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 

  Iziko Museums of Cape Town, 2008

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