Physiology of Finfish and Shellfish (2+1)

2.1.11. Cartilaginous fish

Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Gas exchange occurs at the gills and oxygenated water must always be flowing over the gill filaments for respiration to occur. Water enters through the mouth of the animal, into the pharynx, over the gills and exits through the gill slits. Respiratory gas exchange takes place on the surface of the gill filaments as the water passes over and out the gills.

Adjacent slits are separated by a cartilaginous gill arch from which projects a long sheet-like septum, partly supported by a further piece of cartilage called the gill ray. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water.

A smaller opening, the spiracle , lies in front of the first gill slit . This bears a small pseudobranch (that resembles a gill in structure , but only receives blood already oxygenated by the true gills.

(The spiracle is a vestigial first gill slit. It appears as an opening behind the eye, as in the spiny dogfish photo below. It is absent or reduced in many sharks, especially the fast swimming sharks and is usually larger and present in sedentary or bottom dwelling sharks. The spiracle in sharks is used to provide oxygenated blood directly to the eye and brain through a separate blood vessel. In the rays, the spiracle is much larger and more developed and is used to actively pump water over the gills to allow the ray to breathe while buried in the sand).

Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.

(Ram ventilation The production of respiratory flow in some fish in which, the mouth is opened during swimming, such that water flows through the mouth and across the gills. In fish, which have a reduced or no ability to pump water buccally, such as mackerel and sharks, perpetual swimming is required to maintain ventilation).