64 Sharks: The Animal Answer Guide
If sharks are so well camouflaged by countershading, you can reason-
ably ask why photographs of sharks and rays often make them seem quite
visible. The answer is that photos are usually taken with artificial lighting
distributed in unnatural ways, at least as far as the evolution of shark col-
oration is concerned. Underwater photographers usually use strobe lights,
which shine or flash very bright light on the side of a shark and thus il-
luminate it unnaturally, and conspicuously. The distribution of light on a
dock or boat deck also misrepresents the real visual world of sharks. We
live in a terrestrial world where light is more evenly distributed above and
to the sides, and is often brighter from the side than from above. Underwa-
ter light, by comparison, is predominantly (95%) downwelling, with only
about 4% of light coming from the side. Countershaded sharks are colored
to match this light distribution and thus become invisible (see “Why are so
many sharks dark on top and light on the bottom?” above). Add to this the
fact that photographers seldom take pictures of well-camouflaged sharks,
the ones that you can’t see. Or at least those photos don’t make it into
books.
What color are a shark’s eyes?
Eye color in sharks and all vertebrates is primarily a function of struc-
tural colors in the iris, the area surrounding the pupil. The iris of many
sharks (Tiger, White, Porbeagle, makos) is large and black, or whitish
(many carcharhinids), although a few species (Milk Shark, Rhizoprionodon
acutus) have a yellow iris, and others (Leopard Shark) have a lighter, gray
iris around a black pupil.
The pupil itself in most sharks is black and round (sixgill sharks, Blue
Shark, White Shark, hammerheads, Sand Tiger), although some sharks
have a vertical, slit-like pupil (Lemon Shark, Blacktip Reef Shark, Silky
Shark), and in others the slit is horizontal or nearly so (some heterodon-
tid bullhead sharks, squatinid angel sharks, Leopard Shark, Small-Spotted
Catshark). Exceptions to black pupils are found in several deep-sea sharks
that have large green pupils, such as centrophorid gulper sharks, etmop-
terid lanternsharks, and somniosid dogfish and sleeper sharks.
The “eye color” of some batoids is confusing because the pupil may be
covered by a variously pigmented curtain called the operculum pupillare
(literally “pupil cover”), an eyelid-like finger of tissue that expands over
the eye in bright light conditions. Examples include the Banded Guitarfish
(Zapteryx exasperata), many skates, sawfishes (Pristidae), and dasyatid sting-
rays. The highly reflective eyes of holocephalan chimaeras generally have a
dark pupil surrounded by a lighter iris.
http://www.ebook3000.com