There are about 400 species of sharks and most sharks are slender, muscular and fast animals that reach a length of about 1 to 2 meters.
The smallest shark species is Etmopterus perryi with a size of about 17 centimeters, and the largest living shark is the whale shark (Rhincodon typus) that reaches an average length of 12 meters. Sharks are found in all seas. Most species do not occur below about 1,500 meters, but there are exceptions up to 3,000 meters.
Sharks are best known to the general public for being dangerous (to humans), but out of about 400 species, only four or five species qualify as dangerous. Almost all are predators (predatory fish) that live on vertebrates (mostly fish but also marine mammals and birds), large crustaceans and mollusks.
Species of sharks
An exact number of species of sharks is difficult to give because it depends on the author’s concept of species. Furthermore, new species are found every year. There are about 400 species of sharks divided into 8 different orders.
Sharks do not form a monophyletic group. The rays together with the orders of gray sharks, spiny dogfish, sea angels and saw sharks form the clade Squalea, while the other orders (pig sharks, baker sharks, mako sharks and ground sharks) share a different common ancestry and belong to the clade Galeomorphii.
The various species of sharks are found almost worldwide in all seas and oceans except the very cold waters around the north and south poles. Most species have a small distribution range, such as the long-nosed whitetip shark (Centrophorus harrissoni), which occurs only along the eastern coast of Australia and around the island of New Caledonia. The whitefin lantern shark (Centroscyllium ritteri) is found only around Japan. Other species have a worldwide distribution and occur in the Pacific, Atlantic and Indian oceans as well as in the Mediterranean Sea, an example being the thresher shark (Alopias vulpinus).
The head of a shark is large and cone-shaped, the head contains small eyes on the side and nostrils in the front at the top of the snout. Along the jaw edge are one or more rows of strong saw teeth with many more rows of spare teeth on the inside of the jaw. Because the teeth are regularly lost and replaced by the spare teeth, they are always razor-sharp. On either side of the head are usually 5 gill slits, rarely 6 or 7. With these, the shark filters oxygen from the water. It is true that many sharks have to keep swimming to continuously provide themselves with enough oxygen, but there are also species that open and close the mouth regularly, which also allows water to pass by the gills. This still allows them to take in oxygen while not moving.
A shark’s skin consists of a kind of skin teeth, which are tilted backward. This makes the skin look somewhat like sandpaper in texture and was used that way in the past. This is because the skin has many small hard inclusions that serve as armor, which is useful in fighting with prey animals and the skin is also difficult for parasites to penetrate. The torso of a shark consists of the internal organs, and also carries a number of fins, paired or not. The pectoral fins are paired and are located directly behind the gill slits, the single dorsal fin is positioned on the middle of the back. The ventral fins, second dorsal fin and anal fin are located more to the rear of the body. The ventral fins are modified as the sexual organ in the male.
The caudal fin is posterior and is often very large in sharks. Many species can be distinguished by the tail fin alone because of the different shapes associated with the shark’s lifestyle.
A shark’s buoyancy relies on a large amount of oily substance in its liver called squalene. This substance is lighter than water; sharks do not have a swim bladder like fish. Squalene, by the way, is extracted and used in the pharmaceutical industry for its supposed rejuvenating effects.
The intestinal tract is short and thick. The internal surface is equipped with a spiral spiral fold, which greatly slows food transport and increases the absorbent intestinal surface.
Like all fish, sharks have a single circulatory system, unlike mammals, among others, in whom the heart pumps blood in two separate loops to the lungs and other organs, respectively. Yet the heart consists of four, in this case consecutive chambers: the sinus venosus, the atrium, the ventricle and the conus arteriosus.
From the heart, a system of arteries branches into the capillaries of the gills, this is the branchial arterial system. Sharks, rays and some bony fish have coronary arteries as part of the branchial circulation to supply the heart muscle.
The arteries that carry blood from the gills to the tissues form the systemic circulatory system,’ further divided into a posterior caudal system and an anterior cranial system. The veins then carry the deoxygenated blood back to the heart at lower pressure. Sharks also have three sinus cavities that act as blood reserves for short-term strenuous efforts.
