Do sharks really have teeth on their eyeballs? Not quite - but one species does have tooth-like scales on its eyes. A paper published in 2020 reported that the eyeballs of the whale shark, the world's largest living fish, are each covered with thousands of dermal denticles.
These are essentially tiny, flat, modified teeth, similar to scales but covered in enamel. Found on the skin of sharks and rays, and on the nictitating membrane (translucent third eyelid) of some species, dermal denticles reduce drag and turbulence while swimming, and possibly also provide protection and deter ectoparasites.
The shape and structure of dermal denticles on whale-shark eyeballs are different from those on skin. It's thought that they protect the eyeballs from damage, which supports a hypothesis that vision is particularly important for whale sharks.
In the same paper, the authors reported that whale sharks are able to retract their eyeballs into their sockets - which some other animals, though no other sharks, can also do.
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Megan Shersby
