There are over 460 known species of sharks, ranging in size from the gigantic whale shark to the miniature dwarf shark, and they come in all manner of shapes, each adapted to its own peculiar niche in the sea. There are, for instance, flat-packed angelsharks that hug the sea bed, torpedo-bodied blue sharks that wander the open ocean, weird-looking goblin sharks with long, pointed snouts that dwell in the deep sea, eel-shaped frilled sharks, wobbegongs that resemble seaweed-encrusted rocks, highly-manoeuvrable hammerheads with bizarre-shaped heads, megamouths with cavernous mouths and thick lips, deep-sea sharks that glow in the dark and thresher sharks with amazingly long, scythe-shaped tails.
The partly open mouth of this raggedtooth shark seems to be overflowing with sharp, pointed teeth that it uses to grasp slippery squid and fish. (Image: Dan Burton, www.underwaterimages.co.uk)
The 'business' end of the shark is its head, with powerful jaws and row upon row of formidable teeth. The mouth of most sharks is situated on the underside of the head and can be thrust forward when biting prey. For example, the lemon shark is an active hunter capable of rapid acceleration. When approaching its target at speed, it brakes with its pectoral fins, raises its snout, drops its lower jaw, protrudes its upper jaw and teeth, and then jabs forwards several times to get a good grip. The protruding upper jaw retracts under the head, pulling the prey into the mouth. It then rips and tears the flesh of its victim by shaking its head from side to side.
Tooth shape varies with diet. The shortfin mako shark and sand tiger shark have sharp, pointed teeth, with which they grab slippery fish and squid. The tiger shark has serrated teeth, resembling those in a chain saw. It can slice through flesh and bone, and even the tough carapaces of sea turtles. The tooth shape of the great white shark changes with age. When young, this formidable predator has pointed teeth for grabbing fish, but on maturing, it develops triangular, saw-like teeth in the upper jaw for carving through the flesh of sea mammals, and awl-like, grasping teeth in the lower jaw for holding its prey steady--rather like the knife-and-fork combination.
Sharks are highly efficient swimmers. The streamlined shortfin mako has been seen to leave the water at speeds estimated to be in excess of 75 km/hour (46mph). It achieves this remarkable feat partly with a hydrodynamic body shape and powerful swimming muscles, but also with the help of its skin. Shark skin is covered with tiny 'teeth' (dermal denticles) and each species has denticles of a different shape. They are the shark's secret weapon. You can feel the denticles if you stroke a shark. If you move your hand from front to back it feels smooth, but if you brush the other way the skin feels like a carpenter's rasp. These 'skin teeth' protrude from the skin and are aligned in such a way that they channel the water across the shark's body in the most efficient way. They also work in the interface between skin and water, reducing drag and allowing the shark to 'slide' through the sea.
In experiments during the 1980s at Scripps Institution of Oceanography, California, the efficiency of a blue shark was compared to that of a submarine and, weight for weight, the shark required six times less driving power. This discovery has led designers of racing yachts, submarines, aircraft and even bathing suits to experiment with 'rough' rather than 'smooth' surfaces for their products. The results in terms of energy saving have been remarkable.
Sharks' teeth drop out at a great rate. For example, the lemon sharkloses a row every eight days. This is far from being a problem for the sharks; it is part of a wonderfully efficient system of tooth replacement for a creature that depends so much on these structures. The teeth are borne on a 'conveyor belt' arrangement, with newly-developing teeth at the back and the full-grown teeth at the front. The new ones move forward when the old ones are damaged, blunted or drop out and in this way are replaced endlessly throughout life.
This emphasis on efficient feeding is reflected also in the shark's digestive system. On average, a shark must eat between 0.6 and 3 per cent of its body weight each day in order to survive. To help this process, the shark has an expandable stomach capable of receiving large quantities of food in one sitting. This is important for any species that scours the open ocean, such as the oceanic whitetip shark whose food is widely scattered and whose next meal is far from certain. Some species, such as the great white and mako, ensure food is dealt with quickly by having a body that is warmer than the surrounding seawater and a warm stomach. A mako, therefore, is able to digest a meal within one or two days, whereas other hunters, such as the blue shark, take an average of three days to digest a similar quantity of food.
All sharks have a relatively short gut, which is equipped internally with a special valve structure. Each species has a valve of a different design, and some closely-related species, such as deep-sea lantern sharks are difficult to tell apart except for the number of twists and turns in their (spiral-shaped) valves. The valve arrangement slows down the passage of food, allowing digestion to take place more effectively and nutrients to be absorbed more efficiently. But there is a downside, too, to this arrangement.
