Shark Colors 63
thin surface tissue. Iridescent colors change depending on the direction of
incoming light and the angle at which a shark is viewed.
The Smalleye Hammerhead (Sphyrna tudes) of eastern South America is
sometimes called the Golden Hammerhead. Juveniles sport both structural
and pigment coloration that makes them gold. Orange and yellow pig-
ments in the skin cells add brightness to the color, which is also enhanced
by iridescent structural tissues. The result is that live animals have glow-
ing, metallic golden heads, sides, and fins that lose their glow upon death.
Even more interesting, the yellow and gold pigments are not produced by
the animal itself but apparently come from its diet, in particular, a kind of
shrimp and a marine catfish and its eggs. The Yellow Smoothhound (Mus-
telus higmani) feeds on the same shrimp and is also yellowish. Horn Sharks
in California are another example of shark “coloration” obtained from an
external source. Horn sharks feed heavily on purple sea urchins and often
have purplish-pink teeth and fin spines.
After death, the drying and shriveling of the skin alters the thin surface
tissues. Hence, iridescent colors quickly disappear in dead sharks. Pigment-
based colors fade more slowly as the pigment granules decay with time.
The fairly uniform yellowish-brown color (with some darker and lighter
areas) of sharks that have been dead for a while, such as those in museum
displays, doesn’t do justice to their colors in life.
One interesting twist on color change occurs in deepwater sharks like
the Velvet Belly Lantern Shark. Lantern sharks are uniformly dark but
have small light organs (photophores) on their bellies (see “Which sharks
aren’t countershaded?” above). Lantern Sharks change the amount of light
they emit in much the same way that shallow-water sharks change their
color by controlling melanin distribution—except that Lantern Sharks use
melanin to control light emission. On top of each photophore is a closeable
diaphragm filled with melanin that works like the iris diaphragm in our
eyes. The light stays on all the time, but the Lantern Shark contracts and
expands the melanin in the overlying diaphragm to allow light to shine out
or not.
Is there a reason for the color patterns of sharks?
Most fishes, either predator or prey, are colored in a way that makes
them difficult to see underwater. “Invisibility” is a good explanation for
the dominant color patterns of sharks, whether countershaded or bottom-
matching. Countershaded sharks blend into the background of the water
column, and mottled-back sharks and rays are camouflaged against the
background of the ocean floor. In benthic sharks, skin growths and folds
increase the resemblance to the bottom, as in the orectolobid wobbegongs.