World's Biggest Shark Has Teeth on Its Eyeballs, Scientists Discover

The biggest sharks in the world have tiny teeth all over their eyeballs, scientists have discovered. In analyzing the eyes of whale sharks, which can reach up to 60 feet in length, researchers found them to be covered in "dermal denticles" that help protect them from damage.

Dermal denticles are known to cover shark skin. They are tiny v-shaped scales that are structurally minute teeth. According to the Smithsonian Institution's Ocean Portal, shark skin is covered in dermal denticles to decrease turbulence and drag, helping them to swim quieter and faster.

In a study published in the journal PLOS One, researchers led by Taketeru Tomita, from the Okinawa Churashima Research Center, Japan, found dermal denticles on the eyes of whale sharks—a filter-feeding species known for its unique spotted patterned skin.

Researchers say that many species have eyelids to protect their eyeballs. Some sharks have "third eyelids" that cover their eyes during feeding, the outer surface of which is covered with dermal denticles. Other species, they say, instead have retractable eyeballs.

Whale sharks have eyes that project out of their obit—a feature that could result in an increased risk of injury. They do not have eyelids, and the only known protection mechanism is that the species can rotate "the entire eyeball back into the eye socket," the researchers wrote.

Studying large ocean creatures like whale sharks has traditionally been difficult as a result of their small population size. In the latest study, the team took advantage of the recent care of whale sharks in aquariums, along with dead specimens. They used a range of techniques to examine their eye protection morphology and compared them to other shark species. Findings showed whale sharks have unique "armored eyes."

Its teeth-covered eyeballs, the team say, are a novel form of eye protection among vertebrates. "The eye denticle differs in morphology from that of the dermal denticles distributed over the rest of the body," they wrote, saying they are for abrasion resistance, rather than speed and noise reduction.

"As far as we know, eye denticles have not been found in other elasmobranchs [sharks, rays, and skates], including species closely related to the whale shark," they wrote. "It seems likely, therefore, that eye denticles are a characteristic unique to the whale shark."

Their findings also show whale sharks are able to partially retract and rotate their eyeballs, offering them additional protection.

Previously, it was thought whale sharks depend very little on their eyesight compared with other senses. These newly identified features suggest otherwise, the team said. They say future research should look at other aspects of vision, including color range, visual field and sensitivity.

whale shark
Representative image of a whale shark. Scientists have found the species has tiny teeth all over its eyeballs. iStock