With slow food throughput, the intestinal valve is a favourite lodging site for internal parasites, most notably tapeworms. They spend all of their lives not just in one species of shark but only in a single section of its valve. The dusky smoothhound or smooth dogfish, for example, has a spiral valve with eight chambers. The first four chambers are home to different, chamber-specific species of tapeworm. By the time food has reached the fifth chamber, much of the nutrients have been removed, so the rest of the chambers are unoccupied.
Parasites are a fact of life for sharks. There are parasitic copepods that saunter in and out of their gills and nasal passages or attach themselves firmly to the edges of their fins, and marine leeches that make a nuisance of themselves around the cloacal area, but surprisingly, sharks are usually untroubled by serious natural diseases.
Anal fin Unpaired fin located posterior to the anus, which is present on the underside of most, but not all, sharks.
Dorsal fin The large triangular fin on a shark's back. Some sharks have a large anterior and a small posterior dorsal fin, while others have just a single dorsal fin.
Pectoral fins Pair of fins positioned just behind the gills that resemble aircraft wings in many species of shark.
Pelvic fins Pair of fins positioned on the underside of a shark, between the pectoral fins and the anal fin (if present).
Sharks were most probably among the first creatures to have developed an effective immune system, and during the past several hundred million years their bodies have evolved all manner of defence systems against disease. They have high levels of an immuno-globulin circulating in the blood, for instance, that is ready to destroy or disable virtually any invading particle at any time. Immune cells are produced in the spleen, thymus, gonads and oesophagus, rather than just in the bone marrow as with mammals. They circulate in the blood, ready to go to work with hardly any lag between infection and response. As a result, sharks can live long and relatively healthy lives if left unmolested, but they are slow to grow and mature. Tagging studies in the northwest Atlantic have shown that the sandbar shark, for example, may live for 40 to 50 years but does not reach sexual maturity until it is 30 years old.
Whatever their size, like any animal, all sharks must breathe. It is sometimes said that sharks must keep moving in order to breathe or sink to the bottom and drown, but this is only partly true. Some sharks, such as the whitetip reef shark and nurse shark and especially, bottom-dwelling species such as carpetsharks, wobbegongs and angelsharks, are able to rest on the sea floor and pump water over their gills, where they extract the oxygen it contains. Whitetip reef sharks open their mouths, expand the walls of the pharynx, and cause the water to rush in. By closing the mouth and raising the floor, the water is propelled over the gills and out through the gill slits. Other species, such as the oceanic whitetip shark and grey reef shark, really do have to keep moving or they are in trouble. They adopt a system known as 'obligate ram ventilation' in which oxygen-rich seawater passes freely through the open mouth and pharynx, over the gills and out through the gill slits as they swim forwards. Most modern sharks possess five pairs of gill slits, but more primitive species, such as the appropriately-named sixgill and sevengill sharks, have more.
Sharks swim by passing a series of waves down the body. First the head oscillates from side to side, and then the amplitude of the movement becomes progressively greater towards the tail. This pushes a series of inclined surfaces outwards and backwards against the water which, when pushed aside, causes the shark to move forwards. The tail shape varies with lifestyle. Fast-swimming sharks, such as the makos and the great white, have tail fins with an almost equal upper and lower lobe, a feature they share with other fast swimmers, such as swordfish and sailfish. Others, such as the tiger shark, have a tail with a long upper lobe, its function a subject of debate amongst shark scientists. Some think that it creates a thrust that is directed ventrally through the shark's centre of gravity, its body rather than its side fins providing lift. Others believe that it produces a downward thrust that is countered by the shark's pectoral fins.
The pectoral fins are like an aircraft's wings, and together with the slightly flattened underside of the body, they help provide the upward force of lift when the shark moves forward. In cross-section, they have the leading edge slightly thicker than the trailing edge, and as the water passes above and below the fin it moves more quickly over the top than the bottom surface. A partial vacuum forms on the upper side and a slight pressure pushes up on the underside that is sufficient to pull the fin (and the shark attached to it) upwards. The pectoral fin is also used for steering.
The shark also remains buoyant thanks to an oil-filled liver. Some bottom-dwelling sharks, such as dogfish sharks, have livers that are 90 per cent oil. The oil is less dense than water (indeed, it floats at its surface) so the unusually large liver in most sharks helps keep them afloat. The basking shark, which can grow to over 9 m (30 ft) long, has a gigantic twin-lobed liver that is a quarter of its body weight and runs the entire length of its abdominal cavity.