A brittle star’s whole body is an eye.

Scientific American looks at how these starfish relatives don’t need eyes to see:

And yet now there appears to be something far stranger about the biology of at least one species: the entire body of Ophiocoma wendtii appears capable of forming a blurry but serviceable image, like a squirmy but strangely cute Eye of Sauron.

They tested this by placing the brittle stars in the center of arenas surrounded by walls with black bars on one side to simulate shelter. More brittle stars ended up at the black bars than would be expected by chance. Dark-adapted beige O.wendtii, however, were not capable of finding these black bars at rates exceeding chance whether they were supplied with fluorescent lights or daylight, seemingly ruling out light intensity or circadian rhythms as an explanation.

This behavioral combination was particularly puzzling because a close relative, O. pumila, has all the same light-detecting equipment, but when placed in the same arenas, these animals end up randomly distributed at any time of day. They are blind.

However, the team knew that the bodies of both brittle stars are studded with light receptors called opsins. O. pumila may not be able to see, but it can sense light; when exposed to light, it hides in sand or crevices right where it is. O.wendtii, on the other hand, skitters to the nearest shelter. The only obvious difference is that O. pumila is not ever red. What difference could being red make to seeing?

Pigment is natural sunscreen, and O. wendtii is covered in red pigment packets called chromatophores by day. At night, the chromatophores retract. When the scientists scanned both brittle star species and measured the field of view from each animal’s light sensors, they discovered deployed pigment packets narrow the field of vision by physically blocking light. The angular aperture narrows from about 118 degrees to 68 degrees in O. wendtii with chromatophores blazing.

If the light sensors of O. wendtii are directional, that explains how the array widely scattered over their entire body could form an image.

You can read the original research here, in Current Biology.