Chondrichthyes (cartilaginous fish,
including sharks, rays and chaemeras) >
Viscera, buoyancy, swimming and respiration in cartilaginous
fish
Cartilaginous fishes have characteristically
short guts, with a baglike stomach and spindle-shaped intestine with
an internal spiral valve that increases its inside surface. The
liver is generally very large and oily in cartilaginous fishes, and
is their prime energy storage and hydrostatic organ. Some coastal
sharks have liver oil saturated with vitamin A, while deepwater
sharks and chimaeras have livers densely charged with a long-chain
hydrocarbon molecule, squalene, which forms a light, viscous oil. In
many deepwater sharks the body cavity is often greatly elongated to
accomodate the huge, oily liver, which reduces the specific gravity
of the shark to neutral buoyancy, so that it can swim and hover off
the bottom without having to generate lift from its fins and body.
Some deep-water chimaeras have large oily livers, compacted in their
relatively short body cavities. No cartilaginous fishes have
hydrostatic swim bladders like bony fishes, but the
Spotted
raggedtooth shark (Carcharias taurus) gulps air and uses its stomach
like a swim bladder to attain neutral buoyancy. It can hover
motionless in midwater. Some pelagic sharks require only slight lift
from foward motion to attain neutral buoyancy, but some
bottom-dwelling sharks and rays have well-calcified skeletons and
smallish livers. These have strong negative buoyancy and sink to the
bottom when not swimming.
Sharks and some rays use their strong tails and
caudal fins to swim with, but
skates and
stingrays, with reduced
tails and large pectoral disks, use their pectoral fins as
propellers, and can achieve fine control by differential undulation
of these fins. Chimaeras move slowly over the bottom, propelling
themselves with their fanlike pectoral fins. Some bottom sharks such
as the
bullhead sharks (family Heterodontidae) and various
carpet
sharks (order Orectolobiformes) have muscular pectoral and pelvic
fins, and use them to walk on rocks and coral. Other sharks and rays
swim in midwater and at the surface, and the acme of their prowess
is seen in highly advanced types such as the
mackerel sharks (family
Lamnidae) and devil rays (family Mobulidae), which are extremely
active, cruise for long distances, and can jump high out of the
water. The mackerel sharks have specialized circulatory systems that
allow accumulation of heat in their bodies, so that they are
effectively warm-blooded and can produce greater power from their
muscles.
A pervasive legend has it that some sharks must
continuously swim to respire, and are dependent on the flow of water
through their gills as they swim. We are unsure how this legend
originated, but note that there are many sharks that can sit
motionless on the bottom or hover in midwater and actively pump
water through their gills. This includes large
requiem sharks
(family Carcharhinidae) such as the
Tiger shark (Galeocerdo cuvier),
which have been observed `sleeping' in caves on the bottom. Some
sharks routinely pump water through their gills as they swim, and it
is uncertain if any sharks have abandoned gill-pumping and
exclusively use the passive flow of water for respiration.
Text by Leonard J.V. Compagno, David A. Ebert
and Malcolm J. Smale
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