A shark is equipped with an extraordinary array of sensors, probably the most diverse of any known predator. Low-frequency sounds tend to travel great distances underwater, so a shark's auditory system could be the first sense to pick up an interesting target. Rapid, irregularly-pulsed, broad-band sounds at frequencies below 600 hertz, similar to those made by injured prey or a group of spawning fish can alert a hunter such as a bull shark from over 1.6 km (1 mile) away.
At a distance of 0.5 km (1/3 mile), it is able to smell blood in the water and follow a trail back to the source. It can detect one part of fish extract in 25 million parts of seawater, the equivalent of ten drops of blood in an average-sized municipal swimming pool. At 100 m (330 ft), the shark's lateral line system kicks in. This row of fluid-filled sensory canals on either side of its body responds to pressure changes and movements, so a shark can almost feel the presence and location of something moving in the water--a kind of 'touch-at-a-distance.'
At 25m (82 ft) in relatively clear water, the shark can spot movements. It can see even in very dim light, for sharks, like domestic cats, have a tapetum lucidum, a layer of shiny plates behind the retina at the back of the eye that reflects light back onto the retina's light sensitive cells. It ensures that the maximum amount of available light falls on the retina, enabling the animal to see almost in the dark. In sharks, the tapetum can be masked. A curtain of melanin-filled cells, known as melanoblasts, migrates into channels that cover the reflective plates. Using this, a shark is able to rise rapidly from the dimly-lit depths into the bright light of surface waters without being blinded. Not all sharks have the screening--deep-sea sharks have a tapetum but no melanoblasts, an adaptation to a world of inky darkness where the only light is often from bioluminescent fish and other marine creatures.
The most extraordinary sensory system is one that discerns electricity. In the snouts of most sharks are small, jelly-filled pits, known as the ampullae of Lorenzini, and in each pit is a sensor that detects minute electric fields associated with contracting muscles, such as those in a pumping heart. The system is remarkably sensitive. A shark is able to detect a change in intensity of a hundred-millionth of a volt per centimetre, the equivalent of a flashlight battery creating a field between two electrodes set 1600 km (1000 miles) apart. In this way, a shark is able to detect muscular activities in its prey, even if the target is hiding under the sand or gravel of the sea bed.
Hammerhead sharks of the genus Sphyrna have their electrical sensors spread across the width of their curiously shaped heads. By sweeping its head from side to side, like a person with a metal detector, a hammerhead is able to scan a larger area of the sea floor and more accurately locate any flatfish, skates or rays buried in the sand. The great white shark uses its electrical sense not for locating prey but in the moment just before contact. In order to protect its eyes from the claws or teeth of seals and sea lions, it swivels them back into special sockets and is effectively swimming blind. The electrical field generated by the seal's muscles guides the shark the last few centimetres to its target.
Having taken a bite out of its victim, a shark may then use its sense of taste to determine its palatability. Taste receptors are located on swellings in the mouth and gullet, and it is with these that a shark decides whether to swallow or spit. Great whites sometimes spit out human flesh. They prefer animals that are insulated with a thick layer of high-energy fat, and even the most obese amongst us do not have sufficient blubber to interest a great white. So as long as a major artery is not severed and there is not too much loss of blood, some attack victims at least survive an assault by an extremely powerful predator that is quite capable of biting us in half and swallowing the pieces whole.
A blue shark employs its integrated sensory system to detect and then feed on a shoal of anchovies near San Diego, California, on the Pacific coast of the USA. (Image: Richard Hermann/Seapics.com)
While a tiger shark, the ocean's garbage collector, will take a chunk out of virtually anything, including humans, most other sharks prefer to eat fish, squid, crabs and lobsters, sea urchins and marine worms, eating whatever is abundant locally. The diet of populations of smoothhounds of the genus Mustelus of Tomales Bay on the US Atlantic coast, for example, is determined by which part of the bay an individual lives in. Those near Hogg Island seek out slender crabs and small fish, while others at Indian Beach feast on yellow shore crabs and polychaete worms.
Some shark species have a preference for particular foods. Australia's sicklefin weasel shark is an octopus specialist, while the great hammerhead has a penchant for stingrays--and an unusual way of catching them. It pins them down using the front of its head, and then turns rapidly to take a chunk out of the ray's 'wing.' The immobilised ray is unable to escape, so the shark can take leisurely bites out of its quivering body until it has swallowed the lot. One individual was seen to have 96 stingray barbs stuck in its head--surely a world record.
