Like sharks, the giant guitarfish doesn’t have eyelids that close all the way, so it can’t blink. That might guarantee a win in a staring contest, but it does pose problems for eye protection in the sandy, tropical waters where the creature lives. So when thrashing prey kick sand or bits of coral its way, the guitarfish protects itself with an eye-catching method: retracting its eyes almost completely into its head, leaving a craterlike depression. Now, new research shows that guitarfish can thank a specialized eye muscle for that ability. Using high-speed video, researchers found a guitarfish could sink its eye nearly 40 mm. That’s almost as much as the diameter of the eyeball itself and likely more than any other vertebrate, the researchers reported online before print in Zoology. A muscle known as the obliquus inferior appears to be key; when the researchers electrically stimulated the muscle in a dissected guitarfish, the eyeball sank. Other animals, including frogs, bottlenose dolphins, and mudskippers, also retract their eyes, but employ a different mechanism. Some rays and skates, however, have the same muscle arrangement as guitarfish, so researchers are eyeing them for future studies.
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