Most fish, including most shark species, are cold-blooded, more specifically ectothermic,’ that is, they use environmental factors (variable water temperature) to regulate their body temperature. However, two families of sharks, porbeagle sharks (Lamnidae) and thresher sharks (Alopiidae), are regionally endothermic and thus warm-blooded: some parts of the body are maintained at a higher temperature than others. In Lamnidae, this is done by a heat exchanger by which blood coming from the gills is warmed by blood flowing to the gills.
Bony fishes have a skeleton made of bone that is sturdy enough to handle large forces from muscles. In cartilaginous fish such as sharks and rays, the skeleton consists of more flexible cartilage to which the way they swim is adapted. Most sharks swim anguilliform, with the exception of mackerel sharks, among others, which swim thunniform.
In anguilliform swimming, apart from the head, the entire body is used. A relatively large amount of energy is lost in the lateral force and the additional shape resistance provided by the curved body. Consequently, it occurs mostly in fish that swim slowly, especially in dense vegetation and sediment. For an active hunter like the shark that travels long distances, this mode of locomotion seems at odds with the low efficiency of the spindly body that must make the long winding motion along its entire length. However, efficiency is improved in several ways. Most sharks have two dorsal fins, with the first usually being a lot larger. With the heterocercal caudal fin, the upper lobe can be thought of as a dorsal fin. These three are widely spaced and each generates a wake that is slightly out of phase with the winding motion of the shark. At the appropriate phase difference – depending on the distance between the fins which in turn is based on the size of the fins, their shape and the waveform of swimming – this tracking current increases the thrust of the second and especially the third fin.
In addition, muscle contractions are amplified by the skin. The stratum compactum in the lower skin consists of collagen fibers that act as tendons. In addition, in the skin, hydrostatic pressure increases as the shark swims faster. This ranges from about 20 kPa in slow swimming to 200 kPa in fast swimming. This makes the skin stiffer, allowing it to store more elastic energy. The muscles are connected via collagen septa to both the spine and the inside of the skin. When the muscles on one side are tightened, muscles and skin on the other side are stretched. The muscles there are very elastic, but the skin is not. As soon as the muscles on one side relax again, the skin on the other side therefore already contributes to the initiation of the movement at a time when the muscles on the other side have yet to begin tightening. This effect increases with increased speed.
The heterocercal tail fin is not symmetrical, and given the larger upper lobe, it would be expected that the thrust from the tail would be directed backward as well as upward. According to this classical model, this produces a moment that would cause the shark to spin forward, but this is prevented by the flattened underside of the front body having a small angle upward, generating an elevator force. All this prevents the shark from sinking while providing a forward force.
However, this is accompanied by additional resistance through the forelimb and pectoral fins, so an alternative model was proposed in which the lower lobe generates sufficient counterforce so that the resultant force produces a forward motion. Further research seems to support the classical model, but in which only the front body without the pectoral fins provides elevator force. The alternative model does seem to hold true for sturgeon, which also have a heterocercal tail fin and can move the angle of the thrust beam up and down about 10° to dive and rise, something sharks do with their pectoral fins.
Speed in most fish increases by increasing the frequency of the tail stroke, but sharks additionally vary the stroke size and length of the propulsion wave through the body. With the combination of a flexible body with elastic skin, well-placed dorsal fins and a heterocercal tail fin, the anguilliform shark’s swimming form has managed to make a good compromise between speed, acceleration and maneuverability.
The outer surface of the skin of cartilaginous fish contains dermal teeth homologous to dental teeth, with enamel around the dentin. These generate vortices that reduce turbulence, thereby lowering drag and also making the shark quieter. It also contributes to the lotus effect that acts as an anti-impact agent.
Large hammerhead sharks swim sideways a lot with angles of about 50 to 75° up to 90% of the time, including during ascents and descents. The front dorsal fin, unlike all other sharks, is larger than the pectoral fins, so that when swimming sideways it can provide the elevator force with the upper pectoral fin and body that in other sharks is provided only by the front body and, to a lesser extent, the pectoral fins. When swimming sideways, the wingspan is larger, providing more elevator force, so the angle of attack can be reduced. As this angle becomes smaller, the induced drag also decreases. As resistance decreases by about 10%, energy consumption also decreases by about 8%.
The sexual organs of male sharks (called class presses) are located between the ventral fins. During copulation, these organs fold forward (sometimes one of the two) and penetrate the female cloaca (a slit-shaped opening in the abdomen that serves for urine, feces and reproduction). In the process, males sometimes bite into the female’s pectoral fin. In sharks, mating with internal fertilization takes place. In this process, there are three forms of development before birth. Some shark species (such as the catshark and dogfish) are oviparous. These produce a few firm eggs packed into leathery sacs, whose thread-like appendages entwine seaweed or rocks.
