Sharks

Sharks have long held a special awe and fascination for humans, possibly something to do with their unmatched ability as predators and their position at the top of the ocean food chain. Just the thought of sharks still induces a primeval fear in many people, but of those fortunate enough to see live sharks in their own element, most cannot help but have a feeling of admiration for their beauty of movement and their supreme efficiency as predators. Sharks are one of the most physically perfect lifeforms on this planet.

While sharks are found almost everywhere in the world’s oceans, they are most common in tropical waters at depths above 2000m, the Indo-pacific region being particularly rich in shark species.

There are two major ‘groups’ of fishes; those with bony skeletons, which form the largest group, (Osteichythes) and those with cartilaginous skeletons (Chondrichthyes), which make up only around 1% of living fishes. Sharks possess cartilaginous skeletons, and this is their main defining feature, shared also by the rays and the Chimaera family. All three groups fall within the class Chondrichthyes, (cartilaginous fish), with sharks and rays belonging to the subclass Elasmobranches.

Chondrichthyes have been around for nearly 400 million years largely unchanged, having early on become superbly adapted to their ecological role. All sharks are carnivorous, and almost all are predators, with highly developed sensory systems and a large area of the brain assigned to processing sensory information. While people tend to imagine all sharks as being active hunters of other animals, in fact they occupy a huge range of ecological niches, from the huge, planktivorous Basking and Whale sharks to the bottom dwelling Carpet sharks, and probably the ultimate pelagic predator; the Great White shark.

Scientists currently recognise around 30 families and 370 species of sharks, plus 38 species of ghost or Chimaera sharks. Chimaeras (Holocephali) differ in several ways from the elasmobranchii, having a fixed upper jaw and fused plate-like teeth for crushing molluscs; only one external gill slit, long whip-like tails and being mostly deep-water benthic feeders.

Physical Adaptations of Sharks

Elasmobranches possess a number of unique features suiting them to their existence as apex predators, from their shape, to their sensory systems and impressive dentition! While individual species vary considerably in their physical appearance, there are many adaptations they have in common:

Unlike bony fish that have a swim bladder to regulate buoyancy; sharks control their position in the water column by having a light cartilaginous skeleton and large oily liver (oil being lighter than water). Not having a swim bladder also allows a shark to make fast changes in depth without having to wait for gas pressure to equalize, a necessary adaptation for ‘pursuit predators’ hunting in depths from the surface to hundreds of metres deep.

Although sharks have a diverse range of body shapes, the ‘typical’ shark that most people are familiar with has a very streamlined fusiform (torpedo-shape) body, and a tailfin with a longer upper lobe. This long upper tail lobe necessarily provides a downward driving force to balance forward lift generated by the pectoral fins and the flat ventral surface of the snout.

The fusiform body plan of a Gray Reef shark (Gray Whaler)

Most ‘typical’ shark species cruise at around 2-5kmph (1-3mph), but are capable of short bursts up to 30kmph (18mph) when needed in the pursuit of prey. Dermal denticles (‘teeth’ on the skin) channel the surrounding water to produce laminar flow, which lowers friction and makes a shark ‘hydrodynamically quiet’…another adaptation that aids in approaching prey.

An interesting adaptation of many predatory shark species is a nictitating lower eyelid, which slides across the eyeball allowing the shark to protect its most vulnerable organs at the time of attack.

Mackerel sharks (family Laminidae) are especially adapted for a pelagic predatory existence having a ‘conico-cylindrical body shape, caudal fins for added stability, and a ‘heat exchanger system’ which allows them to maintain a body temp up to 10ºC above ambient temperature, making muscle operation more efficient. The great white shark is a prime example of the Laminidae family, and can be found in almost all latitudes.

A silky Epaulette shark in Lawadi, PNG.

In addition to the ‘typically’ shaped shark species, there is a diverse range of body forms to be found among the shark family; from elongated sinuous species such as the Epaulette shark which hunt among nooks and crannies in coral reefs, to broad and laterally flattened Guitar sharks, adapted to life on the sea bottom. As well as body shape, tooth type varies considerably among elasmobranches, depending on their predominant prey and hunting technique.

Sharks have probably the most efficient dentition of the animal world and are one of the only creatures able to remove large pieces of tissue (up to 10kg or more) from their prey with a single bite. The teeth are arranged in rows, (up to five rows in some species) which are continuously replaced as those in use are damaged or lost. This process of tooth replacement continues throughout the lifespan, allowing the teeth to get bigger as the shark gets bigger. The bite-force of a shark is enormous, with a large individual able to exert around 18 tons per square inch!

