Look at a starfish in a tidal pool and you may think: ah, there’s one of those pretty, multi-armed sea worms that crawl around and don’t do much. But look deeper and your views might change.
Hundreds of feet below the ocean’s surface, some starfish make their own light. And they can look right back at you too, with a teeny eye on the tip of each bendy starfish arm.
Scientists, who didn’t even know if deep sea starfish had eyes, did not expect to find this.
They collected starfish from Arctic waters off Greenland’s coast to determine which species had eyes, and for those who did, how well they could see. The researchers found that some starfish are far more biologically complex than previously thought, after looking at the structures of their eyes, their behavior in a simulated environment, whether they glowed in the dark and what tasks in the wild would make eyesight useful.
“Even in places where the sun don’t shine, it’s far from dark,” said Anders Garm, a marine biologist at the University of Copenhagen who led the study published Wednesday in Proceedings of the Royal Society B. “There are animals there making their own lanterns and finding their way.”
In deep sea habitats, life-forms acquire bizarre adaptations to living under high pressures and without light. By the time you get to the bottom, most fish and crustaceans lose their eyes and rely on other senses. Or they modify them, sacrificing a sharp picture, for a bigger one — kind of like what we see when our eyes adjust to a dark room.
Scientists thought deep sea starfish would also be eyeless, or at the most have very simple eyes. But as they examined specimens they retrieved, they found that many starfish had unexpected visual tools.
They sampled 13 species of starfish, which were representative of the diverse ecologies of underwater life. They found that all but one species that burrows in the ocean’s sediments had eyes. Two from the deep glowed in the dark. One, Novodinia americana, had a whole body that lit up when stimulated, and its eyes, with bigger pupils, were capable of detecting even sharper images than their shallow water relatives.
Because their eyes are on the underside of their arms, starfishes must do some aquatic yoga to direct their gazes, the researchers found. Some starfish bent their arms like a periscope at a 90-degree angle. Others, from the twilight zone, bent their arms completely backward, as if looking up to gauge day or night.
But why go through the trouble of making light and creating better eyes for detecting it when most starfish navigate using their sense of smell?
Different tasks place different demands on the eye, said Dr. Garm. And tuning eyes for only what matters in an environment saves energy.
In shallow water, the Crown-of-Thorns Starfish uses vision alone to travel short distances to the coral they eat. But its sight is slower than any other observed in the animal kingdom and lacks detail. Shutter speed doesn’t really matter when it’s trying to locate a stationary reef.
In the deep sea, starfish may make light for a few tasks.
Starfish often find mates using pheromones. But in the deep sea, a starfish even just a few inches upstream from a potential mate won’t know it’s there. So they may light up to signal to each other, while remaining invisible to scent-detecting predators.
Seeing light could also help them locate glowing food sources or hide from bioluminescent predators.
In order to better capture this light, just as you would widen a camera’s aperture to take a photograph of a dark place, starfish species deeper down have larger pupils to collect light from a larger space.
While they appear to be communicating with light, “who knows what they’re saying,” said Dr. Garm. To find that out, he plans to observe deep sea starfish in the wild with cameras on a remote operated vehicle. “If we can show that they communicate with light in the deep sea, that would be pretty astonishing.”
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