Most sharks are ovoviviparous; they produce live young but through an embryo connected to a yolk sac. Not infrequently, these embryos eat the still unfertilized eggs in the maternal uterus, resulting in fewer young being born. This is oophagy (literally, egg-eating). The survivors are formidable hunters immediately after birth. Some shark species are viviparous, such as Carcharhinus (of the requiem sharks) and hammerhead sharks (Sphyrna). Like mammals, these species develop an embryo with a placenta.
Compared to bony fishes, which usually produce a huge amount of eggs (fish roe), sharks have relatively few offspring. They are carrying capacity strategists, matching the amount of offspring to carrying capacity of the environment.
Sharks can reproduce asexually by parthenogenesis: a female shark can have offspring without contact with a male shark.
A team of 10 Australian scientists found hybrid sharks on the coasts of Australia for the first time, using DNA testing and random fishing of specimens across 2,000 km of coastline. 57 specimens were actually found to be a hybrid. They are a cross between an Australian blacktip shark and the blacktip shark. The Australian blacktip shark lives close to shore, exclusively in tropical waters and is slightly smaller than the related common blacktip shark, which lives more in deeper and colder waters. However, its offspring were found 2,000 kilometers south of tropical waters. The water there is much colder, which scientists say may indicate that sharks are adapting to global warming. It is also possible that the hybrid sharks are reproducing.
Sharks have a good sense of smell, with some species able to smell a millionth of blood dissolved in seawater. They are like “swimming noses.” Sharks find their prey by focusing on the nostril where the prey’s odorants first enter their nose.
The olfactory regions in the brain are also relatively highly developed in sharks. Some species such as the nurse shark have external bearded threads with tactile senses for certain substances, thus enhancing the sense of smell. The sense of smell is mainly used to locate prey at long distances. For short distances, the sensory systems described below are used.
Sharks possess a subtle ability to detect electric fields caused by static or moving objects (such as fish or people) in their immediate environment. This occurs through tiny sensory pores in the head, called Lorenzini’s Ampoules. These electroreceptors can range in number from several hundred to several thousand. They can also be used to detect prey hidden under the sand. Sharks can also use changes in the magnetic field caused by ocean currents relative to Earth’s magnetic field for orientation and navigation.
Also for further distance, they use electroreception to detect prey, with the so-called sideline organ. For the attack phase, however, this organ is not accurate enough.
Contrary to popular belief, most sharks possess well-developed eyesight. The eyes of sharks are similar to those of vertebrates and have a retina (with rods and cones), eye lens and cornea. However, the eyes are adapted to the sea by means of the tapetum lucidum, a tissue behind the retina that we also find in dogs and cats. It reflects light back to the retina, sharpening vision especially in dark environments. The eyes of sharks have eyelids, but they do not blink because the surrounding water cleans the eyes. They also have an extra thick eye membrane called the cuticle or third eyelid that covers the eyes when they attack or consume prey. Other sharks such as the great white shark do not have this membrane, but roll the eyes back into the eye sockets when they attack their prey.
The ears of sharks are much simpler than in land animals and consist of small openings on either side of the head that are connected directly to the inner ear through a narrow canal. They have no eardrum because the surrounding water conducts sound excellently and very quickly. For shorter-distance orientation, they use the lateral line on the side of the body. This consists of a long series of small bowl-shaped organs with hairs with which small pressure differences caused by movement or vibrations in the water can be picked up. Especially vibrations in the range between 25-50 Hz are perceived. Sharks can also sense natural disasters such as a hurricane by registering changes in water pressure. They then flee to deeper water long before the hurricane can do any damage. Sharks also flee to deeper water when thunderstorms arrive.
Sharks are the most sensitive species to electromagnetic radiation. This is because they have special senses used to detect the electric fields generated by muscle contractions and emitted by all living fish. It is believed that this sometimes causes sharks to attack (fishing) boats because the boat’s reaction with the salty seawater creates electric fields similar to those of prey fish, the same is true of undersea power lines.
The shape and number of teeth varies greatly among species, as does the strength of the jaw muscles. A large shark can exert 3 tons of pressure per square centimeter in its mouth. This is equivalent to an elephant standing in a spot the size of a postage stamp.
