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Snorkelling with the Whale sharks off Coral Bay.
Photos courtesy of Kristin Anderson |
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The Biology and
Ecology of the whale shark
(Rhincodon typus)
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The following detailed description is by
courtesy of Geoff Taylor, a Western Australian Medical Practitioner
with an interest in the animals.
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The Whale shark is accompanied by many brightly coloured
fish - as well as snorkellers |
The world of fish is divided into the cartilaginous fishes,
Chondrichthyes, and the bony fish, Osteichthyes. Most of the fish of the reef with which
we are familiar are bony fish, covered in scales, with one gill slit on either side of the
head. Sharks, skates and rays are different; they are cartilaginous and belong to the
sub-class Elasmobranchii. There are three main features distinguishing the Elasmobranchs.
Their skeleton is made of cartilage, their skin does not have scales, but instead has hard
denticles making the skin extremely tough, and they have from five to seven gill slits on
each side of the head. Internally, there is another difference: the sharks do not have
swim bladders to give them buoyancy. The large oily liver of sharks is thought to help
compensate for this lack of an air-filled swimbladder.
There are more than 350 identified species of shark in the world. They come in all sorts
of shapes and sizes from the bizarre goblin sharks Mitsukurinidae, to the flat
bottom-dwelling angel sharks Squatinidae and wobbegongs Orectolobidae, to the fast
swimming predators with which most of us are familiar. One of the fiercest sharks in the
ocean is the tiny cookie cutter shark Isistius brasiliensis. Measuring only 500mm, it is
bold enough to attack some of the largest animals in the ocean such as the baleen whales.
With its razor sharp teeth it gouges the flesh from its victims. At the other end of the
biological scale, the largest shark in the ocean has tiny teeth which are vestigial, is no
threat to other large creatures, and feeds on some of the smallest creatures in the ocean,
the zooplankton. It is of course the Whale Shark.
Where does the whale shark fit into the world of sharks? The sharks are divided into eight
Orders according to their anatomical make-up. Such features as the presence or absence of
an anal fin, and the number of gill slits determines their classification. The whale shark
fits into the order of Orectolobiformes which have 5 gill slits, an anal fin, 2 dorsal
fins, no spines on the fins, and the mouth well in front of the eyes. The word literally
means "extended tail lobes", and some of the species in the order have very long
tails. They are generally known as the carpet sharks because they are bottom dwellers. The
Wobbegong sharks are also in this order.
There are certain features of the whale shark that distinguish it from other sharks. The
most obvious is the very broad head. Like other Orectolobiform sharks, it has a wide mouth
which is right at the front of the head, rather than being slung underneath the head as in
other species. The olfactory opening of the shark is just above the upper lip, hidden in a
nasal groove. There is a small barbel on the inside of the groove, reminiscent of another
carpetshark family, the Nurse sharks, Ginglymostomatidae. The eyes of the shark are small
and located behind the angle of the jaw. The shark closes its eye by rotating it and
sucking it back into its head.
Just behind the shark's eye is a round hole called the spiracle. This orifice is present
in many other sharks and rays, and is a vestigial first gill slit. The species with large
spiracles are generally bottom-dwellers, and rays sometimes respire through the spiracle
when resting on the bottom. It is particularly a feature of Carpet sharks. Behind the
spiracle are five large gill slits, the large size being a feature of filter feeders. The
gills essentially have two functions. Not only do they extract oxygen from the water, they
are equipped with fine sieves to filter out tiny creatures from the water. There are two
other distinctive features of the whale shark: it has three prominent ridges along each
side of the body, and its colouring with white spots, and stripes all along the body.
