Starfish can see … with their arm-eyes

The starfish (Diplopteraster multipes) often bends her arms so that her eye (the red dot) is more or less straight up.

(Marie Helene Birk/University of Copenhagen)

If you look at this small, funky starfish, there is a chance that the well-armed sea creature would look back at you (can already see a blurry version of you) — with a maximum of 50 eyes — already attached to the ends of the squishy limbs. And if you catch at the right moment, the small starfish can glow a vivid blue tone.

This scenario comes from a new discovery. Scientists have discovered that starfish — previously thought to largely rely on scent to navigate through the ocean floor — actually have the ability to see all around them, even in the deep sea, where there is no sunlight, the researchers said.

But although they have in mind the skate, starfish have no 20/20 vision, said the study’s senior investigator, Anders Garm, a professor in neurobiology at the Marine Biological Section of the University of Copenhagen in Denmark. [Take our Vision Quiz: What Can Animals See?]

“Even the best starfish vision is still pretty rough around 500 times less acute than human vision,” Garm told Science in an e-mail. He added that starfish only see in black and white and not in color.

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Star vision

Researchers have known for about 200 years that most of the starfish species sport compound eyes at the tip of each arm. These eyes have multiple lenses, such as an insect peepers. Each small lens, known as ommatidia, to create a single pixel of the total image of the animal shown. But scientists do not test the visual acuity of these creatures until 2014, Garm and a colleague showed that the tropical starfish Linckia laevigata eyes had ” the true image formation, although with low spatial resolution,” and that the starfish uses its vision to navigate in the ocean floor, on the basis of their research, published that year in the journal Proceedings of the Royal Society B.

In 2016, Garm and his team showed that another starfish — the crown of thorns starfish (Acanthaster planci) — can also see images with its advanced compound eyes, according to a study published in the journal Frontiers in Zoology.

However, Garm had never been tested in the vision of the deep-sea starfish that lives under water in the inky darkness. So, in the new study, Garm and his colleagues studied 13 different starfish species in shallow to deep water off the coast of west -, south-and southeast Greenland, the arctic ocean.

One of the starfish had no eyes, they have found. This critter (Ctenodiscus crispatus) live in the sediment, like other blind starfish, and probably used his sense of smell to navigate, Garm said.

The other 12 starfish, even those who lived in a zone with no light — known as the aphotic zone — “still possessed eyes, and some of them are just as good or better the spatial resolution of the shallow water species living in a lot of light,” Garm said.

Two of the considered species — Diplopteraster multipes and Novodinia americana — were also luminous, which means that they can glow on their own. (This is different from biofluorescence, in which an organism absorbs the light from an external source, and glows by letting go of that light at a lower wavelength.)

It is likely that the bioluminescent starfish use their vision to see the glowing signs of other starfish, Garm said. “In other words, they are likely to flash at each other to communicate things such as reproductive state,” he said.

It is also possible that D. multipes uses her vision to help find tasty, deep sea bacteria mats, which emit a weak light, the researchers said in the study.

The study is to be published online Feb. 7 in the journal Proceedings of the Royal Society B: Biological Sciences.

Original article on Live Science.

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