Temporal range: 50–0Ma Early Eocene to Present
J. P. Müller & Henle, 1837
Péron & Lesueur, 1822
|Tiger shark range|
The tiger shark, Galeocerdo cuvier, is a species of requiem shark and the only member of the genus Galeocerdo. Commonly known as sea tiger, the tiger shark is a relatively large macropredator, capable of attaining a length of over 5 m (16 ft). It is found in many tropical and temperate waters, and it is especially common around central Pacific islands. Its name derives from the dark stripes down its body which resemble a tiger's pattern, which fade as the shark matures.
The tiger shark is a solitary, mostly nocturnal hunter. Its diet includes a wide variety of prey, ranging from crustaceans, fish, seals, birds, squid, turtles, and sea snakes to dolphins and even other smaller sharks. Due to its tendency to swallow virtually anything it encounters, including non-edible manmade objects that linger in its stomach, the tiger shark has been nicknamed "the garbage can of the sea". The tiger shark is considered a near threatened species due to finning and fishing by humans.
The shark was first described by Peron and Lessueur in 1822, and was given the name Squalus cuvier. Müller and Henle in 1837 renamed it Galeocerdo tigrinus. The genus, Galeocerdo, is derived from the Greek galeos, which means shark, and the Latin cerdus, the word for the hard hairs of pigs. It is often colloquially called the man-eater shark.
The tiger shark is a member of the order Carcharhiniformes, the most species-rich order of sharks with more than 270 species also including the small catsharks and hammerhead sharks. Members of this order are characterized by the presence of a nictitating membrane over the eyes, two dorsal fins, an anal fin, and five gill slits. It is the largest member of the Carcharhinidae family, commonly referred to as requiem sharks. This family consists of mostly slender but powerful mid-to-large-sized sharks and includes some other well-known sharks, such as the blue shark (Prionace glauca), lemon shark (Negaprion brevirostris) and bull shark (Carcharhinus leucas).
Range and habitat
The tiger shark is often found close to the coast, mainly in tropical and subtropical waters throughout the world. Its behavior is primarily nomadic, but is guided by warmer currents, and it stays closer to the equator throughout the colder months. It tends to stay in deep waters that line reefs, but it does move into channels to pursue prey in shallower waters. In the western Pacific Ocean, the shark has been found as far north as Japan and as far south as New Zealand. A Tiger Shark tagged in the Caribbean has been tracked migrating Cape Cod: although Tiger Sharks are a tropical species, the warm Gulf Stream brings the coast of Cape Cod to within the extreme north of its range during the summer.
Tiger sharks can be seen in the Gulf of Mexico, North American beaches and parts of South America. It is also commonly known in the Caribbean Sea. Other locations where tiger sharks are seen include Africa, People's Republic of China, Hong Kong, India, Australia and Indonesia.
Certain tiger sharks have been recorded at depths just shy of 900 metres (3,000 ft) but some sources claim they move into shallow water normally thought to be too shallow for a species of its size. A recent study showed the average tiger shark would be recorded at 350 metres (1,100 ft), making it uncommon to see tiger sharks in shallow water. However, tiger sharks in Hawaii have been observed in depths as shallow as 10 ft and regularly observed in coastal waters at depths of 20 to 40 ft.
Anatomy and appearance
Among the largest extant sharks, the tiger shark ranks in size only behind the whale shark (Rhincodon typus), the basking shark (Cetorhinus maximus) and the great white shark (Carcharodon carcharias). Very large specimens of some other species such as megamouth sharks (Megachasma pelagios), Pacific sleeper sharks (Somniosus pacificus), Greenland sharks (Somniosus microcephalus) and Bluntnose sixgill sharks (Hexanchus griseus) can attain similar sizes as tiger sharks. The tiger shark commonly attains a length of 3–4.2 m (9.8–13.8 ft) and weighs around 385–635 kg (849–1,400 lb). Sometimes, an exceptionally large male tiger shark can grow up to 4.5 m (15 ft). Females are larger, and exceptionally big ones can reportedly measure over 5 m (16 ft). Per Guinness World Records, one female specimen caught off Australia reportedly measured 5.5 m (18 ft) long and weighed an exceptional 1,524 kg (3,360 lb), although her weight is thought to have been bolstered by her pregnant state at the time. Even larger specimens have been reported but are unconfirmed.
The skin of a tiger shark can typically range from blue to light green with a white or light yellow underbelly. The advantage of this is that when it is hunting for its prey, when prey looks at the shark from above, the shark will be camouflaged since the water below is darker. And when prey is below the shark and looks up, of course because of the sun, it is lighter so that the light underbelly will also camouflage the shark. This is known as countershading. Dark spots and stripes are most visible in young sharks and fade as the shark matures. Its head is somewhat wedge-shaped, which makes it easy to turn quickly to one side. They have small pits on the snout which hold electro-receptors called the ampullae of Lorenzini which enable them to detect electric fields, including the weak electrical impulses generated by prey,which helps them to hunt. Tiger sharks also have a sensory organ called a lateral line which extends on their flanks down most of the length of their sides. The primary role of this structure is to detect minute vibrations in the water. These adaptations allow the tiger shark to hunt in darkness and detect hidden prey.
