6
Sharks
SENSORY PERCEPTION
Contact
& Taste
Ampullae
(< .1 m)
Vision
(< 100 m)
Lateral Line
(100 m)
Olfaction
(< 1,000 m)
Sound
(> 1,000 m)
Figure 8: Range of a shark's senses
(i) Vision: The eyes of a shark are very similar to those of a human having a cornea, iris,
lens and retina. The shark's retina contains sensory cells sensitive to both low intensity
light and colour. The shark also possesses a reflective layer behind the retina called the
tapetum lucidum. This structure also found in the eyes of a cat, enhances the shark's
ability to see in extremely low light. In fact, research has shown that the eyesight of a
shark is ten times more sensitive to light than human eyesight. However, although the
shark's vision is very good, it is often limited by the clarity of the surrounding water.
7
Sharks
(ii) Hearing: Sounds produced underwater can be detected by a shark up to 2.5
kilometres away. Consequently, sound is often the first stimuli that reaches the shark and
they rely heavily on hearing to accurately assess their surrounding environment. Sounds
are detected primarily by the shark's internal ears, located on either side of the head.
To perceive stimuli such as low frequency vibrations generated by an injured fish, the
shark has evolved a highly specialized organ called the lateral line. The lateral line is a
unique system of sensory cells designed to detect low frequency sound waves and water
movement. These cells, called neuromasts, are arranged in a canal along each side of
the shark's body, just beneath the surface of the skin. The canals are connected to the
surface by a series of small tubules or pores. Disturbances in the water are detected by
the neuromasts and transmitted to the central nervous system. The functions of the
lateral line include: (1) orientation; (2) the detection of low frequency sound waves; (3) the
detection of potential prey; (4) the detection of changes in pressure. A modification of the
lateral line, called the ampullae of lorenzini, surround the head and snout of the shark
and are capable of detecting minute electric fields.
(iii) Smell and Taste: Sharks have an extremely keen sense of smell. Research has
shown that the Blacktip shark (Carcharhinus limbatus) is capable of detecting 1 millilitre of
tuna blood in 10 million litres of water, a ratio of 1 part per 10 billion (10,000,000,000).
The sensory cells responsible for detecting odours are located on fleshy flaps of skin
inside the nostrils called the schneider folds.
The nostrils of the shark are located on the underside of the snout, in front of the mouth.
The location of the nostrils and their unusual shape allow water to continually flow across
the schneider folds, helping to identify elusive odours. Once a shark encounters an
interesting smell, it starts to swim back and forth seeking the highest concentration of the
odour, which allows it to follow the scent trail to its source.
Having located its prey, the shark relies heavily upon taste to assess its quality. Taste
buds are present in the mouth and throat of the shark and are ultimately responsible for
final acceptance or rejection of the food.
8
Sharks
BOUYANCY
The body of a shark is heavier than water; consequently a shark would sink to the sea
floor if it remained stationary. Unlike bony fishes, sharks have no swim bladder to assist
flotation and they have evolved many techniques to remain neutrally bouyant. The three
major forms of buoyancy utilized by sharks are:
(i) a skeleton composed entirely of cartilage, which is lighter and more flexible than
bone;
(ii) a greatly enlarged liver containing squalene, an oil much lighter than the surrounding
water;
(iii) rigid pectoral fins which act as hydrofoils, giving the shark lift as it moves forward.
The Grey Nurse shark (Carcharius taurus) has developed an additional method of
buoyancy control which is quite unique. This shark swims to the surface and swallows air
which it stores in its stomach. This allows it to maintain neutral buoyancy at any depth,
even while remaining stationary.
SHARKS and MAN
Man's first encounter with the shark gave rise to horrific tales of sea monsters reaching
unbelievable proportions. These creatures were reported to have dragged ships to the
sea floor and devour the crew as they struggled to the surface. Today, people realise that
'sea monsters' do not exist but many still fear these graceful hunters of the deep. In
reality, out of approximately 370 species of shark, only five are considered dangerous to
humans. They are the White Pointer (Charcaradon carcharias), the Tiger shark
(Galeocerdo cuvier), the Mako (Isurus spp.), the Whaler (Charcharinus spp.) and the
Hammerhead (Sphyrna spp.). Fortunately, these sharks do not normally hunt human
beings.
Attacks are generally the result of a shark mistaking a person for its natural food source,
defending its territory from intruders or investigating an unfamiliar object. The term 'maneater' is a fallacy. Less than 100 people throughout the world are attacked by sharks
annually. No species of animal could survive on such a small supply of food. Research
has shown that the chances of being killed by a shark are approximately 300 million to
one. It is possible to reduce these chances further by following these simple precautions:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
If possible swim in areas where shark meshing is employed.
Avoid murky water.
Avoid excessively deep water.
Do not swim at dusk, dawn or after dark.
Do not swim where people are fishing.
Do not swim with dogs or other domestic animals.
Do not swim near trawlers or where fish have been 'cleaned'.
If a shark is in the vicinity, leave the water.
9
Sharks
Throughout history we have utilized sharks for many purposes (see below). Although
sharks have often been killed for these products, a much larger number are caught as bycatch of other fishing industries and their unwanted carcasses discarded into the sea.
Compared to the 30 humans killed by sharks each year, we kill over 100 million sharks
annually. As sharks reach a reproductive age slowly and only reproduce a small number
of offspring, we are jeopardising the existence of many shark species.
Some of the more enlightened authorities have protected species such as the Grey Nurse
shark (Charcarias taurus) in New South Wales and the White Pointer (Carcharadon
carcharias). We must cease the mistreatment of these misunderstood animals before we
drive them to the edge of extinction. The loss of these magnificent creatures would have a
disastrous effect upon marine ecology as they play a vital role removing sick and injured
and dead animals from the ecosystem. It would be extremely ironic if we were to destroy
the shark only to discover that its cartilage could produce a cure for the disease cancer
which is responsible for killing thousands of people every year.
Table 1: Some of the products obtained from sharks:
Anatomy
Eyes
Fins
Flesh
Skin
Jaws and Teeth
Cartilage
Liver
Product / use
cornea transplant
shark fin soup
food and fertilizer
leather and sandpaper
weapons and ornaments
burn treatment and cancer research
Vitamin A, lubricants and cosmetics