Most sharks reach an age of 20 to 30 years, although there are some shark species, such as the whale shark and the bowhead shark, that can live to be more than 100 years old. The latter are even the longest-lived vertebrates on Earth; researchers found a female that was 400 years old. These sharks grow very slowly and only reach sexual maturity after 150 years.
Relationship to humans
The chances of a human being losing their life due to the attack of a shark are incredibly small. Bathers and swimmers in the sea are much more likely to be killed by a box jellyfish or by an octopus or stonefish, sea snake or sea crocodile. The shark is at the very bottom of a list of dangerous animals that can fatally injure humans. The species considered dangerous to humans are:
- The white shark or man shark (Carcharodon carcharias)
- The tiger shark (Galeocerdo cuvier)
- The bull shark (Carcharhinus leucas)
- The sand tiger shark (Carcharias taurus).
These four sharks account for 69% of all shark attacks.
According to the ISAF, during the 10-year period from 2010 to 2019, a total of 779 unprovoked attacks were reported and investigated for accuracy, of which 54 or 6.8% were fatal.
Many shark attacks result from a mistake by the shark, mistaking a swimming human for a seal or other prey animal. A human may also enter its territory. This danger exists particularly during dusk when the shark is looking for food. In general, sharks are not dangerous if they are not stimulated by, for example, the smell of blood. Many accidents in the past have been caused by sport divers without diving equipment hunting fish with harpoons. The floundering or entrained bleeding fish then attract sharks.
It is quite possible to dive with sharks without cages provided they are not fed and are treated with respect. Feeding with dead fish is sometimes practiced by sport divers and underwater photographers to attract the sharks. Presumably much fear of the shark, especially among children, is caused by movies and TV productions. The best known is the Jaws series in which a white shark makes a beach unsafe. In Deep Blue Sea, basking sharks are bred by genetic engineering to extract substances from their brains.
Remarkably, the very largest shark species are harmless to humans; the whale shark (Rhincodon typus, up to 13 meters), the basking shark (Cetorhinus maximus, up to 10 meters) and the basking beaked shark (Megachasma pelagios, up to 5 meters). These species live on small animals (plankton) that they filter out of the water with their large mouths.
Sharks and swimming
The skin of the shark is not covered (as in bony fish) by scales, but by an epidermis in which there are small teeth of the same material as the large teeth in the mouth. These are called skin teeth. It has been discovered that the profile of the skin teeth found on the shark’s flanks increase its speed. The profiles act like spoilers, preventing turbulence that would otherwise act as a brake. This phenomenon has been used in the swimwear of top athletes. During the 2000 Summer Olympics in Sydney, many world-class swimmers dressed in these suits (fastskins), a type of synthetic sharkskin that reduces flow resistance. In 2010, the FINA banned the use of these fastskin swimsuits.
The other side of the coin is that several shark species are in danger of extinction due to bycatch from being caught in fishing nets and in long-line fisheries, and from finning: the cutting off of shark fins for culinary purposes. According to an IUCN report, 32% of the species of sharks and rays living in the high seas are endangered and are therefore on the international red list. Also on the Dutch red list are two species of sharks (spotted smooth shark and rough shark) and two species of rays (common stingray and thornback ray) that occur as endangered fish species in the North Sea.
Recent research in the Atlantic Ocean shows that large shark species in particular are drastically declining in numbers. This study showed that the decline for species such as the tiger shark and hammerhead shark, for example, exceeded 90%. Possibly this leads to shifts deeper in the ecosystem.
Because sharks are carrying capacity strategists, and differ greatly from (common) bony fish in this respect, they are especially sensitive to fishing. Because of their low number of offspring per individual, overfished populations recover very slowly (if at all).
The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) lists the following shark species in Appendix II (species that are not necessarily endangered, but whose international trade is managed to avoid uses contrary to conservation): basking shark (Cetorhinus maximus), whale shark (Rhincodon typus), white shark (Carcharodon carcharias), oceanic whitetip shark (Carcharhinus longimanus), porbeagle shark (Lamna nasus), scalloped hammerhead, (Sphyrna lewini), great hammerhead (Spyrna mokarran), smooth hammerhead (Sphyrna zygaena), silky shark (Carcharhinus falciformis) and thresher sharks (Alopias).