Teeth of the Mako (left) and Great White.
[ drawing - Dave Abbott ]

Tooth-shape in sharks vary according to their different functions and the prey type pursued. Mako’s and Sand tiger’s have fang-like teeth for seizing and holding prey to be swallowed whole such as small fish; whereas Tiger’s and White sharks have large triangular teeth for cutting large fish, squid, and even mammals into ‘bite-size’ pieces.

At distances less than 15m (45ft) vision is an important sense for sharks and many have an adaptation which allows them to optimise all the available light in dimly illuminated water. A mirrored reflecting layer behind the retina known as the tapetum lucidum acts as a photomultiplier, or ‘light magnifier’. At distances greater than 15m, smell and taste takes over in importance; and sharks can detect blood and food in the water at levels as low as 1 part per 10 billion parts of water! All fish, including sharks, have a lateral line that is sensitive to vibrations and subtle changes in water-movement around them...an important means of locating potential prey (or predators). Sharks however have an additional adaptation known as the ampullae of Lorenzini; a group of specialised sensory receptors (teleceptors) that can pick up the weak ‘bio-electric’ signals that is given off by all living organisms. To top off this formidable arsenal of ‘detection organs’, sharks have an acute sense of hearing that is extremely sensitive to low frequency vibrations, as is made by a struggling fish…or something splashing on the surface!

As mentioned, sharks occupy a variety of ecological niches, and the different species pursue a diversity of prey; from minute plankton and small invertebrates right up to large marine mammals. Some of the smaller reef sharks such as the Epaulette shark have narrow flexible bodies and strong pectoral fins that allow them to hunt through small crevices in coral reefs in pursuit of ‘cryptic’ prey. Such species occupy an ecological niche similar to the Moray eel.

A Whale shark encounter in Isla San Benedicto.

Species such as the Swell shark and the Angel shark are ‘lie in wait’ or ambush-predators, relying on camouflage to hide them until their prey comes within striking distance. Basking sharks and other filter feeders like the Whale shark suck water in through their mouths as they swim and purge it through their gill slits, filtering out the food in each mouthful, occupying a similar niche to the baleen whales and the Manta ray.

Others like the Thresher shark and Sawshark have adapted themselves to hunting schooling prey, flailing their elongated tail or tooth-studded rostrum amongst a school of fish until sufficient are stunned or impaled. Generally larger sharks take larger prey, and feeding activity increases at certain times of the day, and at certain tidal phases. Most sharks are nocturnal, and flood tide is a favourite time to feed for many reef species.

A Scalloped Hammerhead shark.

Mako sharks, Seven gill’s and Hammerheads can all be classed as pursuit predators, relying on speed and agility to catch their prey, whereas the infamous Great White is a stalking predator; usually attempting to approach its prey undetected until the final moment of attack.

Most of the predatory shark species employ countershading, which is when the dorsal (upper) surface of the shark is a dark colour that seen from above blends with the blue of deep water, and the ventral (underside) is white to merge with the bright surface light apparent from below.

Interestingly, sharks have not yet been found to exhibit territoriality, that is, defence of a particular area. They do however defend a ‘personal field’ or ‘space’ and will first engage in a threat display, and then attack any creature entering this space. ‘Might is right’ in sharks and a smaller individual will always give way to a larger individual, although sex also plays a role, and females will often give way to males.

Schooling Hammerheads.

Some species like the Scalloped Hammerhead and the pelagic Blue shark form large aggregations at times, whereas others like the Tiger shark and the Oceanic Whitetip are usually solitary. Many species travel vast distances making research into their behaviour very difficult. One Blue shark for example was recaptured 6000km from where it was first tagged!

Although once thought to be ‘automatons’, sharks are now recognised as having a level of intelligence not before realised. Even though ‘intelligence’ is not an easy quality to measure in any animal, as it must be related to their level of existence rather than to a ‘human interpretation’.

Bony fish generally produce millions of small eggs in order to overcome the high mortality resulting from their vulnerability to predation. Sharks on the other hand lay fewer, but larger eggs that are fertilised internally, and so suffer a much lower mortality rate. To accomplish this internal fertilisation, the males have modified pelvic fins, or ‘claspers’, with which they transfer sperm to the female; these organs are a good way to distinguish males sharks from females.

During courtship and mating male sharks often bite the female, sometimes quite severely, and to combat this, in some species the female has much thicker skin than the male.

