42 Sharks: The Animal Answer Guide
the ridges and flat-topped pedestal, decrease turbulence around the body
and thus serve as streamlining. In fact, the keels of high-performance sail-
boats mimic this design in their “winged keels,” a design adaptation that
led to Australia’s winning the America’s Cup sailboat races in 1983. (The
Ozzies’ winged keel innovation was a closely guarded secret.) Swimsuits
designed for competitive swimmers, such as the Speedo Fastskin, have
built-in ridges emulating a shark’s denticles to reduce drag across a swim-
mer’s body. Critics claim that these swimsuits have contributed to record-
breaking swim times, and such “performance-enhancing” suits have been
banned from some competitions.
The reduced turbulence created by a shark’s denticles also allows a
shark to swim through the water quietly, a definite advantage for a preda-
tor trying to sneak up on its prey. Hydrophones (underwater microphones)
detect much less noise from swimming sharks than from swimming bony
fishes. Not surprisingly, the scales of bottom-dwelling and slow-swimming
sharks (Basking Shark; Bramble Shark, Echinorhinus brucus; Thorny Skate,
Amblyraja radiata), as well as many rays, lack apparent streamlining features
and are instead enlarged for protection.
The roughness and toughness of shark scales have long been exploited
by humans. Dried shark skin was used as sandpaper before modern tech-
nology perfected the human-produced variety. Swimmers who get bumped
by sharks are often scraped and even cut because of the roughness of a
shark’s body.
Scales appear on different parts of a shark’s body in modified forms. You
could say that the scales extend along the body and into the mouth because
shark teeth are actually modified scales, growing from the same embryo-
logical structures. Another evolutionary modification of scales is the barb
of a stingray, which is a modified, elongated placoid scale with sawtooth
edges and a venom gland at its base.
Although scales and teeth arise from the same embryonic structures,
sharks do not shed old scales and grow new ones the way they grow new
teeth. However, if a shark loses a scale because of an injury, it can replace it.
New scales are obvious, having a clear appearance rather than the opaque
look of older scales.
How do sharks breathe under water?
Most selachian sharks breathe passively: as they swim, water enters their
mouth and passes over their gills, exiting through the five (or six or seven)
gill slits. This kind of breathing is often called “ram ventilation.” Oxygen
is taken in and carbon dioxide released at the thin membranes of the gill
filaments, where blood vessels are a few cell layers away from the outside
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