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.”