When food is concentrated, such as a whale carcass or a tightly packed shoal of fish or squid, many sharks must feed together and, while feeding frenzies (when sharks bite at anything and everything including others of their own kind) have been initiated by artificial feeding, in the wild there is some degree of order amongst diners. There are rules in shark society. Generally, small sharks defer to large sharks, but there is more to it than that.
Baby sharks receive little or no parental care when they enter their perilous underwater world, so shark mothers ensure they receive a good start in life before they are born. Unlike some bony fish that produce millions of eggs and larvae that are left to the mercy of the sea, sharks produce relatively few eggs and embryos (in most species less than 100) and protect them well. They do so in one of two ways: by protecting their embryos within egg cases that are deposited somewhere safe; or by retaining their embyros inside the body.
The more primitive system is oviparity, meaning 'born from an egg', which is adopted by the dogfish, the nursehound, the swell shark and the Port Jackson shark. Each of their eggs is enclosed in a tough capsule known as a 'mermaid's purse.' Other species are viviparous, meaning 'live birth'. The embryos develop in the uterus (a shark's uterus is divided into two branches), and obtain their nourishment directly or indirectly from their mother. A hammerhead shark embryo, for example, relies on a yolk sac for food at first but later the sac begins to grow branches that fasten to the uterine wall, like a placenta, and extract nutrients directly from the mother's blood system. The most bizarre form of viviparity must be that of the sand tiger shark. When the embryos have used up their yolk sac they take to eating each other. Two dominant babies eventually survive, one in each uterus, and they are fed yolk-rich unfertilised eggs until the two intra-uterine cannibals are born.
Great white sharks, for example, were once thought to be solitary nomads but it is now recognised that they are probably more social animals showing complex relationships with other individuals. Pairs or small groups of sharks of roughly the same sex and age return to the same area each year. Off the Farallon Island, to the west of San Francisco, two recognisable females, known to local shark researchers as 'The Sisterhood', always turn up in the same patch of sea each winter to stake out the breeding rookeries of northern elephant seals and California sea lions. They arrive at the same time, patrol the same bays and are present at each other's kills. They appear to communicate not with sounds, as whales and dolphins do, but with a body language of movements and postures. This behaviour is shown dramatically by another species, the grey reef shark.
Grey reef sharks patrol the drop-off into deep water at the edge of coral reefs and atolls, and they have a clear way of expressing their displeasure. When approached too closely, a shark will perform a distinctive threat display accompanied by exaggerated swimming movements. Its back is arched, its pectoral fins point downwards and its snout is raised with its jaws slightly open. It swims in a rather stiff and awkward fashion following a figure-of-eight loop. If the perceived threat fails to back down, the shark then attacks, slashing at the offender with the teeth in its upper jaw. The Galapagos shark and silky shark perform similar threat displays. The bonnethead, blacknose shark and great white shark have been seen to adopt the arched body and raised snout posture, but do not swim so erratically as the grey reef shark. These revelations must mean that there is a complete shark body language waiting for researchers to unravel.
At some point in their lives, sharks must meet to procreate. Courtship and mating behaviour is rarely observed in the wild, but it is thought receptive females release pheromones (natural chemical stimulants) into the water to attract males. At Cocos Island, in the eastern Pacific, female whitetip reef sharks have been seen pursued by gangs of eager males. At Cape Hatteras, on the US Atlantic coast, sand tiger sharks are a little more reserved. They gather in groups of five or six, each shark stacked one on top of another. Both males and females hang motionless in the water until the time comes for them to mate.
Mating in sharks can be a violent affair. The male grabs a female by her pectoral fin or the skin behind her head and hangs on tightly. The female's skin is thicker than the male's (in some cases as much as three times as thick) so he inflicts less damage. Once he is firmly attached, and the other males have drifted away, he introduces his sperm into her cloaca using one of a pair of sausage-shaped claspers, each a penis-like extension of the pelvic fin. A spur or a bank of terminal ridges at the tip prevents it from slipping out. The sperm is then transferred in a jet of seawater squirted out by a pair of muscular-walled siphon sacs in the male's abdominal wall. Mating complete, male and female part company.