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SHARKS AND RAYS

Content Updated: 25th June 2006

CONTENTS:

-- Taxonomy
-- Length and Weight
-- Colour
-- Distribution
-- Longevity
-- Sexing
-- Activity
-- Home Ranges
-- Predators
-- Anatomy and Senses
  -- Vision
  -- Smell (Olfaction)
  -- Taste (Gustation)
  -- Touch (Tactility)
  -- Electroreception
-- Food and Feeding
-- Breeding Biology
-- Behaviour
-- Interaction with Humans

Blacktip Reef sharkSharks and rays form an incredibly diverse group of fishes. Check out the Links page for some of my highly recommended shark sites that go into far greater depth than I plan to here. On this page, I intend to cover the basic biology and ecology of elasmobranchs, with a particular slant towards those found in UK waters

Taxonomy: Sharks, rays and their kin the chimaeras are part of a group of fish called Chondrichthyes (fish with cartilage skeletons). If we remove the morphologically ‘odd’ chimaeras, we are left with the elasmobranchs (literally meaning ‘strap gill’) or the sharks and rays. Elasmobranchs can then be divided into the selachians (sharks) and batoids (skates and rays). At the moment, the world of elasmobranch taxonomy is in a tumultuous state, and it is rare to come across two shark taxonomists that agree on the same classification scheme. I personally like to follow a classification part way between those proposed by elasmobranch taxonomy guru, Professor Leonard Compagno at the South African Museum and former fisheries biologist, now ReefQuest executive director, R. Aidan Martin. As such, in my view there are ten orders, 39 families, 104 genera and about 487 species of shark currently known to science. With regard to batoid classification, I again follow a scheme part way between that of R. Aidan Martin and Brian Mould of the University of Nottingham. Thus, I perceive there to be six orders, 22 families, 72 genera and about 560 species of skate and ray. This takes the total elasmobranch families to 61, and the species to around 1047. While this may seem a lot of fish, it is a drop in the metaphorical ocean compared to the Osteichthyes (fish with skeletons of bone) that encompass about 95% of all fish species with somewhere in the region of 435 families and more than 23, 500 species!

Of these 1000+ species of elasmobranch, 59 of these are known from UK waters (i.e. up to 300 miles off the coast), and a few other species are in the “maybe” category because of anecdotal sightings or the presence of their egg cases. Of these 59 species, 35 are sharks and 24 are rays (see UK Elasmobranch Species Checklist). The UK elasmobranch fauna is diverse, ranging from the well-known bottom dwelling Smallspotted catshark (Scyliorhinus canicula) to the deepwater Greenland shark (Somniosus microcephalus) right up to the second largest fish species in the modern day oceans, the Basking shark (Cetorhinus maximus). We even have some of the more striking species as occasional visitors, with the Bigeye Thresher (Alopias superciliosus), Common Thresher (Alopias vulpinnus) and Smooth Hammerhead shark (Sphyrna zygaena) being a few examples. There are even reports of the infamous Great White shark (Carcharodon carcharias) from UK waters (see UK Great White Shark). That which follows is a basic taxonomic hierarchy for perhaps the best known and most thoroughly studied of all shark, the Spiny dogfish. (Back to Menu)

Kingdom: Animalia (Animals)
Phylum: Chordata (Possess a basic 'backbone')
Class: Chondrichthyes (Cartilaginous fish)
Order: Squaliformes (Dogfish sharks)
Family: Squalidae (Dogfishes)
Genus: Squalus (meaning 'sea fish')
Species: acanthias (meaning 'thorny')

For more information about how we classify living organisms, see my Taxonomy page.

Whale SharkLength and Weight: Sharks show an extreme diversity in their size. The largest shark species (and indeed fish species) is the Whale Shark (Rhincodon typus - right), reaching a staggering size. The largest accurately recorded specimen was tagged and tracked with telemetry in June 1996 at Banco Gordo in the Sea of Cortez; this specimen was 18m (59ft) long. Prior to this, the largest faithfully measured individual was landed alive at Cuffe Parade on Bombay’s coastline (in the Arabian Sea, part of the Northwest Indian Ocean) and measured 12 metres (almost 40ft). However, there are reports of Whale sharks attaining lengths as great as 20 metres (almost 66 ft). A paper currently in press reports on a male whale shark landed in Taiwan during March 1987, which was 20 metres long and weighed 34 tons (68,000lbs or 30,844kg). Several popular general science books relating to sharks cite the whale shark as attaining a maximum length of 18m, a length based on a communication to the journal Science back in 1925 by Hugh Smith. Smith wrote of a huge shark that became wedged in a bamboo stake-trap set on the east side of the Gulf of Siam (now the Gulf of Thailand, part of the west South China Sea in the Pacific) back in 1919. No official measurements were taken of the fish, but Siamese fishermen estimated it to be 10 wa (Siamese fathom) long. Unfortunately, there is a considerable elasticity involved in translating the Siamese fathom into metric units – averages range from one fathom equalling 1.7 to two metres (5 ½ to 6 ½ ft). The average in this instance was 1.8m (6ft), hence an 18m (59ft) fish; several authors have since argued that 18 metres was probably an overestimation. Despite the considerable length that this species obviously has the potential to reach, whale sharks commonly attain 12 to 14 metres (40 to 46 ft) in length.

Second in the ranking for largest shark species is held by the Basking shark (Cetorhinus maximus). The largest accurately measured specimen was 11.5 m (38 ft) and weighed in at 4,500kg (9,900 lbs). Prior to this, the largest individual came from Norway and measured 9.8m (32 ft). Other authors have reported greater lengths; Parker and Scott (1965) reported a theoretical maximum of 12.26 m (40.22 ft), which was later corrected to 12.76m (41.86 ft). Furthermore, the late Mike Holden reported a maximum length of 13.72m (45ft) in his paper for Sea Fisheries Research (1974). Generally speaking, Basking sharks typically reach between seven and nine metres (23 to 30 ft). Indeed, in a paper presented to the first meeting of the European Elasmobranch Association at the Novotel, Birmingham in October 1996, Sam Pollard reported that of more than 3,000 Basking sharks spotted off the UK, only eight percent (i.e. about 240 individuals) were over eight metres (26ft) in length.

The Megamouth shark (Megachasma pelagios) is oft cited as the third largest shark species, but this crown actually goes to the Great White (Carcharodon carcharias). Shark literature is rife with overestimations of Great White size, some suggesting that these fish may reach more than 11 metres in length. A paper by John Randall in the journal Science back in July 1973 challenged these reports, and suggested a maximum size of eight metres (26ft) based on bite marks on whale carcasses from South Australia. Today, most white sharks encountered fall into the 3.7 to 4.9 m (12 to 16 ft) range, although ReefQuest’s CEO, R. Aidan Martin, notes that White sharks may reach a theoretical maximum of 7.1 m (23ft) and weigh as much as 2,300kg (5060 lbs). According to the Canadian Shark Research Centre, the largest accurately measured Great White shark was a female caught in August 1989 at Prince Edward Island off the Canadian coast (North Atlantic) and measured 6.1 m (20 ft).