It has been placed into a family of its own - Rhincodontidae. While some might wonder at
the whale shark being grouped with the bottom-dwelling sharks, there are other carpet
sharks that have obvious similarities, though they are much smaller. The leopard shark
Stegostoma fasciatum is one. Not only does it have a large spiracle, a feature of carpet
sharks, but it also has the distinctive ridges along its body, and like the whale shark is
covered in spots: black spots on a yellow background. The Leopard shark is normally a
bottom-dwelling shark but like the whale shark at Ningaloo is sometimes seen basking on
the surface. The Nurse sharks Ginglymostomatidae also have many similarities, including a
nasal groove on the upper jaw, which in the Nurse shark has a prominent barbel. The nurse
shark is thought to have a similar pattern of reproduction, with similar shaped eggs to
the whale shark. (Some authorities have suggested that the Nurse sharks Ginglymostomatidae
and Leopard sharks Stegastomatidae should be included in the Rhiniodontidae family.)
The whale shark has been found in all the major oceans, but appears not to enter the
Mediterranean sea. It lives in the warm waters of tropical and sub-tropical seas. Its
range extends further polewards on the east coast of continents where warm tropical water
is carried away from the tropics by ocean currents. Thus, on the east coast of Australia
the whale shark has been seen as far south as Montague Island south of the 36th parallel
of latitude, and Wolfson records a sighting from Point Hicks in Victoria, 38° south .
Similarly the sharks have been seen off Cape Town in South Africa. In the northern
hemisphere whale sharks are carried north in the warm Kuroshio current to the islands of
southern Japan, and the range is extended north by the Gulf stream on the east coast of
America. In Western Australia it has been sighted as far south as the Kalbarri cliffs,
latitude 28° South.
It is not known how far whale sharks migrate. At one time it was suggested that they might
undertake huge migrations, breeding in the Indian Ocean, and drifting south in the
Mozambique current around Cape Horn into the Atlantic, where they might be carried across
the Atlantic in the Southern equatorial Current, reaching the Caribbean. (Gudger, 1931).
Our studies at Ningaloo would not support this concept, as we have been able to recognise
sharks from one year to the next.
The whale shark is said to prefer waters of surface temperature 21-26 degrees centigrade,
in areas where there are upwellings of nutrient rich colder water. These conditions favour
blooms of plankton on which the sharks feed. They are often seen with schools of pelagic
fish, especially trevally (Jacks).
It is one of three species of shark that are filter feeders - they are all massive sharks.
The Basking shark Cetorhinus maximus is a gray coloured shark with a pointed nose that
lives in more temperate seas. It grows to a length of 9 metres and has very large gill
slits. A fishery for the sharks exists on the west coast of Scotland. The basking shark
cruises at the surface with mouth gaping wide filtering the water with its gill rakers.
The megamouth shark Megachasma pelagios is an elusive creature that is only known because
of dead specimens periodically washed up on the shore. A specimen was washed up at
Mandurah, Western Australia in 1989. It grows to at least 4.5 meters. It is thought to be
a filter feeder but has larger teeth than the other two species, and does not have the
huge gill slits of the others.
The dimensions of a really large whale shark are awe inspiring. The largest specimen ever
measured accurately was caught by accident in 1983 off Bombay, in a gill net by a 20 foot
boat. For a while the crew were terrified as they were towed around the ocean by the
shark, which was more than twice the length of the boat. The 12.18 meter male specimen
weighing in at 11 tonnes. The mouth was 1.36 meters wide. The dorsal fin was 1.37
meters
(over four foot) high. The pectoral fins were over 2 meters long. An even larger specimen
estimated at 18 meters in length was reported from the Gulf of Siam in 1925, but may have
been overestimated - see chapter 3. This specimen was larger than many of the great
whales.
Although the largest member of the shark family, the whale shark has very small teeth, for
its size. This distinguishes it from the predatory sharks which are renowned for their
large, sharp teeth. Fossils found on old coral reefs at Ningaloo show that these same
waters were once the home of a predatory monster the same size as the whale shark.
