Form and Function of Sharks 37
eyeball backward when attacking active prey. This moves the more delicate
parts of the eye out of harm’s way, exposing the tougher back of the eyeball,
called the sclera, to flailing seals or surfers.
An additional feature behind the retina is a layer of reflective crystals
called the tapetum lucidum (literally, “bright carpet”). The tapetum is re-
sponsible for the “eye glow” that occurs when a light is shone at a shark,
a cat, or a crocodile at night. Many nocturnally active animals have a ta-
petum, which acts as a light amplifier. Light rays pass through the retina,
activating the receptor cells. The tapetum reflects the light that wasn’t cap-
tured in the retina back through the retina, stimulating additional rod cells
on the way back out. (The eye shine we see is the tapetum-reflected light
that wasn’t captured moving in either direction.) A tapetum therefore max-
imizes the amount of light captured. As a result, sharks are about 10 times
more sensitive to light than humans, and even twice as sensitive as a cat.
Sharks tend to have larger eyes than skates and rays, and sharks that live
in the open ocean closer to the surface have larger eyes relative to body size
than sharks that live in the deeper, darker waters, where vision would be ex-
pected to be less important because of the comparative lack of light. Sharks
that feed on active prey (fishes, squids) also tend to have larger eyes than
more sluggish benthic feeders that prey on less mobile prey such as crabs
or clams. Chimaeras have even larger eyes relative to their body size than
sharks, skates, or rays. Their large, bulging eyes (and buck teeth) are what
gave them the alternative name of rabbitfish.
Can sharks see color?
The question of whether sharks see color has intrigued scientists for
decades. Testing for color vision is more difficult than it would seem. Some
animals that have visual structures that should allow them to see color—
namely, the right kinds of cells in their retinas—do not respond behavior-
ally to different colors (cats are an example). Sharks respond to different
colors, but the difference might be to brightness and contrast rather than
to color.
Recent advances in microscope types appear to have finally answered
the question: sharks cannot see colors, but rays and chimaeras can. Tests
on the retinal cells of 17 shark species, including Tiger and Bull sharks,
indicate they lack the necessary cell types. These species possess cones,
but to see color an animal needs more than one cone type, with each type
most sensitive to different wavelengths of the visible light spectrum. The
sharks tested either possessed only rods or had only one cone type. This
means they can’t see color but instead are sensitive to brightness and con-