Since the Megamouth Shark (right) was first discovered – purely by accident off Oahu in November 1976 – only 18 specimens have been confirmed. The largest of these was a male caught off the Philippines in February 1998, and then a female caught off the Japanese coast two months later, both measuring 5.49 m (18ft). It is wholly possible that these sharks attain lengths greater than 5.5 m, but there are simply no data to verify this.

Dwarf LanternsharkSo, if the Whale shark, Basking shark, Great White and Megamouth are the largest selachians in the oceans today, what are the smallest, and where do the British species fit into this? The smallest species of shark is still a matter of some contention among biologists. Leonard Compagno attempted to answer this question in his 1988 review of the systematics of the Carcharhiniformes (the whaler sharks). The problem here seems to be that we currently don’t have sufficient data on most of the smallest shark species to establish a clear winner. There have been several contenders for the world’s smallest shark title, and most popular literature has considered the Spined Pygmy Shark (Squaliolus laticaudus) the smallest. In 1974, a paper in the Indian Journal of Fisheries, reported the capture of some Smooth dogfish (Eridacnis radclffei) from the Gulf of Mannar. Of the 15 adult females found, a 15cm (6 in) individual had large ovarian eggs. Unfortunately, the presence of ovaries is not necessarily a sign of maturity, and more often than not, detailed histological analysis of the eggs is required. In his review of this topic, R. Aidan Martin proposed that the aptly named Dwarf lanternshark (Etmopterus perryi - above) should be tentatively considered the smallest extant (living) shark species, based on developing embryos found in one 19cm (7.5 in) individual. (Image courtesy of WikiMedia Commons)

Sharks inhabiting UK waters tend not to reside at either size extremity, the exception being the Basking Shark. With the Basking shark as the largest British shark, the Greenland shark (Somniosus microcephalus)probably follows in second place. These sluggish deepwater sharks can reach lengths of 7.3m (24ft) and weigh a hefty 760kg (1,675 lbs). Common Threshers (Alopias vulpinus) are also a well-known UK shark species, and these sharks can reach 7.6m (25ft) in length and weigh 348kg (767 lbs). However, the Thresher (also referred to as the Fox shark) has an epicaudal lobe (i.e. the upper part of its tail) that is almost as long as its body, which is kind of cheating in the largest shark stakes! UK waters also play host to several reasonably large summer visitors. The Blue Shark (Prionace glauca) is known to reach lengths of 4m (13ft) and weigh 200kg (441 lbs), and the Shortfin Mako (Isurus oxyrinchus) also reaches 4m (13ft), but can weigh over 500kg (1,102 lbs). Another summer visitor, the Porbeagle shark (Lamna nasus) grows to 3.5m (11.5 ft) and can weigh almost 230kg (507 lbs). Our year-round residents tend to be smaller than the summer visitors, with the flattened Common Angel Shark (Squatina squatina) attaining a maximum of 2.5m (8ft) and weighing 80kg (176 lbs); the Tope shark (Galeorhinus galeus) reaches 2m (6ft) and 45kg (99 lbs) whilst the bottom-dwelling Smallspotted Catshark (Scyliorhinus canicula) and Greater Spotted Catshark (Scyliorhinus stellaris) reach one and 1.7 metres (3.3 and 5.5 ft), respectively.

UK ray species tend, overall, to be smaller than their selachian kin. The Common Skate (Dipturus batis) is probably the UK’s largest batoid, reaching almost 3m (10ft) and weighing in at a maximum of 97kg (214 lbs). The Blonde (Raja brachyura) and Thornback Rays (Raja clavata - left) both attain maximum lengths of about 1.2m (4ft) and weigh a maximum of 14.3kg (31.5 lbs) and 18kg (40 lbs), respectively. Undulate Rays (Raja undulata) reach about one metre (just over 3ft) in length, whilst the Common Stingray (Dasyatis pastinaca) and Smalleye Ray (Raja microocellata) reach about 90cm (3ft). (Back to Menu)

Blue SharkColour: Sharks can range in colour from grey through most shades of blue and even into yellows and pinks. The UK shark fauna also vary in colour. The Basking shark – along with many of the catsharks – tend to be brown, with white or black markings. At the other end of the scale are the Shortfin Mako and the aptly named Blue shark (Prionace glauca, pictured right, which are usually a striking cobalt-blue colour with a white belly. One distinctive feature of shark colouration is that the dorsal (back) surface is always darker than the ventral (underside) surface. A dark dorsum helps the shark to blend into the seabed -- when viewed from above -- while a white dorsum blends the shark with the bright, sunlit, surface. The point of all this is to make the shark more cryptic, and thus less obvious to a potential prey item or predator. Hence, White sharks have a dark grey back and bright white ventrum (the boundary between the two is referred to as a line of demarcation) and are, therefore, rarely seen hunting over sandy bottoms (where a dark shape would be easily detected against the light sand. Rays too have a broad colour range, although most of the species seen around the UK coast tend to be a sandy brown colour, with darker brown or black markings. The Smalleye (or Painted) Ray is an exception to this, being almost pale pink with white spots and a red rostrum (nose). The Undulate Ray is a muddy-brown colour with dark green-brown lines and spots. (Back to Menu)

Distribution: Elasmobranchs are known from almost every ocean on our planet, with some even found in freshwater lakes and rivers. In their 1995 paper in the Journal of Aquariculture and Aquatic Sciences, Leonard Compagno and the late Sid Cook listed 68 elasmobranch species that were either obligate freshwater dwellers (i.e. were found only in freshwater), penetrated far into freshwater but were essentially marine, or were only found marginally in freshwater. Of these 68 species, 17 were sharks and 51 were batoids. In terms of marine distribution, sharks and rays are found from the tropics to the poles, with some eight shark species having been recorded from the chilly waters of the Arctic. The most cosmopolitan of our shark and ray species inhabit all British territorial waters (i.e. out to 12 nautical miles – 22km – from shore), including the North Atlantic, English Channel and North Sea. These species are the Lesser and Greater Spotted Catsharks, and the Spiny Dogfish (Squalus acanthias) along with the Common Skate and the Thornback Ray. Most of the other species seem to be found only in small pockets, or narrow strips around the UK. (Back to Menu)