Carcharodon megalodon probably became extinct about 12,000 years ago. Its teeth measured
over 15 cm (6 in) in length, twice the size of the great white shark - a formidable
predator indeed. The whale shark rarely uses its six thousand teeth which are arranged in
11-12 rows on each jaw, each tooth being only 2 mm long and angled in towards the mouth.
The appearance is like a large rasp on each jaw - hence the name of the shark: Rhineodon,
the original spelling, means "rasp-tooth". It is thought that the harsh noise
sometimes heard by pearl fishers when the shark is close by, could be produced by grinding
these teeth together.
The whale shark feeds by filtering small planktonic organisms out of the water through its
5 gill slits which have fine gill rakers. The gill slits are relatively large, reflecting
its method of feeding - only the basking shark has bigger gills. It is remarkable that the
sharks are able to filter prey that are only one mm in diameter. When actively feeding the
gills flare out as the shark pumps large volumes of water through the gills at speed.
There has been some confusion over the feeding of the whale shark and at Ningaloo it has
been seen feeding in different ways. Traditionally the whale shark is described as a
suction filter feeder. By suddenly opening its mouth it is able to draw a large amount of
water inside, which is then expelled through the gill slits. In the normal process of
respiration the whale shark rhythmically opens its mouth about 200mm. When feeding it is
able to open it 2-3 times as wide. Sometimes the sharks feed passively, cruising along
beneath the surface with mouth agape, feeding in the same manner as the basking shark. At
other times it feeds much more actively charging around on the surface as when attacking a
swarm of Krill.
The internal organs of the whale shark have rarely been examined by marine biologists, as
most of the specimens that have come to scientific attention have rapidly putrefied. The
shark is said to have a small gullet, and while it might be able to take large objects
into its mouth it would be unable to swallow them. The liver of the shark is huge, and in
a twelve meter shark the liver alone weighs 1 tonne, 9% of the total weight of the shark.
Extracts of the liver of the whale shark are reported to have strong anti-tumour activity.
The skin on the back of the whale shark is thicker and tougher than any species in the
world. The outer layer is covered in overlapping dermal denticles, a veritable suit of
armour. Each denticle is 0.5 mm wide, and 0.75mm long. The point of each denticle projects
backwards along the body. Underneath is a layer of connective tissue that is up to 140mm
thick. The belly of the shark is much softer and the thickness of the connective tissue
here is only 2/3 of the thickness on the back of the shark. It is interesting
that when swimming past divers, the sharks often "bank" onto their sides to
present their tough dorsal skin to the diver, rather than their soft underbelly. It is
said that sharks have extraordinary powers of healing. We have evidence of this at
Ningaloo. One shark filmed in 1986 had two deep gashes down one side of the body. It was
easily identifiable because of a "shark bite" out of the left pectoral fin,
scars on its flank and the lateral markings. When re sighted in 1993 the two gashes had
completely healed without any scar whatsoever.
The eye of the whale shark is relatively small for a creature that spends much of its life
in the dark depths of the ocean, suggesting that eyesight is not an important sense. It
would be hard for the shark to locate its tiny prey with eyesight alone. The shark does
not have any eyelids, but is able to withdraw the eye into the head, rotating it as it
does so, as a protective measure.
Some sharks have an extremely sensitive sense of smell, and studies have been made of
chemicals that might attract or repel sharks. Sharks often forage and feed at night when
vision is of little use, and the ability to detect chemicals in the water is thought to be
of paramount importance in location of prey. The nasal grooves above the mouth of sharks
such as the whale shark allow a continuous stream of water to flow past the olfactory
apparatus.
Probably the most important sense in the shark is the puzzling sixth sense. Like other
species of shark, the whale shark is equipped with nerve endings that can detect
electro-magnetic fields in the water and they can locate prey even in the dark. Along both
sides of the shark are a row of small pores which communicate with the lateral nervous
system. These openings are known as the Ampullae of Lorenzini. We have plotted these pores
on the whale sharks at Ningaloo |
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