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Instead of Eyelids, This Fish Retracts Its Eyeballs

The giant guitarfish can pull its eyes nearly 1.6 inches inside its head

Giant guitarfish (Rhynchobatus djiddensis) (Brian Gratwicke via Flickr (CC-BY-2.0))
smithsonian.com

The giant guitarfish is named for it’s rather guitar-like shape—the creature looks a bit like a ray stuck to the front of a shark. But that’s not the oddest thing about this fish. Giant guitarfish don’t have eyelids and instead, they've developed one particularly strong muscle to pull their eyes back into their head, according to new research.

The giant guitarfish can retract is eyeballs nearly 1.6 inches back into its heads, almost as far as the diameter of the eyeball itself, reports Emily DeMarco for Science

So why does the guitarfish have such a dramatic skill? Because it hunts for prey, like crabs and small fish, in the sandy bottoms of tropical oceans. "[W]hen thrashing prey kick sand or bits of coral its way, the guitarfish protects itself with [this] eye-catching method," DeMarco writes.

As alien as it sounds, other animals can retract their eyeballs as well. Frogs can retract them about half the diameter of the eyeball, the second most impressive distance measured after the guitarfish. Surprisingly, you can also retract your eyeballs when you close your eyes, but it's only about 0.04 inches.

For most jawed vertebrates, from fish to hippos, six simiarly sized eye muscles control each eye’s movements. So the team investigated the muscles that allowed the guitarfish to accomplis this funky feat. They used a variety of methods to watch guitarfish eye movements from both inside and outside their heads—dissections, CT scans, digital video recordings and an ultrasound.

Though this misfit has all six muscles, only one enlarged muscle is responsible for the guitar fish's eye-retracting trick, according to the results recently published in the journal ZoologyThis muscle also attached to the underside of the fish's skull as opposed to its eye-socket.

Though the retaction is pretty nifty, there is more to this study than just poking guitar fish in the eyes. Scientists trace these odd morphologies through evolutionary lineages to learn how specialized traits evolve and how creatures are related. 

Next the researchers hope to look at other skates and rays—the group to which the guitarfish belongs. 

Time to add oddball eyeballs to the list of amusing evolutionary adaptations.

Fisheye
The guitarfishes' eyes retracts nearly 1.6 inches into its head. (Taketeru Tomita, Zoology 2015)
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