Longevity: The longevity of sharks and rays still remains an enigma for the vast majority of species. In 1998, Susan Smith, David Au and Christina Show published a paper in the journal Marine and Freshwater Research looking at the intrinsic rebound potential (i.e. how rapidly they recover from intense fishing pressure) of 26 species of pelagic sharks. Their results showed that many of these sharks didn’t reach maturity until they were more than ten years old, leading to a considerable length of time being required to rebuild over-exploited populations. Indeed, it is now well known that elasmobranchs grow and mature slowly, and probably grow continually throughout their lives. It is believed that many elasmobranch species may replace as little as two-percent of their population annually. The task of ageing elasmobranchs is also more difficult than it is for bony fishes – elasmobranchs don’t have otoliths (ear stones) – and continual replacement of their teeth means that dental wear-and-tear can’t be used as an indication of age (as it is in many mammals). As such, most elasmobranch ageing studies rely on staining of vertebrae. It seems that many sharks lay down two distinctive bands; dark bands are deposited in winter and light in summer. Staining the vertebrae -- often with silver nitrate -- and counting the bands in a similar fashion to gauging the age of a tree, is a popular technique for ageing elasmobranchs. Tagging studies are a big help in contributing to our data pool on shark longevity, especially when tagged sharks are injected with tetracycline upon landing; tetracycline is a broad-spectrum antibiotic that fluoresces under ultraviolet light. Consequently, if that shark is re-captured two years later, a sample of vertebrae will show how many bands have been deposited in that time and thus allow us to gauge an estimate of the shark’s age. Of course, there are some inherent pitfalls with this method, primarily the fact that band deposition is almost certainly related to the growth rate, which itself is related to environmental conditions. Thus it is possible that a shark will lay down more bands during plentiful years than in years when pickings are slimmer. Another way of estimating age is using mathematical equations (e.g. von Bertalanffy growth curves), but the mathematical details of these are well out of the scope of this website!

Smoothhound SharkPitfalls aside, we do have age information for some of the more frequently encountered species. The Great White shark, for example, is estimated to live to 60 years old, and calculated to reach maturity at 15 years old. Of the UK shark species, probably the longest-lived is the Spiny Dogfish (Squalus acanthias), the oldest recorded individual being 75 years old. Tope sharks (Galeorhinus spp. - left) can reputedly live for as many as 55 years, whilst the oldest Smoothhounds (Mustelus spp.) live for about 25 years. The large pelagic Blue shark (Prionace glauca) has been known to live as long as 20 years, and probably longer. A recent paper in Fishery Bulletin by Lisa Natanson and two colleagues, gave the maximum age for Porbeagle sharks (Lamna nasus) based on vertebral band readings as 24 to 25 years old. However, the scientists found that – on the basis of longevity calculations – Porbeagles in an un-fished population may live as long as 46 years. Age data for batoids are more difficult to come by. However, estimations for the Common stingray (Dasyatis pastinaca) have been as high as 100 years, although Ali Ismen, of the Canakkale Onsekiz Mart University in Turkey, estimated a maximum age of ten years -- using longevity calculations -- in his recent paper in the journal Fisheries Research. Tagging studies involving the Common Skate () off Scotland are suggesting a maximum age of as much as 40 years, an age close to the 50 years proposed by Marie-Henriette Du Buit in her 1972 paper. (Back to Menu)

Shark sexingSexing: Sexing sharks is a relatively simple task. Male sharks have elongated edges to their pelvic fins that roll up forming sausage-like protrusions called claspers -- or more accurately, myxopterygia (pronounced: mix-op-ter-ridge-ee-a) as they are not actually used for clasping the female -- the front opening of which is the apopyle and the rear opening (as it is a tube) the rhipidion. The claspers are intromittant organs that are used to transfer sperm from the male’s ductus deferens to the female’s uterus. Some species have small hooks and a spur on the clasper to keep it in place during sperm transfer. Female elasmobranchs don’t have claspers, only the opening into which the clasper fits and the waste products exit (called the cloaca – pronounced clow-ache-kah). Females generally grow to larger sizes than males of the same species and some species exhibit sexual segregation for much of the year. (Back to Menu)

Activity: Many shark species are considered nocturnal, although propuscular (active at dawn and dusk) would probably be a better description. During the afternoon, some species can be found resting on the seabed or in caves (e.g. the Whitetip Reef shark, Triaenodon obesus), while others swim casually around their preferred hunting or socialising grounds. It should be pointed out that although most species of shark increase their activity rhythms during the twilight hours, they are opportunistic feeders and will not pass-up the opportunity of a meal, merely because the clock says that it’s not dinnertime yet. Indeed, groups of sharks can be seen feeding in the middle of the day off Australian or South African beaches if conditions lead to concentration of prey species. It was originally thought that most of the species of shark frequently encountered were nocturnal hunters, but we are gradually beginning to get a different picture emerging through ongoing research. For example, many considered that the Great White shark was a nocturnal predator; work on White shark vision by Prof. Samuel Gruber at Bimini Field Station has since revealed that White sharks have a good colour visual acuity, suggesting that this species is actually a diurnal (daylight) predator. However, evidence that certain species of Hammerhead shark exhibit an increased sensitivity to bioelectric fields at nightfall, suggests that some sharks do hold nocturnal predatory behaviour. (Back to Menu)

Home Ranges: The home ranges of elasmobranch fishes have only been studied in detail for a few species; probably the best-studied example is the Lemon shark (Negaprion brevirostris). A study by Sam Gruber, John Morrissey and the late Donald Nelson tracked nine sharks for varying lengths of time during the early 1980s. From the results, it appears that this shark has a home range that may vary considerably on an individual basis. A mature female that was tracked for almost five days occupied an area of 93 square kilometres (36 sq-miles), while a juvenile female spent the majority of the 19 hours for which she was tracked within an area of 18 square kilometres (7 sq-miles). These home ranges have also been found to overlap with other species and members of the same species. Indeed, territoriality has yet to be conclusively demonstrated in any species of shark.

Portjackson SharkStudies on other sharks have revealed quite the opposite to the results found for Lemon Sharks. It was always considered that Great White sharks off California were largely coastal species, which bred off Southern California and then migrated a short distance south to feed on pinnipeds (seals and sealions). However, a paper by six marine biologists spanning three institutions found that this is not the case. The paper, published in January last year (2002) found that these sharks were, in fact, highly migratory; the furthest ranging of the six sharks tagged travelled from where it was tagged off California west to Hawaii – a distance of some 4,500km (2,800 mi). (Photo: A juvenile Portjackson shark, Heterodontus portusjacksoni)

An interesting paper presented to the American Elasmobranch Society’s 1999 Annual Meeting in Pennsylvania by Michael Robinson at the University of Miami suggested that home range in sharks may be related to body size and metabolic rate. After doing the math's for the Horn shark (Heterodontus fransisci), Scalloped Hammerhead (Sphyrna lewini), Grey Reef shark (Carcharhinus amblyrhynchos) and the Lemon shark, Robinson found that the only species for which this scenario seemed to apply was the Lemon shark. He suggested that, as this shark has a high metabolic rate -- for an elasmobranch, at any rate -- it requires a relatively larger area over which to forage for food.