The young of elasmobranches develop in one of three distinct ways depending on their species. The simplest, known as Oviparity, is when eggs are laid in leathery case made of ‘keratin’. In some cases the eggs are laid straight away; in other cases they are retained in the uteri until the embryos are nearly developed, generally between 2 and 15 months.

The majority of shark species are Ovoviviparous; that is the eggs are retained in the oviducts where they hatch and remain until the young are fully developed. There are different forms of nourishment according to the type of ovoviviparity; in ‘yolk-sac viviparity’, the young feed off a yolk-sac attached to their digestive tract, while with Uterine viviparity the embryo relies on a rich secretion from the uterine lining for nourishment. In Intra-uterine cannibalism, the strongest embryos eat the weaker ones while they are still in the uterus! Gestation can vary between 6 and 22 months.

Viviparity, when the young are nourished through a placenta, occurs in only 9% of chondrichthyes. Only the more advanced species, such as the Great White are viviparous and they have fewer but larger young which are born alive. Gestation in these species is approximately 8-12 months.

In general, growth in sharks is relatively slow and their average lifespan is somewhere around 20-30 years; although some dogfish live as long as 70 years. Large pelagic species such as the Great White, Mako, and Blue whaler can grow approximately 30cm per year.

A small Whitetip Reef shark on the Great Barrier Reef

Sharks and Man

Although sharks have long been feared by man, it is common nowadays for divers to actively seek out encounters with almost all species of sharks and the almost non-existence of problems resulting from these frequent encounters has changed many people's perception of sharks. Only a small percentage of the 370 species of shark have the potential to harm man and even less have been actually implicated in attacks. Usually when an attack does occur it is a case of mistaken identity, sharks certainly aren’t the ‘oceanic assassins’ the media generally depicts them to be. Only around 6 people are killed each year by sharks; this despite the millions of people around the world that enter the water. On the other hand around 15 million sharks are killed by humans each year, so while people tend to think of human-shark interrelationships from the point of view of the danger sharks pose to man; in reality it is the other way around…humans are far more dangerous to sharks. We use their teeth for jewellery, liver oil for high-grade lubricating oil, pharmaceuticals and vitamins, cartilage for health food supplements, flesh for food and fertilisers; and there is a large market in Asian countries for shark fins to be used as aphrodisiacs and in shark fin soup (US$100 kg).

Threats and Management

Sharks are currently under threat from all sides. As well as target fishing, they also form a huge 'by-catch' in other fisheries. Additionally, there is a large recreational fishery and in many cases, those sharks caught are killed, weighed and ‘scored’, and then wastefully dumped at sea. Coastal development is reducing shark ‘nursery areas’, and environmental threats such as pollution, and bio-amplification of organ compounds and heavy metals, directly or indirectly impact on sharks just as they do on all marine species.

As sharks are generally slow growing, late maturing, and produce relatively few young, the fishery requires careful management. For example, species that live to 50 or more years can only sustain a harvest of around 2-3% of the population, and fishing beyond this limit results in a ‘population collapse’.

Even shark-netting bathing beaches accounts for high numbers of sharks; many of which are harmless anyway. In Queensland Australia alone, over 20 000 sharks died in beach nets over 16 years, (not to mention 468 dugongs, 317 porpoises, 10,889 rays, and 2,654 sea turtles).

Sharks generally occupy the top position in the food chain and perform a necessary function within the marine ecosystem in regulating populations of other species, culling the weaker individuals and maintaining ‘genetic fitness’ of fish populations. By reducing their numbers excessively we are tampering with the health of the ocean we so heavily depend upon.

Unfortunately, because sharks are not ‘cute and cuddly’ and many people are ignorant of the important role they play, they do not attract the same level of attention and protection that many ‘charismatic’ terrestrial animals enjoy. This is a shame, as seen in their own environment sharks are incredibly beautiful, and more deserving of our admiration and respect. When you stop to consider the minute number of attacks as a percentage of the millions of people entering the water each year around the world, it seems ridiculous to do what seems to be our best to exterminate these incredible creatures.

Many traditional cultures revered sharks, including the Haida Indians of the Pacific Northwest, the Polynesians and Melanesians. Even Aristotle in 380BC wrote of his impressions of shark biology and behaviour. It is to be hoped that with increasing environmental awareness in the new millennium sharks will once more regain the reverence and respect they formerly held.

REFERENCES : 
Taylor L, Cons.Editor ‘Sharks and Rays’ 1997, The Nature Company; 
Michael S W, ‘Reef Sharks and Rays of the World’, 1993, Sea Challengers.