A reflective layer behind the tiger shark's retina called the tapetum lucidum allows light-sensing cells a second chance to capture photons of visible light, enhancing vision in low light conditions. A tiger shark generally has long fins to provide lift as the shark maneuvers through water, while the long upper tail provides bursts of speed. Tiger sharks normally swim using small body movements. Its high back and dorsal fin act as a pivot, allowing it to spin quickly on its axis, though the shark's dorsal fins are distinctively close to its tail.
Its teeth are specialized to slice through flesh, bone, and other tough substances such as turtle shells. Like most sharks, its teeth are continually replaced by rows of new teeth.
Tiger sharks serve important roles in several habitats and in relationships with other species. In one study, experimenters examined the relationship between the habitat use of foraging dugongs, a large relative of the manatee, and the threat of tiger shark predation in Shark Bay, Australia. Shark abundance was determined by the rate at which they were caught in the target habitats, and habitats were categorized as shallow or deep. When tiger sharks were not present, dugong foragers used both shallow and deep habitats according to their relative food availability. When shark presence was high, dugongs overused deeper habitats which reduced their risk of predation. 
Using many of the same techniques, another study found similar results in bottlenose dolphins. When tiger shark density is high, dolphins rest in deeper waters to minimize the risks of predation. This behavior is exhibited despite the fact that food is more available in shallow environments.
The risk of predation by tiger sharks for relevant species fluctuates dramatically during and between years. One study of the tiger shark population in Shark Bay revealed a pattern of significant variation in shark density with a peak in the warm season and a trough in the cold season. Thus, tiger sharks are intricately involved with their local habitats and dominate some of the resources in those habitats.
Direct observation of foraging, using attached cameras, has revealed that tiger sharks prefer shallow habitats where their prey can be found. Despite ample encounters with prey, tiger sharks engage in relatively few fast chases after prey. This may be indicative of vigilant prey, in which case it is unsurprising that the sharks would not pursue as stealth is their primary predatory method.
This foraging method may account for the tiger shark’s diet, which includes a number of air-breathing animals. At the surface, during breathing, these animals have more difficulty detecting a predator below, which offers the tiger shark an opportunity to attack. The sharks are best suited to swim along the bottom to avoid detection.
Tiger sharks also feed on benthic prey (bottom-dwelling), which are most likely to detect the shark when they swim along the ocean floor. Swimming in surface waters may reduce detection from bottom-dwelling prey. To accommodate these different foraging strategies, tiger sharks have a variable swimming pattern when foraging that bounces from shallow to deep waters to maximize detection on benthic and air-breathing prey.
This bouncing swimming pattern may also be accounted for by a minimization of energy cost. Sharks are negatively buoyant and as a result, an energy-requiring ascent paired with an energy-less descent can produce significant energy savings. Interestingly, tiger sharks also power-stroke on their descent indicating that this energy hypothesis cannot be the sole purpose for their bouncing swim.
The tiger shark is an apex predator and has a reputation for eating anything. Young tiger sharks are found to prey largely on small fish as well as various small jellyfish, cephalopods and other mollusks. Around the time they attain 2.3 m (7.5 ft), or near sexual maturity, their prey selection expands considerably and much larger animals become regular prey. Numerous fish, crustaceans, sea birds, sea snakes, marine mammals (e.g. bottlenose dolphins (Tursiops ssp.), common dolphins (Delphinus ssp.), spotted dolphins (Stenella ssp.), dugongs (Dugong dugon), seals and sea lions), and sea turtles (including the three largest species: the leatherback (Dermochelys coriacea), the loggerhead (Caretta caretta) and the Green sea turtles (Chelonia mydas),) are regularly eaten by adult tiger sharks. The tiger shark also eats other sharks (including adult sandbar sharks (Carcharhinus plumbeus)), as well as rays, and will sometimes even eat other tiger sharks.
Due to high risk of predatory attacks, dolphins often avoid regions inhabited by tiger sharks. Tiger sharks may also attack injured or ailing whales and prey upon them. A group was documented attacking and killing an ailing humpback whale (Megaptera novaeangliae) in 2006 near Hawaii. The tiger shark also scavenges on dead whales. In one such documented incident, they were observed scavenging on a whale carcass alongside great white sharks.
Evidence of dugong predation was identified in one study that found dugong tissue in 15 of 85 tiger sharks caught off the Australian coast. Additionally, examination of adult dugongs has shown scars from failed shark attacks. Finally, dugong microhabitats shift similarly to those of known tiger shark prey when the sharks are abundant.