Studies tracking the movements and looking at home ranges in batoids are rare, as these fish are largely over-shadowed by their more formidable cousins, the sharks. (Back to Menu)

Predators: See related Q/A (Back to Menu)

ANATOMY AND SENSES

Sharks have an unmistakable fusiform (bullet-shaped) body form, with eight fins attached: two dorsal (back) fins, two pectoral (side) fins, two pelvic fins (underneath towards the tail), a single anal fin (underneath, just in front of the tail) and a caudal (tail) fin. The dorsal and anal fins prevent the shark from yawing (i.e. moving side-to-side about its vertical axis), while the pectoral and pelvic fins guard against rolling and pitching (i.e. head up and tail down, then v/v cyclically). The caudal fin provides the primary propulsive force, although it is also associated with lift in some species. There are two types of caudal fin observed in sharks: heterocercal (the top lobe is larger than the bottom) and homocercal (symmetrical) types. The sharks with homocercal tails tend to be the large, fast swimming species including the Great White, Shortfin Mako (Isurus oxyrinchus) and Porbeagle (Lamna nasus). Heterocercal tails tend to be associated more with bottom-dwelling species, although they are also a trait of the Carcharhinoid (Whaler and Reef) sharks.

The caudal fin of most extant batoid species has been demoted to almost vestigial status, with most skates and rays swimming via undulatory movements of their expanded, wing-like pectoral fins.

I suspect that when asked what the fastest animal in the world is, most people’s reply would be the Cheetah (Acinonyx jubatus) but they’d be wrong, even with the top speed of 60mph (100kph) over short (less than 550m or about a third of a mile) distances on level ground! The Cheetah is the fastest mammal on earth -- closely followed by the Pronghorn Antelope (Antilocapra americana) -- but the fastest animal on the planet is a fish. In speed trials off Florida, a Sailfish (Istiophorus platypterus) took 91m (almost 300ft) of line in three seconds, equivalent to a swimming velocity of 68mph (109kph). The argument of precisely how fast elasmobranchs can swim has been a centre for debate in the popular literature for many years. The purposed ‘top speed’ for the Shortfin Mako shark has been variously reported from 22 to 55mph (35 to 88kph). Indeed, in his 1984 (and subsequently 2001 revision) catalogue of extant shark species, Leonard Compagno described Isurus oxyrinchus as “the peregrine falcon of the shark world”. According to ReefQuest Director and shark biologist R. Aidan Marin, a relative of Isurus -- the Great White shark -- packs a considerable amount of propulsive power, attaining estimated speeds of 30mph (48kph). Such bursts of speed draw on white muscle mass -- so-called because the lack of blood vessels makes the muscle appear white -- that burn carbohydrate in the absence of oxygen. The lack of oxygen means that the oxidation of the carbohydrate substrate is incomplete and glucose is oxidised to produce lactic acid. When the lactic acid reaches a threshold, it must be detoxified (which requires oxygen) and the intense activity of the white muscle stops (in humans, this build up of lactic acid causes a ‘stitch’). Thus, these intense bursts of speed cannot be maintained for prolonged periods of time; for most of the time elasmobranchs (and other fish) rely on red muscle masses for normal cruising speeds -- frequently between 0.2 and one metre per second (half to 3ft per second) depending on species -- and use white muscle only during hunting or predator escape manoeuvring. (Back to Menu)

Senses
Elasmobranchs have an awe-inspiring battery of senses that help them locate and catch their food.

Blue shark eyeVision: In his 1974 bestseller Jaws, the late Peter Benchley wrote of the white shark: “The eyes were sightless in the black and the other senses transmitted nothing extraordinary to the small, primitive brain.”  Contrary to this popular misconception (which remained dogma for many years), sharks have good eyesight, and many even have colour vision. The structure of an elasmobranch eye is similar to that of most vertebrates, although there are some differences. Probably the most obvious difference is that elasmobranch lenses are larger and almost spherical in shape (most vertebrates have a smaller, stretched lens that appears oval) because -- as the seawater and the cornea (the transparent membrane in front of the eye) are roughly the same density (unlike land-based vertebrates which have a cornea more dense that the surrounding air) -- the cornea cannot contribute to the focussing of an image, leaving that entirely up to the lens. Consequently, a large powerful lens is required, although rather than changing the shape of the lens (as humans do), elasmobranchs change its position – the lens moves back to focus on distant objects and vice versa. Studies conducted by Samuel Gruber and funded by the Office of Naval Research in the US during the mid-to-late 1970s were instrumental in dispelling the myth that sharks have poor eyesight. One specific paper of Gruber’s, published in 1985, looked at the visual system of the Great White shark. Gruber and his co-workers found that the White shark had the lowest rod (sense shades of light) to cone (involved in detection of colours) cell ratio of any shark studied to that point. The Great White apparently has a rod:cone ratio of 4:1 (i.e. for every four rod cells, there is a cone cell); most sharks studied to that point had ratios around 10:1. The upshot of this is that the Great White seems to have the best colour vision of the 15 elasmobranch species for which Gruber et al. had data. Sharks are also able to see distances of up to 20m (almost 66ft) depending on water conditions.

As one might expect for a crepuscular predator, shark’s eyes are adapted for low-light conditions. Lying immediately behind the retina is a layer of cells covered in a guanine-like crystalline substance. During daylight, special cells called melanoblasts slide from the base of each of the plates, covering it completely. During dark or poorly illuminated conditions, the melanoblasts are drawn back, exposing the silvery plates. This layer of cells is referred to as a tapetum lucidum (“bright carpet”) and acts as a kind of mirror to reflect light that would otherwise pass through the retina (and be lost) back into the eye. Therefore, the shark or ray is able to utilize more of the available light, giving it a significant advantage during nighttime hunts. The tapetum structure is also found in many nocturnally predatory species (e.g. cats, dogs, crocodiles, etc.) and some nocturnally herbivorous species (e.g. deer) and causes the phenomena known as eye shine.

Most of the studies looking at vision in elasmobranchs have used restrained sharks, and these fish have always been shown to be slightly hyperopic (longsighted) and thus were thought to have difficulty focusing on close objects. However, more recent studies, by Robert Hueter at the University of Florida, have shown that restraining sharks may cause them to contract their lenses giving a false impression of farsightedness. By bouncing beams of infrared light off the retina of free-swimming juvenile Lemon sharks, Hueter and his colleagues were able to demonstrate that the sharks were able to focus on both near and distant objects. (Back to Menu)

Smell (Olfaction): Sharks are notorious for their sense of smell, and seldom is a discussion on sharks complete without some mention of the irksome “a shark can smell a single drop of blood in an Olympic-sized swimming pool” statement. An Olympic swimming pool contains about one megalitre (one million litres, or almost 265, 000 gallons) of water.  Side-stepping the indefinite volume of a drop, and assuming a large drop of 1ml, sharks should thus be able to detect a singe “drop” of blood in 1000 million drops of water! More commonly, the figure presented is closer to a single drop in about 90 litres (25 gallons) of water – i.e. one molecule of blood in a million molecules of water. There are few laboratory studies that have looked at just how accurate such statements are, and most have been carried out on batoids. On his ReefQuest site, R. Aidan Martin notes that certain rajoids (skates) can detect concentrations as low as one molecule of serine (one of the non-essential amino acids involved in the production of Cystine, an amino acid involved in detoxification of the body) in 1015 molecules of water. Indeed, it was originally thought that as much as 70% of a shark’s brain was put aside for olfactory (smell) purposes. However, a more recent study has shown that the true value is closer to 18% in the White shark -- the largest proportion of any shark studied to date -- and can vary considerably between species. A dogfish (Squalus) was shown to have 6%, catsharks (Scyliorhinus) 14% and a whaler shark (Carcharhinus) 3% of olfactory area by Leo Demski and Glenn Northcutt in their 1996 paper on the brain and cranial nerves of the White shark. A shark’s nostrils are blind sacs (i.e. they don’t connect to the respiratory tract and have no function in breathing as they do in mammals) -- referred to as nares -- that are covered by a flap of skin (Flaps of Schneider) that direct the water through the nare in a sigmoidal (S-shaped) motion. The nare is lined with convoluted lamellae (plates of tissue) onto which free amino acids attach and elicit an electrical impulse to the shark’s brain. (Photo: Nasal slit of a Sixgill shark, Hexanchus griseus)

