Form and Function of Sharks 47
How do a shark’s fins function during swimming?
While pectoral fins are providing lift and some turning ability as ex-
plained above, the propulsive force for swimming comes from the dorsal
fins and tail. Most sharks have two dorsal fins, the first usually larger than
the second, separated by a considerable gap. The upper lobe of the asym-
metric tail can be thought of as a third fin in line with the dorsal fins, again
separated from the second fin by a considerable gap. The distances be-
tween the three fins are apparently determined by the size of the fins, their
shapes, and the form of the muscular waves passing from head to tail of the
swimming shark. Each fin tapers posteriorly, leaving behind it a wake as
it moves through the water. This wake is displaced laterally by the waves
passing down the fish, so the wake itself follows a snaking path that moves
posteriorly as the fish moves through the water.
Calculations of the timing differences and wave nature of the wake sug-
gest an ideal distance between fins that would maximize the thrust of the
second dorsal and particularly of the tail. If timed correctly, the trailing fins
can push against water coming toward them from the leading fins. Such
an interaction between flows would increase the thrust produced by each
trailing fin. Measurements of swimming motions and fin spacing in six spe-
cies of sharks indicate that this is exactly what happens. Unlike bony fishes,
which use their foldable dorsal fins primarily to speed up or brake, or fold
them to reduce drag while cruising, sharks use their dorsal and tail fins as
additional interacting thrusters.
Sharks have taken the relatively inefficient eel-like swimming mode im-
posed by their flexible bodies and combined elastic skin, their rigid but
carefully spaced median fins, and an asymmetric tail that produces a con-
stant direction of thrust to achieve an efficient compromise between cruis-
ing, acceleration, and maneuverability.
How long can a shark live out of water?
An interesting question, one we hope few people will try to answer
experimentally. The shark literature contains statements such as “There
appear to be few examples of hypoxia-tolerant elasmobranchs,” meaning
few sharks can tolerate low oxygen conditions. Available evidence suggests
that the few sharks best able to survive periods of exposure in air are those
that live in shallow areas such as tidepools, where they may get stranded
when the tide goes out. These include the Brownbanded Bamboo Shark
(Chiloscyllium punctatum) and the Blind Shark (Brachaelurus waddi), both of
which are reported to survive 12 to 18 hours of air exposure. Blind Sharks
get their name from their habit of closing their thick lower eyelids when in