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Sharks: perfectly constructed for predation

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Sharks have been around since the age of the dinosaurs and are perfectly evolved marine predators with no competition at the top of the food chain until humans arrived. Dr Vic Peddemors, from the NSW Department of Primary Industries, examines some important features of shark physiology that contribute to their long standing success.    
Most sharks are ram ventilators, swimming all the time with their mouth slightly ajar, directing the water over the gills. All bottom dwelling sharks, such as carpet sharks, wobbegongs and rays, also have a spiracle in the top of their heads and it is this hole that acts as the water inlet for the gills whilst they are lying on the seabed. Interestingly, whale sharks also have spiracles, a mark of their evolution from bottom dwellers.

Sharks have dual hearing, consisting of lateral lines running along the body and actual ears in the head.    
Left: Lateral line of a seven gill shark
The lateral line grooves contain tiny cells with minute hairs which move in relation to sound pressure waves. When the shark moves its tail, for instance, a wave is emitted which bounces back off any object it meets. This echo causes the hairs to move and the shark can determine its distance from that object.
Sharks also have tiny pin prick holes in the tops of their heads which are the external openings to their proper ears.

These are believed to be used for far field hearing and balance. When turned upside down, sharks go into tonic immobility so that is why scientists turn sharks up-side-down when tagging or performing surgery.
Sharks have several features that set them apart from bony fishes. They do not have scales but rather denticles which are small tooth-like structures, each one attached to a blood vessel and a nerve ending. This “skin” is not flat but is a series of ridges and grooves which cause vortices of water to flow along the side of the body, making their swimming motion perfectly slip-streamed. So effective is this, that NASA copied the design for use in the tiles on the outside of the space shuttle.
The controversial swimming costumes used in the last Olympic Games also had a similar concept.

Sharks have very good eyesight. Unlike humans who need time to adjust when moving from dark areas into light areas, sharks adjust instantaneously. They also have a reflective layer at the back of the eye, similar to that of cats, which is exposed when they enter very dark spaces. In this reflective layer are a whole lot of grooves within which sit tiny dark pigment cells called melanoblasts. These move over the reflective layer when the shark moves back towards the light and therefore act much like built-in sunglasses.

Above: Dissection of a shark eye shows the reflective layer (tapetum lucidum) that increases the sensitivity of the eye in low light conditions, plus the lens lying outside the eye.     
Most sharks also have a nictitating membrane which comes up from the bottom of the eye to cover the eye. This occurs when they are feeding and is a protective mechanism. These highly adaptive modifications further underlie the importance of eyesight in sharks.

A shark also has a good sense of smell. The olfactory organ has increased surface area much like an air filter in a car. Water comes in one side, goes over all the surface area and leaves on the other side.
Sharks do move along scent trails, working from side to side to find the source using these effective scent corridors.
Right: Shark nostril lamellae dissection
Sharks also have tastebuds on the base of their tongues and some species like the Great White have tastebuds at the bottom of their teeth.

It is believed, that these help the shark make a final decision about edibility of intended prey. It is thought that this is probably why Great Whites tend to spit humans out, preferring animals such as dolphins and seals who have a thick layer of rich blubber.

Sharks have a sixth sense, which is the ability to detect minute electrical pulses. The organ, called the ampullae of Lorenzini, sits inside a shark’s snout, the external openings of which are the pin prick pore holes all over the snout. A gel-like substance carries the pulse from the pore to the sensory organ. It is thought that these same ampullae can also detect electromagnetic fields which would help them navigate.

Above and right: Mako shark snout showing pores leading to the ampullae of Lorenzini
There are hundreds of species of sharks in the world’s oceans and new species are being found month by month, with scientists from the CSIRO at Hobart at the forefront of these discoveries. Curiously, these many different species of predators manage to live side by side without competing for food. This is because each species has a unique set of teeth so they catch different prey in different ways.

Shark teeth are also embedded in cartilage (there is no bone) and work their way to the front of the mouth like they're on a conveyor belt.

Sharks keep replacing their teeth throughout their lives. It is only the front row of teeth that is functional, the other rows originally being soft and hollow teeth, strengthening as they slowly move forward. These teeth are therefore in reserve when needed, such as if a shark loses a tooth when biting into something that is too hard.

There are a few myths that have been historically attributed to sharks. Shark fin soup gained a reputation as having an aphrodisiac and, while extremely difficult to obtain, was only served by the very wealthy in Far East Asia. Nowadays it is less expensive but the myth is still believed, even though the shark’s fin itself is relatively tasteless, contributing to the gelatinous structure of the soup. Another myth, this time in the USA, was that pills made from shark cartilage kept cancer at bay if taken daily. Such was the demand that the shark population of Costa Rica was wiped out. Indeed, the preservation of sharks is still their greatest need.

Images from Vic Peddemors
Text: V.B. April 2010

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