Form and Function of Sharks 41
It has been speculated that such divers bumped into a torpedo ray, received
a strong shock, became disabled, and inhaled water.
Some skates (Rajidae) emit a weak electric discharge from specialized
organs in their long tails. The output differs among species and may help
identify conspecifics (members of the same species) during mating. Weak
electric discharges are unknown among any other marine fishes, bony or
cartilaginous.
Do sharks sleep?
We really don’t know whether sharks sleep. Sleep in other vertebrates,
including bony fishes, involves a reduction in activity and metabolism, and
a decreased sensitivity to external stimuli. Sharks that lie on the bottom
(see “Must a shark keep swimming to breathe?” below) are less active but
are still consuming energy. In fact, juvenile Lemon Sharks that lie on the
bottom and must pump water over their gills to breathe use about 10%
more energy than does a juvenile swimming slowly with water passing
over its gills simply because of its forward movement. However, Lemon
Sharks are usually active, unlike several species that do “rest” on the bot-
tom for extended periods (but still actively pump water in their mouths
and out their gills). This “resting” group includes Nurse, wobbegong, and
Whitetip Reef (Triaenodon obesus) sharks as well as angel sharks and many
batoid skates and stingrays.
The idea of sleeping sharks gained credibility with the discovery of nor-
mally active sharks lying on the bottom in caves off the Yucatan (Carib-
bean) coast of Mexico. These “sleeping sharks of the Yucatan” were mainly
Caribbean Reef Sharks (Carcharhinus perezi). Over the years, another four
species of sharks have been seen at this cave locale, including Tiger, Lemon,
and Bull. Low-salinity water flows out of the caves, and the sharks are re-
markably quiet and approachable. Why they are there and whether they
are sleeping remains a matter of conjecture.
Do sharks have scales like other fishes?
Sharks have scales but they are very different from the familiar scales
of bony fishes. Bony fish scales are flat, circular, relatively smooth, and
overlapping. Shark scales, called placoid denticles, have ridges and val-
leys and bumps, vary greatly in shape among species, and seldom overlap.
They can be close together or scattered almost randomly around the body.
Seen from the side, many shark scales have one or several pedestals, often
with a flattened top and sometimes with a recurved hook facing backward.
Form follows function: fast swimming sharks have scales that, because of