Most animals that live in perpetual darkness tend to stop using their eyes or light-sensitive organs, but some deep-water starfish seem to be going the other way. Researchers have looked at 13 species of Arctic starfish living hundreds of meters below the sea surface and discovered that all but one retained the use of its eyes.
As reported in Proceedings of the Royal Society B, the 13 species were picked at various depths, from shallow waters to the deep sea below 1,000 meters (3,280 feet), off the coast of Greenland. Three of the species live below 320 meters (1,050 feet), which is the aphotic zone, the region where almost no light from the Sun can reach.
Starfish vision is based on ommatidia, small individual units that make up a compound eye. These eyes are located at the end of each arm and they tend to be very similar between different species. Starfish use them to know where they are going although their main sense is smell.
These deep-water species were also tested for bioluminescence. If the species glowed in the dark, that could explain the presences of functioning eyes. When the only thing you can see is the abyss staring back at you, perhaps you don’t want to spend energy on vision.
The researchers found that two species could emit some light signals, Novodinia Americana and Diplopteraster multipes, and maybe not surprisingly, these two species had the best visual organs. D. multipes was the species most sensitive to light while N. Americana had the largest spatial resolution known for starfish.
Having great vision (for a starfish) and being able to emit light is probably related. This is the working hypothesis the researchers have put forward. They think that the starfish use bioluminescent flashes to communicate and for reproductive purposes.
The species without eyes is called the Ctenodiscus crispatus, which is a burrowing type of starfish. Not much to see within the seabed, we guess. Starfish are curious organisms and they are found in most marine environments. They are often considered simple animals as they don’t have a central nervous system but recent studies have highlighted much more complex behaviors, vision included, suggesting that it’s not the brain that maketh the being.