Video courtesy of Mariana Tombini
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