The broad, heavily calcified jaws and nearly terminal mouth, combined with robust, serrated teeth, enable the tiger shark to take on these large prey. In addition, excellent eyesight and acute sense of smell enable it to react to faint traces of blood and follow them to the source. The ability to pick up low-frequency pressure waves enables the shark to advance towards an animal with confidence, even in murky water. The shark circles its prey and studies it by prodding it with its snout. When attacking, the shark often eats its prey whole, although larger prey are often eaten in gradual large bites and finished over time.
Notably, terrestrial mammals, including horses (Equus ferus caballus), goats (Capra aegagrus hircus), sheep (Ovis aries), dogs (Canis lupus familiaris), cats (Felis catus) and brown rats (Rattus norvegicus), are fairly common in the stomach contents of tiger sharks around the coasts of Hawaii. Because of its aggressive and indiscriminate feeding style, it often mistakenly eats inedible objects, such as automobile license plates, oil cans, tires, and baseballs.
Swimming efficiency and stealth
All tiger sharks generally swim slowly, which, combined with cryptic coloration, may make them difficult for prey to detect them in some habitats. They are especially well camouflaged against dark backgrounds. Despite their sluggish appearance, tiger sharks are one of the strongest swimmers of the carcharhinid sharks. Once the shark has come close, a speed burst allows it to reach the intended prey before it can escape.
Males reach sexual maturity at 2.3 to 2.9 m (7.5 to 9.5 ft) and females at 2.5 to 3.5 m (8.2 to 11 ft). Females mate once every 3 years. They breed by internal fertilization. The male inserts one of his claspers into the female's genital opening (cloaca), acting as a guide for the sperm. The male uses its teeth to hold the female still during the procedure, often causing the female considerable discomfort. Mating in the Northern Hemisphere generally takes place between March and May, with birth between April and June the following year. In the Southern Hemisphere, mating takes place in November, December, or early January. The tiger shark is the only species in its family that is ovoviviparous; its eggs hatch internally and the young are born live when fully developed.
The young develop inside the mother's body for up to 16 months. Litters range from 10 to 80 pups. A newborn is generally 51 centimetres (20 in) to 76 centimetres (30 in) long. This shark typically reaches maturity at lengths of 2 to 3 m (6.6 to 9.8 ft). It is unknown how long tiger sharks live, but they can live longer than 12 years.
The tiger shark is captured and killed for its fins, flesh, and liver. It is caught regularly in target and nontarget fisheries. Several populations have declined where they have been heavily fished. Continued demand for fins may result in further declines in the future. Tiger sharks are considered a near threatened species due to excessive finning and fishing by humans according to International Union for Conservation of Nature.
While shark fin has very few nutrients, shark liver has a high concentration of vitamin A which is used in the production of vitamin oils. In addition, the tiger shark is captured and killed for its distinct skin, as well as by big game fishers.
In 2010, Greenpeace International added the tiger shark to its seafood red list, which is a list of fish commonly sold around the world, and which have a very high risk of being sourced from unsustainable fisheries.
Relationship with humans
Although shark attacks are a relatively rare phenomenon, the tiger shark is responsible for a large percentage of fatal attacks and is regarded as one of the most dangerous shark species. They often visit shallow reefs, harbors and canals, creating the potential for encounter with humans. The tiger shark also dwells in river mouths and other runoff-rich water. While the tiger shark is considered to be one of the sharks most dangerous to humans, the attack rate is low according to researchers. The tiger is second on the list of number of recorded attacks on humans, with the great white shark being first. On average, three to four shark attacks occur per year in Hawaii, and most attacks are not fatal. This attack rate is surprisingly low considering thousands of people swim, surf and dive in Hawaiian waters every day. Attacks by tiger sharks in Hawaiian waters have been shown to increase in the autumn, between September and November, when tiger shark females are believed to migrate to the islands to give birth. 
Between 1959 and 1976, 4,668 tiger sharks were culled in an effort to protect the tourism industry. Despite these efforts, attacks did not decrease. It is illegal to feed sharks in Hawaii, and interaction with them, such as cage diving, is discouraged. South African shark behavioralist and shark diver Mark Addison demonstrated divers could interact and dive with them outside of a shark cage in 2007 Discovery Channel special.
The tiger shark is considered to be sacred na ʻaumakua (ancestor spirits) by some native Hawaiians who think their eyeballs have special powers of visual perception. This aligns with the general known facts about sharks and their highly developed senses.
- For a topical guide to this subject, see Outline of sharks.
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|Wikimedia Commons has media related to Galeocerdo cuvier.|
|Wikispecies has information related to: Galeocerdo cuvier|
- "Galeocerdo cuvier". Integrated Taxonomic Information System. Retrieved 7 April 2006.
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