It was originally considered that the bizarre laterally-expanded prebranchial cephalofoil (i.e. hammer-shaped head) of the Hammerhead sharks (of the Sphyrinidae family) may have evolved to provide these sharks with an increased area over which to detect smells (the so-called “enhanced olfaction” hypothesis). However, recent research by scientists at the University of California has suggested this is unlikely to be the reason. The researchers found that none of the eight extant (living) species of hammerhead shark were better at detecting scent that their ‘smaller headed’ Carcharhinid relatives, and that the sphyrinids and carcharhinids had the same size area of olfactory epithelium (despite the hammerheads having more structural lamellae).

Taste (Gustation): A more detailed variant of smell -- hence if you get a cold and lose your sense of smell you also tend to lose your taste sense -- sharks have taste buds located just behind their teeth. In a similar manner to that observed in humans, dissolved chemicals attach to the epithelium of the taste bud and generate an electrical impulse to the brain, which the shark then interprets on the basis of its potential edibility. (Back to Menu)

Touch (Tactility): Elasmobranchs and all other fish have two types of touch sense: contact tactility and distant tactility. The contact tactility, as the name suggests, is where nerves in the skin fire upon the application of pressure (a similar way to our sense of touch). The distant tactility is achieved via a Lateral Line system. This lateral line is a row of pits (pores) that encircle the fish’s head and continue in a single line along the body to the tail on either side. These pits have a sensory neuromast inside that senses changes in water pressure through the distortion of a hair. Thus, if the fish approaches an object, the water between the two objects gets shoved into an ever decreasing space, the fish can detect this and judge their proximity to said object. This lateral line system can be used to detect vibrations from objects up to 200m (656ft) away.

There is also some evidence to suggest that the pit neuromasts may also be involved in the taste process. Back in 1967, Yasuji Katsuk and a team of scientists from the University of Hawaii were able to demonstrate that shark pit organs responded, not only to changes in ambient salinity, but also had a dramatic neural response to meat and blood. The teeth of sharks are also highly innervated, and some consider that -- without the hands possessed by their hominoid counterparts -- they use their extremely dextrous jaws to investigate unfamiliar objects in their environment.

Ampullae of LorenziniElectroreception: Elasmobranchs possess the rather unique ability -- shared by only a few other creatures on the planet, such as the Duck-billed Platypus (Ornithorhynchus anatinus) -- of being able to detect electric fields generated by other living organisms. The precise mechanisms and details of this get complicated, so I will drastically simplify it for the purpose of this summary. The sense works via a series of pores distributed over the shark’s head -- referred to as the Ampullae of Lorenzini (left), named after Stefano Lorenzini who first described them in 1678 -- and, according to a paper presented to the 2003 AES Meeting in Brazil by Darryl Whitehead of the University of Queensland, these pits can average as many as 2052 in the Bull Shark (Carcharhinus leucas). Each pore leads to a canal lined with a potassium-rich jelly and into a sac containing a receptive hair cell. These cells are receptive to weak DC (direct current) and low frequency (1 to 4 Hz with rapid high-frequency drop of at 16 to 20 Hz) AC (alternating current) fields. Various studies by Adrianus Kalmijn at the Scripps Institution of Oceanography in California have shown that elasmobranch electrosense is very sensitive, with some elasmobranchs being able to detect electrical activity down to five billionths of a volt at a distance of up to 33cm (1ft). To put this into perspective the movement of a Plaice’s (Pleuronectes platessa) operculum (gill cover) generates an electrical signal 5 million times higher than the minimum threshold of detection for elasmobranchs. Electrical charges tend to dissipate readily in seawater, meaning that the ampullae are only accurate to a distance of 20 to 30cm (8 to 12 inches) away from the object. As well as detection of potential food, it is also believed that sharks can use their ampullae for detecting the Earth’s magnetic field (which they may use to navigate an otherwise feature-poor ocean) and may also be used in mate recognition. An intriguing new paper, published recently in Nature, suggests that sharks are also able to detect changes in temperature with their ampullae. (Back to Menu)

Food and Feeding: It would take the rest of this website to list all the species of animal, plant and the inanimate objects that sharks have been documented to consume. Elasmobranchs feed on bony fishes, annelids (worms), crustaceans (crabs, lobsters, shrimps, copepods, etc.), molluscs (clams, mussels, cuttlefish, octopuses etc.), reptiles, mammals (marine and terrestrial), birds, other elasmobranchs; algae and seagrass have been found in the stomachs of some sharks (although it is not clear whether it was ingested intentionally of during the consumption of another desired prey item). Popular literature is also rife with stories of sharks having consumed some of the more bizarre items to be found in the sea. Pieces of wood and plastic, cow and sheep bones, licence plates, sacking, jars, sheets of rubber and even bizarre items like suits of armour have apparently been swallowed by various sharks. Many of these items are purported to have been found in the stomach of the species of shark named a “garbage can with fins” by certain popular science authors, the Tiger shark (Galeocerdo cuvier). Perhaps more disturbing is the fascination with the macabre objects that have been found in the stomachs of some of the more notorious sharks – these include various human appendages. Although I will deal with the notion of shark attack in another section of this site (see Shark Attack), I should stress here that sharks DO NOT actively prey on humans, if they did there would be veritable smorgasbord along the world’s beaches every weekend.

Sharks and rays tend to prey primarily on fish and crustaceans, although the Tiger, Great White, Porbeagle and possibly the Thresher Shark (Alopias vulpinus) are known to consume seabirds. Indeed, Tiger sharks are famed for turning up at the Hawaiian Islands with stark regularity each year to feed on fledging Black-footed Albatross (Dimedea nigripes) that take to the sky for the first time since hatching on the island of Gardener Pinnacles -- a small island about 1600 km (1000 mi) from the main island of Hawaii -- some three months earlier. There are also reports in the literature of the Thresher shark using its highly elongated upper caudal lobe to thwack seabirds resting on the surface.  Perhaps the world’s most notorious shark, the Great White, is famed for feeding on pinnipeds (seals and sealions) and dolphins. It is frequently stated in shark books that as the White shark grows, it changes its diet from fish to mammals. This is erroneous; throughout its entire life cycle, Carcharodon feeds primarily on fish, supplementing its diet with marine mammals when it grows to a large enough size to suitably subdue them. There is no ontogenetic shift in White sharks – as there is in Tiger sharks – whereby they ‘switch’ from feeding on fish as juveniles to feeding on mammals as adults.

