Physical Characteristics of Sharks

Skeleton

Shark skeletons are very different from that of bony fish. Sharks and other cartilaginous fish (such as skates and rays) have skeletons made from cartilage and connective tissue. Cartilage is flexible, durable, and has about half the density of bone. This reduces the skeleton’s weight and also saves energy. Sharks have no rib cage and therefore on land a shark's own weight can literally crush it.

Jaw

The shark's jaw is not attached to the cranium. The jaw's surface needs extra support due to its heavier exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic. This gives these areas much of the same strength found in the real bony tissue found in other animals. Generally there is only one layer of tesserae in sharks, but the jaws of large specimens (such as the bull shark, tiger shark and the great white shark) have two to three layers or more, depending on body size. In the snout, the cartilage can be spongy and flexible to absorb the power of impacts.

Teeth

The teeth of sharks are embedded in the gums rather than directly fixed to the jaw. They are constantly replaced throughout the shark's life. Multiple rows of replacement teeth are grown in a groove on the inside of the jaw and moved forward in a "conveyor belt". Some sharks lose up to 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8–10 days to several months. In most species, teeth are replaced one at a time. The shape of a shark's tooth depends on its diet. Those that feed on mollusks and crustaceans have dense flattened teeth for crushin, those that feed on fish have needle-like teeth for gripping and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting.

Fins

The fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia. Sharks can only drift away from objects directly in front of them because their fins do not allow them to swim backwards

Skin

Sharks' dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming. These work as an outer skeleton, providing attachment for their swimming muscles and also saving energy.

Tails

Sharks have very distinctive tails. The tails of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. They possess a heterocercal caudal fin in which the dorsal portion is usually larger than the ventral portion. This is due to the fact that the shark's vertebral column extends into that dorsal portion. This is in contrast to most bony fishes, which possess a homocercal caudal fin