Sharks can be toothy right up to their eyeballs. At least whale sharks (like Destiny from Finding Dory, above) can, as researchers from the Okinawa Churaumi Aquarium in Okinawa, Japan, recently found out. They already have the freaky ability to retract their eyes into their sockets, like some other sharks, but whale sharks actually have micro-teeth called denticles right on their eyeballs. These protect its eyes from random debris floating around in the ocean. They also prove that this deceptively monstrous plankton feeder has vision sharper than any of its teeth.
“These highly protective features of the whale shark eye seem to emphasize the importance of vision for environmental perception, which contradicts the general, though poorly established, notion of low reliance on vision in this species,” the scientists said in a study recently published in PLOS One.
You would probably think that such a huge creature with such disproportionately tiny eyes wouldn't have evolved a need to see much. The whale shark is the largest known fish in the world. It can grow up to 40 feet and weigh up to 60 tons. Ironically, some of its best defensive weapons are tiny. This is the first time denticles have been found directly on the eyeball of any creature, though some sharks have a clear third lid covered in them. The morphology of the whale shark’s eye denticles differs from that of the dermal denticles found on the skin of many other elasomorphs, or fish with bodies of cartilage, though there has been evidence to suggest some of the functions overlap.
Having teeth on your eyes can be an advantage when you’re constantly swimming and need to resist the abrasion from drag somehow. Eyeballs in general are prone to all sorts of dangers that many animals have evolved a second clear eyelid for, from frogs and lizards to other species of fish. How elasmobranchs like sharks protect their eyes hasn’t really been investigated too much until now. The whale shark’s eyes are already on the sides of its head, and they also bulge out considerably, so that just ups the risk of them being scratched.
Both live whale sharks from the aquarium and a whale shark eye preserved in formalin were examined for the study. Rows upon rows of denticles—about 2,900 of them—surround the iris. This apparently doesn't cause the shark any grief when it retracts its eyes, because it can not only pull them back into its sockets, but roll them downward as it does. CT scans on the disembodied eye revealed how eye denticles, which look kind of like miniature oak leaves when magnified, have branched ridges that differ from the more evenly ridged denticles on the shark’s skin. What really fascinated the scientists was the similarity of the eye denticles of the whale shark to the dermal denticles of the horn shark, which suggested a similar purpose.
“Morphological similarity between eye denticles of the whale shark and body denticles of the horn shark supports our hypothesis that the main function of the eye denticle is mechanical protection,” they said.
Observing live sharks with a portable ultrasound device further caught their eyes retracting. Because water is better able to conduct ultrasound signals than air, the transducer didn’t need to even touch the shark. Imagine an angry shark that has just been poked in the eyeball.
The reputation of shark teeth won’t be going anywhere, but keep the eye teeth in the back of your mind if you ever want to scare someone.