Whitetip Reef shark jawsStudies on captive elasmobranchs have found that many of the reef sharks consume between 0.5 and 1.5% of their bodyweight per day, with juvenile animals consuming more than adults. The percentage is higher for the mackerel sharks (i.e. Great White, Mako and Porbeagle), which need about 3.1% of their body weight per day. It should be noted that these values are averaged over time, and that a given shark need not necessarily feed daily. Indeed, a group of researchers working on Great Whites off California in the late 1970s calculated that 30kg (65 lbs) of whale blubber could sustain the, almost 5m (15ft), shark for as long as six weeks. This is impressive, not least considering that many lamnid sharks are warm-bodied. By this, I mean that they are able to maintain parts of their body (usually the eyes, brain and stomach) a few degrees warmer than the surrounding water. It is important to make clear that this is NOT the same as the -- rather poorly denominated -- “warm-bloodedness” observed in mammals. These sharks do not maintain a stable internal temperature and if the water temperature decreases, so to does their core temperature. Instead, their core temperature dogs the water temperature, although there is a thermal lag (i.e. if the water temperature falls by three degrees, then an hour or so later the shark’s core temperature will drop by three degrees). Studies on the White shark have found that muscle temperature may be three to five degrees Celsius higher than ambient, whilst stomach temperatures may be as high as 14 deg-C (57 deg-F) above that of the surrounding water. Similarly, Shortfin Mako sharks (Isurus oxyrinchus) have been found to maintain body temperatures as high as 10 deg-C (50 deg-F) above ambient (although, curiously, the Longfin Mako –- Isurus paucus -– has the equipment to raise its body temperature, but doesn’t appear to do so) and the Salmon shark (Lamna ditropis), a relative of the Porbeagle, may have core temperatures approaching 13.6 deg-C (56.5 deg-F) above ambient. (Photo: Jaws of a Whitetip Reef shark, Triaenodon obesus)

The method by which sharks achieve this ‘warm-bodiedness’ is via an arrangement of blood vessels called a rete mirable (“wonderful net”), or more commonly, a counter-current heat exchange system. Basically, the system works by passing blood from the muscles on its way back to the gills (warmed by metabolic activity in the muscle masses) very close to blood from the gills on its way to the viscera (internal organs). The respective blood vessels split into numerous smaller capillaries, which intertwine with each other, and the heat from the gill-bound blood diffuses down a thermal gradient into the viscera-bound blood. If this were not the case, the heat generated during metabolism would be lost to the water over the gills (as it is in most other elasmobranchs). By maintaining a higher internal temperature, the sharks have increased muscle power and more rapid digestion; warm-bodiedness also permits them to tolerate greater extremes of temperature, although offloading of heat may cause problems in tropical regions.

The feeding mechanics of elasmobranchs are well studied. In sharks, the bite sequence begins with a lifting of the snout, followed by a lowering of the mandibular cartilage (bottom jaw) and a protrusion of the platoquadrate cartilage (upper jaw) to expose the teeth. The mandibular cartilage then drops further to expose the bottom teeth. After the bite, the snout drops and the jaws return to their normal position. Until this mechanism was fully understood, it was considered that sharks had to roll onto their backs in order to bite, but the lifting of the snout and ‘swinging out’ of the jaws means that rolling is not required. In the smaller sharks, there is often a suction phase to the bite, involving an expansion of the pharynx (mouth and throat). The bite sequence in rays is similar to that of sharks, although -- according to studies on the Cownosed ray (Rhinoptera bonasus) by Philip Motta and Desiree Sasko at the University of Florida -- the bite sequence begins with closure of the spiracles (openings on the head above the eye that leads to the gills). With the spiracles closed the lower jaw drops and the upper jaw protrudes, the lower jaw then raises to pin the prey against the platoquadrate cartilage and the jaws are brought back into their resting position in the mouth; the spiracles are then re-opened. A significant difference in the way the rays feed when compared with their selachian (shark) kin is in their teeth. Many species of batoid feed on shellfish and, as such, have flat ‘pavement-block’ shaped teeth that are used for crushing, rather than biting. Although some sharks also possess these molar forms (e.g. Horn sharks), many have the typical awl-like or stereotypical triangular teeth frequently associated with Jaws image.

Should an elasmobranch ingest something unpalatable, several observations from the wild suggest that they are capable of regurgitating it. Moreover, some species have actually been observed to regurgitate their stomachs, wash them out and then re-swallow them. In a paper published in the journal Nature in April 2000, David Sims and colleague Paul Andrews found that this stomach eversion could be induced in Thornback rays (Raja clavata) with an injection of the emetic (vomit-inducing) chemical veratrine hydrochloride. The authors report that the rays vomited up, rinsed and then swallowed their stomach again; the process of everting and re-swallowing the stomach took about 4.5 seconds. The specific mechanism by which elasmobranchs are able to evert their stomachs is still unknown, although Sims and Andrews suggest that it probably involves the fish relaxing its stomach muscles and rapidly increasing the pressure in its abdominal cavity. Alternatively, peristaltic (i.e. a sort of Mexican-wave of muscle movement) motion along the gastric tract may be involved. Whatever the specific cause, stomach eversion has been observed in several species of elasmobranch, and in some instances seems to be associated with a stressful situation. (Back to Menu)

Undulate RayBreeding Biology: In many ways the reproductive modes of elasmobranchs are more akin to those seen in mammals than in most other fish species. All elasmobranch species practice internal fertilization, which is accomplished with the aid of the claspers. It is important to distinguish that the clasper is only a rolled edge of the pelvic fin, it is not the penis seen in mammals. After what may be a considerably long courtship -- during which the male shark will pursue the female (often with his nose close to her cloaca, suggesting that the female may emit some form of chemical communiqué of her oestrous) and bite at her hind quarters and fins -- the female chooses a sperm donor. Immediately prior to sperm transfer, the male will seize the female’s pectoral fin and rotate his body to allow him to bend one of his claspers forward and insert it into the female’s cloaca; which clasper is used may depend on which of the ovaries is functional. In some species, only one of the female’s ovaries is receptive (often the right-hand one, according to Rodney Steel in his 1995 book Sharks of the World). The male shark has two small sacs located just underneath the skin at the base of the claspers, which are filled with seawater and used to flush the seminal fluid out of the clasper. Indeed, some authors have suggested that these sacs may be used to give the female a contraceptive ‘douche’ (i.e. a jet of water to wash out any competitor’s sperm). Batoids have a muscular gland at the base of the clasper that serves to force out the sperm. The sperm fertilizes the mature ova as they travel along the oviduct to the uterus. Once in the uterus, the ova begin their development, and the male’s job is effectively complete. In some species, the female may retain packets of sperm (referred to as spermatophores) in her uterus, and release the sperm at a time that is best suited to her breeding cycle. (Photo: An Undulate ray, Raja undulata.)

