Basic Shark Physiology & External Anatomy

Skelatal Support

Sharks are cartilliginous fish unlike most fish they have no bones, this is not to say that they squidge around like jelly but instead of bone they have cartillidge. This is a fibrous tissue that can actually be quite hard. Because of this fact and because cartillidge decomposes, it is quite rare to find fossils of sharks.

Bouyancy

As well as a lack of bones, sharks also lack what most bony fish have, a swim bladder. Sharks rely on lift generated by their large dorsal fins in the same way an aeroplane's wings provide lift, however this lift would not be enough to support the shark on its own with the size fins they have (unless moving faster), a lot of the sharks' lift comes from its liver. Sharks have huge livers, these contain oil which is lighter than water and therefore this makes sharks slightly negatively bouyant.

Contrary to popular belief, sharks don't have to swim constantly or they die, however they do have to swim to avoid sinking tot he bottom. This liver as opposed to a swim bladder means that sharks' bodies are incompressible allowing them to move between very deep water and the surface with ease. Those bony fish that live at high pressures usually die when brought to the surface, sharks don't.

Respiration

Again, unlike bony fish, sharks do not have gill covers, most cannot pump water over their gills and must move to breathe. This is not strictly true as many sharks that were thought to need to move constantly have been observed lying motionless. Many sharks and rays can, however pass water over their own gills, these are usually the bottom dwelling sharks such as cat sharks, dogfish, port jackson sharks etc.

Sharks have two methods of reproduction. Basically some sharks lay eggs, usually in an egg pod. Most of the bigger sharks however give birth to live young, the young gestate inside the female shark, when they are born they are fully developed and shoot away fast or risk becoming food. In their early years they are prey to many animals, only a few will survive. Sharks have a long gestation period and do not reproduce frequently.

The external anatomy of sharks varies considerably between the approximately 370 known species. The skin of all sharks posess small dermal denticles known as placoid scales, which give the shark it rough, sometimes sandpaper-like texture. In fact, shark skin has been used in many cultures as a good substitute for sandpaper. You may notice that the scales look like small shark teeth. This is because that is exactly what they are! Actually, it is believed that the teeth of sharks evolved from modifications of scales, so I guess it would be better to say that shark teeth are actually scales! Shark skin is also very tough and pliable. When properly cured, shark skin has been made into fine leather goods such as purses, shoes, boots and wallets.

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