Form and Function of Sharks 43
water. Chimaeras also swim actively most of the time and breathe in this
manner.
Sharks that lie on the bottom (Nurse, wobbegongs, Whitetip Reef),
many skates (including sawfishes), and rays must actively pump water into
their mouths and out their gills. In many such benthic (bottom-living) elas-
mobranchs, water is also sucked in via the spiracles, which are two holes on
the top surface of the head that are derived from structures associated with
the first functional gill arch.
Must a shark keep swimming to breathe?
Sharks that stay up in the water all the time must keep swimming not
only to breathe but also to keep from sinking. Details on breathing are
given in the answer above. But in many sharks the tight link between swim-
ming and breathing goes a step further. In sharks that must keep swimming
to breathe, the contraction of their swimming muscles also helps keep
blood flowing through their arteries and veins. These sharks have relatively
small hearts, or at least hearts too small to pump sufficient oxygen-carrying
blood without the added “muscle pump” provided by swimming.
How do sharks swim?
The manner in which sharks and their relatives move through the dense
medium of water is a fascinating topic worthy of some detailed exploration.
Most sharks swim by passing S-waves of muscle contraction down their
flexible bodies, starting at the head, increasing in amplitude (side-to-side
extent), and ending at the tail. This snakelike movement of waves along the
body limits how fast most sharks can go. A few of the faster sharks—such as
the lamnid White, Salmon, Porbeagle, and makos—keep the front part of
their bodies rigid and sweep their tails back and forth rapidly. The fastest
shark is the Shortfin Mako, which can swim 32 kph (20 mph). Makos feed
on fast-swimming fishes such as tunas that swim in a similar fashion. Wind-
up bathtub shark toys and the helium-filled, radio-controlled “air swim-
mers” (www.airswimmers.com), with their stiff bodies and flapping tails,
actually mimic this type of swimming quite well.
A great deal of effort, and no small amount of controversy, has gone
into the study of shark swimming. Sharks lack the gas-filled swim blad-
ders that make bony fishes neutrally buoyant in the water column, allow-
ing bony fishes to maintain depth without sinking or rising. Sharks instead
have oily livers that provide buoyancy, but the buoyancy provided by these
organs is not enough to make up for all the weight of the muscle and car-
tilage. As a result, sharks tend to sink, albeit slowly, if they aren’t moving