At parturition (birth), the pups exit the cloaca and pull against the umbilical cord until it snaps and they swim away. The second method, aplacental viviparity (also referred to as ovoviviparity), involves the sharks being retained in the uterus, without a placental connection, until they are sufficiently developed to be able to fend for themselves. There are two forms of aplacental viviparity: yolk sac aplacental viviparity (where the embryo is nourished by the yolk of the ovulated egg) and aplacental viviparity with oophagy (where the developing embryos feed on the stream of unfertilised eggs coming from the ovaries)

In many batoid species, the aplacental viviparity with yolk sac is modified to the extent that embryo absorbs a rich uterine secretion called Histotroph. In the Sandtiger shark (Carcharias taurus), the phenomenon of oophagy (“egg-eating”) is taken a step further, to a scenario referred to as adelphophagy (“brother eating”). As the name suggests, the largest embryo in each of the two uteri fights and consumes its siblings – hence only a single embryo is usually born from each uterus. Whilst adelphophagy is only known from the Sandtiger, oophagy is known from many (and suspected in all) lamnoids (i.e. Great White, Porbeagle, Basking shark, etc.) and is also found in two members of the Psueudotriakidae (a hound shark and a catshark) and also in the Tawny Nurse shark (Nebrius ferrugineus).

The third, and final, method of reproduction observed in elasmobranchs is oviparity (egg laying). Here, the developing embryo is encased in a tough theca (egg case) often attached to underwater structures (i.e. seaweeds, corals, etc.) with the aid of sticky tendrils (as in the Smallspotted catshark, Scyliorhinus canicula) or wedged into crevices on reefs (as in the PortJackson shark, Heterodontus portjacksoni). The exception seems to be several skate species (e.g. the Common Skate, Dipturus batis), which seem to just leave their eggs in a pile on the seafloor; there are no oviparous ray species. The shark egg cases are composed of a protein molecule similar to collagen, whilst skate appear to be composed of six major structural proteins. Upon comparison of the structural robustness of skate and catshark eggs, the shark thecas appear considerably more rigid than those of the skate. The egg cases must protect the developing embryo from the corrosive effects of seawater, whilst also staving off attacks by the many predators of the eggs. Elasmobranch thecas are very telolecithal (rich in yolk) and as such represent a delectable source of carbohydrate and protein for a potential predator. David Cox and Thomas Koob have written several papers documenting the predation of elasmobranch eggs by several species, especially Buccinids (whelks). The developing shark or skate will remain in the egg case until it has exhausted its yolk sac, after which it will breakout and swim away. The length of time the young elasmobranch spends in the egg case varies greatly according to species and water temperature; the Smallspotted catshark spends about eight to ten months in its theca (in UK waters), whilst the Heterodontids (Horn sharks) can spend between five months and a year in the capsule. Some species develop a special tooth or spine that they use to pries their way free of the egg capsule; the tooth then falls out a few days post-hatch.

Smallspotted Catshark egg
Eggcase of a Smallspotted catshark (Scylliorhinus canicula), showing the sticky tendrils used to anchor the egg to structures on the seabed. The coin in the photo is about 2cm (just under an inch) in diameter.

It is worth mentioning that these modes of reproduction are not necessarily as well defined as I have made them appear here. In reality, these reproductive traits may blend to the extent that it is possible to differentiate as many as eight variations on these three primary modes.

Whatever the mode of reproduction, most elasmobranchs have a hemimetabolous life cycle -- the young look like small adults, although there may be colouration differences -- and the pups are ready to feed on solid food immediately after birth. The generally conceived notion is that sharks do not exhibit parental care –- unless you consider the mother stopping feeding prior to and during birth in viviparous sharks, so that they don’t eat their young as soon as they’re born, parental care! -– although there are anecdotal reports of young and adult sharks swimming together (e.g. in the Basking Shark, Cetorhinus maximus) in groups which may eventually lead to a change in our perception. One commonly asked question at the aquarium is “why don’t sharks care for their young?” and the short answer is “because they don’t have to”! Having been privileged to be present at the birth of several mammals, the first thing that struck me was how totally dependant the neonates (newborns) were on their parent (or parents). This is not the case in sharks; the neonatal sharks have no requirement for additional parental input (i.e. for food, warmth, or after-dinner conversation!). (Back to Menu)

Behaviour: I know many people who have devoted their lives to the study of shark and ray behaviour, and I concede that I could never hope to do them justice even if I were to devote my entire site to the topic. Hence I shall give a very brief summary of some of the best-documented behaviours, especially those I have personally observed and get asked about at the Blue Reef Aquarium. For a more comprehensive description of various shark and batoid related behaviours, I would recommend a visit to ReefQuest.

Grey Reef sharkProbably one of the most frequently cited behavioural exhibitions is the agonistic display seen in certain reef sharks when pursued by a diver or submersible. This behaviour is perhaps most frequently observed in the Grey Reef shark (Carcharhinus amblyrhynchos - right). Carcharhinus amblyrhynchos is a reef dweller that often aggregates to form loose groups of up to 100 individuals during the day, disbanding at night to search for food. If a Grey Reef shark is relentlessly pursued by a diver or submersible it begins a characteristic swimming display, during which it hunches its back, lowers its pectoral fins and raises its snout. This is accompanied by a more exaggerated swimming style in a wide, sinusoidal path. If whatever is pursuing the shark ceases and moves away, the exaggerated swimming stops and the shark returns to its normal cruising motion. Should the pursuer continue, then the shark will ordinarily attack. Some interesting experiments carried out by the late Donald Nelson on Grey Reef sharks at Enewetak Atoll in the tropical Indo-Pacific Ocean, have helped elucidate the situations that may elicit an agonistic display in these species. In one 1981 paper, Nelson summarizes that which was known about this behaviour in Carcharhinus amblyrhynchos. Nelson notes that partially cornered sharks and lone individuals were more prone to agonistic displays than those in open water or in groups. The swimming motion is highly exaggerated and very obvious, suggesting that it has no relation to feeding (predators rarely give prey warning of an imminent attack), and the observation that the sharks don’t exclude other sharks from their vicinity made the author question whether the display had much to do with territoriality. Interestingly, Nelson also notes a case where a lone Grey Reef shark approached a diver and displayed a “mild or moderate posture” without any provocation on the diver’s behalf. Similarly, there are reports that this shark only displays exaggerated swimming in certain areas, being docile and almost timid in other areas – an observation that does lend some credence to the theory of territoriality, although territoriality has yet to be scientifically demonstrated in any shark species. The reason behind this display is still something of an enigma and, as Nelson points out, under such a variety of conditions, there is no single motivation that can adequately account for all scenarios. My personal belief is that, this display – variations of which have also been reported in the Great White, Silky (Carcharhinus falciformis), Shortfin Mako, Blacknose (Carcharhinus acronatus), Silvertip (Carcharhinus albimarginatus) and Bonnethead Sharks (Sphyrna tiburo) – is probably anti-predatory in most instances, although it may serve as a broad-spectrum communication that has various ‘meanings’ depending on the specific conditions. That said, it is interesting that the other reef sharks that Nelson studied failed to exhibit this swimming behaviour – with the exception of a Silvertip Shark that displayed a “mild threat display” reported by Nelson et al. in a subsequent paper for the Bulletin of Marine Science in 1986 – opting instead to flee from the pursuer.

