The giant guitarfish, Rhynchobatus djiddensis, can’t blink. Like household goldfishes, this guitar-shaped relative of sharks and rays don’t have moveable eyelids. But if you gently poke one near the eye – as researchers did – it retracts its eyeball almost completely into its head. This helps protect them from damages incurred by drifting sand, coral fragments, and the targeted attacks by other fish along the tropical sea floor where they live and hunt. The findings were published in Zoology last month.
Four vertebrate lineages are known to have very retractable eyes: amphibians, cetaceans, amphibious fish called mudskippers, and a group of cartilaginous fishes called the batoids (which include rays, skates, sawfishes and guitarfishes). Contracting what’s called the retractor bulbi eye muscle allows bottlenose dolphins and some frogs and salamanders to retract their eyeballs; mudskippers can sink their eyes into their head completely to keep their peepers moist above water. But how retraction works in batoid fishes, which don’t have retractor bulbi, remained unknown.
So, a team led by Taketeru Tomita from the Hokkaido University Museum lightly touched the skin above and beside the eyeballs of two giant guitarfish housed at the Okinawa Churaumi Aquarium in Japan. They recorded eye movement using a digital video camera and used echo-sonograms to trace movements inside the head. The team also dissected two dead guitarfishes to study the arrangement of the eye muscles and used CT scanning to create a 3D reconstruction.
They discovered that the eye of the guitarfish can move a distance of 37.3 millimeters (1.5 inches) in just a few hundred microseconds. That’s 101% the diameter of the eyeball itself.
This eye retraction distance is among the largest known for vertebrates. Our eyeballs (as well as that of guinea pigs and rabbits) can retract at most 1.5 millimeters (0.06 inches) when we close our eyelids. Bottlenose dolphins can retract their eyeballs up to 15 millimeters (0.6 inches) into their orbits – or 60% of the eye diameter. Frogs can retract about half their eyeball diameter.
This achievement is thanks to the unique arrangement and morphology of the obliquus inferior eye muscle – which pulls the eyeball ventrally towards the animal’s underside. This muscle is highly specialized in guitarfishes, and it’s both larger and longer than that of sharks. Additionally, when the researchers electrically stimulated the obliquus inferior muscle of one of the dead specimens, they caused a 13 millimeter (0.5 inch) depression of the eyeball.
Eye retraction in the giant guitarfish. Total length = 280 centimeters (110 inches). Right eye in normal (i) and retracted (ii) positions. T. Tomita et al., Zoology 2015