food and would eventually starve.
So being able to locate a reef—likely the only big, static object in a starfish's immediate vicinity—is very important for these animals.
Starfish have compound eyes, like the ones on arthropods such as insects or lobsters, but the resemblance ends there, Garm says. For instance, blue sea star eyes lack lenses, unlike arthropods' eyes.
Garm and colleagues combined physical measurements of the eye itself with behavioral experiments to come to their conclusions.
One such measure gave researchers an idea of how wide the sea star field of view was: large enough to pick out a coral reef in front of them.
A second measure looked at the ability of the eyes to resolve images. "A lens can help you create a better-resolved image," said Garm, "or it can help you collect more light." Since blue sea star eyes don't have lenses, the images they form are fairly rough.
Their behavioral observations involved moving individual blue sea stars off of a coral reef near Okinawa, Japan, to see if the animals could make their way back or not.
Starfish displaced about three feet (a meter) from the reef walked back home in pretty much a straight line. But animals placed either six feet (two meters) or 12 feet (four meters) away ended up wandering around randomly.
Starfish placed three feet (a meter) away from their reef at night also wandered around randomly, most likely because they couldn't see the reef, said Garm.
Research on sea star vision is in its infancy, the Smithsonian's Mah said. But he'd love to see whether vision plays a role in other starfish species.
"The large sunflower star (Pycnopodia helianthoides) found on the Pacific coast is a fast and efficient predator which is often observed to chase down and swallow its food," he said. Mah would love to know whether vision plays a part in this large animal's ability to capture food.
Garm and colleagues have their sights set on a large starfish species, but not the sunflower star.
Instead, Garm plans to look at the visual system in the crown-of-thorns sea star (Acanthaster planci), responsible for devouring major areas of coral reefs off the coasts of Australia and Asia.
"It'd be nice to [know] if they use vision to see the reef," Garm said. He hopes to use the information to potentially protect areas like the Great Barrier Reef from this voracious predator. (See "Great Barrier Reef: World Heritage in Danger?")
If Garm can find out how the crown-of-thorns sea star detects the reef, researchers can either prevent the animals from doing so in the first place or devise attractive traps to catch them before they decimate a reef.
"There exists a huge gap in our basic knowledge of ecologically important marine animals, such as sea stars," Mah said. (See also: "Massive Starfish Die-Off Baffles Scientists.")
Now, that gap is seen to be a little smaller.
Follow Jane J. Lee on Twitter.