It is really only in the last decade-or-so that one of the most awe-inspiring sharks has come under the spotlight of rigorous behavioural study. In a series of papers presented in the volume Great White Sharks: The biology of Carcharodon carcharias published in 1996, various biologists put forward papers dealing with some behaviours observed in the world’s largest extant predatory fish. One paper in particular by Wesley Strong of the Cousteau Society in Virginia described a thwart-induced behaviour he termed “Repetitive Aerial Gaping” or RAG. Dr Strong observed RAG in six White sharks (three male and three females) during his study off South Australia. The display was exhibited when the bait was pulled away immediately prior to contact with the shark, and consisted of the shark raising its head out of the water, rolling on its side and opening and closing its mouth in a sequence of slow yet rhythmic partial gapes; the display sequences averaged 10 seconds. Analysing each instance separately, Strong proposed a number of hypotheses including thwarting, aggression reduction, extended feeding attempt, and predator-prey communication. He concluded that RAG is probably a “manifestation of frustration incurred during a series of thwarted feeding attempts and may serve to reduce intra-specific [between individuals of the same species] aggression”.

Another paper in the same volume documented novel, “tail slap and breach” behaviours in White Sharks at the South Farallon Islands off Central California. The authors filmed 129 feeding incidents over the three years between 1988 and 1991, 83 of which involved some variant of the Tail Slap (TS). TS consists of the shark rolling onto its side, lifting its tail out of the water and slamming it down onto the water’s surface with considerable force. The resultant splash of water is usually directed at the other shark.

The authors also documented Breaching behaviour (BR), consisting of the shark raising two-thirds of its body out of the water at a 30 to 60 degree angle, and landing flat. These behaviours were interpreted as a social signal between sharks vying for food, based on the observation that the shark whom the TS was directed at either returned the TS or withdrew to allow the tail-slapper to feed. The BR behaviour, as it displaces more water than TS, was considered to be a more intense form of the TS message.

Possibly the most common behaviour exhibited by the native sharks and rays at the Blue Reef is that of an apparent ‘treading water’. During this display, the fish will turn from its normal horizontal path and swim vertically surface-ward. Upon reaching the surface, it will raise about half of its body out of the water, and continue in an apparent bid to ‘swim into the air’. This behaviour usually occurs when people approach the open-top tank. The precise motivation for this behaviour is something that the aquarium staff are frequently asked to interpret. Interpretation of behaviour is often tricky and fraught with the danger of introducing a human bias. However, I consider that this behaviour probably has several purposes dependant on whereabouts in the tank it is performed. For example, this behaviour is most frequently elicited at the periphery of the tank, and in such instances possibly represents the fish attempting to transverse the barrier it has encountered (i.e. the glass tank walls) by swimming over it. Some visitors consider the behaviour to be investigatory, although given the refractive differences between the water and air, it seems unlikely that the fish would be able to clearly distinguish any aerial objects. When performed in the centre of the tank, it seems unlikely that the fish was attempting an escape from the enclosure. It is possible that the vibrations caused by this ‘treading water’ motion might be involved in some form of inter- or intraspecific communication, but this would require further study. Another possible explanation is that these fish are seeking contact from members of the public at the tank side. Prior to Blue Reef taking over the aquarium, it used to be under the management of the SeaLife Centre chain and the pool was a touch pool, where customers could (under supervision) touch and stroke the elasmobranchs. The protocol under Blue Reef is such that patrons are not permitted to touch the fish. Several observations from aquariums across the globe indicate that the fish may actually ‘enjoy’ the human contact. Consequently, were this the case for Blue Reef’s local elasmobranchs, these fish may be attempting to resume a ‘pleasurable’ interaction. The final, and realistically the most probable is that the sharks and rays approach the visitors looking for food. The elasmobranchs in this specific tank are fed by hand from the surface and may thus consider any visitor to the tank a potential food-bearer. (Back to Menu)

Great White sharkInteraction with Humans: Although many Pacific Island cultures worshiped (and indeed, still do worship) sharks, sadly this demeanour is sorely lacking in the Western world. With the exception of an -- admittedly growing -- body of people, there is still a considerable need to educate people to the plight faced by sharks in the present day – preferably BEFORE we lose these magnificent and keystone (of vital importance to the ecosystem) predators for good. At least part of the problem lies in the fact that Sharks have been part of human myth and legend for centuries, and it is difficult to break such entrenched dogma. For more information on sharks in folklore, I would recommend a visit to Alex Buttigeig’s Sharkman’s World site.

I have devoted a separate area of this site to dealing with the concept of shark attack (see: Shark Attack) and as such will gloss over it here. There are several interactions between sharks and humans that many consider of benefit to both parties. A good example of such an interaction is ecotourism dives in several of the most beautiful parts of the world. For example, about £60 ($90 or €83) will buy a certified diver a dive with some of the spectacular elasmobranch fauna of Maui.  However, recently there has been some arguement that such dives -- especially where feeding of the sharks or rays is involved -- are not good for the fish and, worse still, may ‘train’ them to associate humans with an easy meal! If such arguements are true (and I should point out that there is no conclusive evidence to support such suggestions) then it is unfortunate because, when it comes to trying to dispel the many archaic myths surrounding sharks, shark diving is probably one of our greatest weapons. On dry land, I like to think that we have the power to change people’s perceptions of these magnificent fish at the aquarium, but I realistically I feel that many people don’t come to the aquarium to learn. Even on school trips, most of the children seem painfully unwilling to take in any of the information with which they are presented.

Possibly the biggest threat to the survival of elasmobranchs (and, indeed, many fish species) is that of overfishing. Sharks in particular are largely targeted for their fins, which are something of a delicacy in the Far East. More information about the plight of sharks and shark fining can be found on the Shark Trust and Environmental Literacy Council’s websites and a summary can be found on my Hunting Wildlife page.

It not all doom-and-gloom - much good has come from the introduction of ecotourism and, although this is certainly not without its problems, people are gradually beginning to realise that maybe sharks are worth more alive than they are dead. Hopefully this epiphany has not come too late! We are also learning a great deal about shark behaviour and biology through the various tagging studies happening across the world. (Back to Menu)

Related Q/A:

Q: Are sharks primitive?
Q: How do Whale and Basking sharks grow so big eating such small fodder?
Q: How do sharks and rays control their buoyancy without a swimbladder?
Q: How do elasmobranchs crush hard prey with a 'soft' skeleton?
Q: When did sharks first appear on Earth?
Q: Do sharks have any predators other than Man?
Q: How is it that marine mammals are able to see underwater, while we are not?
Q: How come the largest mammal is bigger than the largest fish?

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