Physical characteristics of sharks

The physical characteristics of sharks are different from those of bony fish, but the large number of species and the diversity of shark habitats means that there are many variations on the "typical" shark body.TOCleft

Skeleton

The skeleton of a shark is mainly cartilage. The sharks have at least 230 bones.

Respiration

Like other fish, sharks extract oxygen from seawater as it passes over their gills. Some sharks have a modified slit called a spiracle located just behind the eye, which is used in respiration. Due to their size and the nature of their metabolism, sharks have a higher demand for oxygen than most fish and they can not rely on ambient water currents to provide an adequate supply of oxygenated water. If a shark were to stop swimming, the water circulation would drop below the level necessary for respiration and the animal could suffocate. The process of ensuring an adequate flow of the gills by forward movement is known as ram ventilation. Some sharks, such as the blacktip reef shark, "Carcharhinus melanopterus", and nurse shark, "Ginglymostoma cirratum", can pump water over their gills as they rest. There are also recorded instances, as in certain caves along the Yucatan coast, where sharks rest on the cave floors and allow the fresh water outflow to pass over them. The outflow is strong enough to still allow for respiration; it is believed that the reason for this behaviour is that the fresh water helps remove parasites. [The Undersea World of Jacques Cousteau Special, "The Sleeping Sharks of the Yucatan," NBC.]