Tiger shark

Tiger shark
Fossil range: 56–0 Ma[1]
Early Eocene to Present
Conservation status

Near Threatened (IUCN 2.3)[2]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: Carcharhiniformes
Family: Carcharhinidae
Genus: Galeocerdo
Müller & Henle, 1837
Species: G. cuvier
Binomial name
Galeocerdo cuvier
Péron & Lesueur, 1822
Tiger shark range
Synonyms

Squalus cuvier Peron and Lessueur, 1822
Galeocerdo tigrinus Müller and Henle, 1837

The tiger shark, Galeocerdo cuvier, is a species of requiem shark and the only member of the genus Galeocerdo. Tiger sharks are relatively large macropredators, capable of attaining a length of over 5 m (16 ft).[3] This shark typically reaches maturity at lengths of 2 to 3 m (6.6 to 9.8 ft).[4][5] It is found in many tropical and temperate oceans, and is especially common around central Pacific islands. Its name derives from the dark stripes down its body, which resemble a tiger's pattern and fade as the shark matures.

The tiger shark is a solitary, mostly night-time hunter. Its diet involves a wide range of prey, including fish, seals, birds, smaller sharks, squid, turtles, sea snakes, and dolphins.

While the tiger shark is considered to be one of the sharks most dangerous to humans, the attack rate is surprisingly low according to researchers[6]. The tiger is second on the list of number of recorded attacks on humans, with the great white shark being first.[7][8][9]. They often visit shallow reefs, harbours and canals, creating the potential for encounter with humans.

Tiger sharks are considered a near threatened species due to excessive finning and fishing by humans.

Contents

Taxonomy

The shark was first described by Peron and Lessueur in 1822, and was given the name Squalus cuvier.[10] Müller and Henle, in 1837 renamed it Galeocerdo tigrinus.[4] The genus, Galeocerdo, is derived from the Greek, galeos which means shark and the Latin cerdus which means the hard hairs of pigs.[4] It is often colloquially called the man-eater shark.[4]

The tiger shark is a member of the order Carcharhiniformes;[10] 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 includes some other well-known sharks such as the blue shark, lemon shark and bull shark.

Range and habitat

The tiger shark is often found close to the coast, in mainly tropical and sub-tropical waters worldwide,[7] though they can reside in temperate waters. Along with the Great White shark, Pacific sleeper shark, Greenland shark and sixgill shark, tiger sharks are among the largest extant sharks.[4] The shark's behavior is primarily nomadic, but is guided by warmer currents, and it stays closer to the equator throughout the colder months. The shark tends to stay in deep waters that line reefs but 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.[11]

Tiger sharks has been recorded at depths just shy of 900 metres (3,000 ft)[4] but is also known to move into shallow water - water that is normally thought to be too shallow for a species of its size.

Anatomy and appearance

Profile photo of shark, accompanied by remora, swimming just above a sandy seafloor
Juvenile tiger shark in the Bahamas

Size

One of the largest sharks in existence, the tiger shark commonly attains a length of 3 to 4.2 m (9.8 to 13.8 ft) and weighs around 385–635 kilograms (849–1,400 lb).[4] The largest specimen yet reported was a gigantic female caught in 1957. This specimen was 7.4 m (24 ft) long and weighed 3,110 kilograms (6,900 lb).[3]

Biology

Tiger sharks' skins can typically range from blue to light green with a white or light yellow underbelly. 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. Tiger sharks have small pits on the side of their upper bodies which hold electroreceptors called the ampullae of Lorenzini, enabling them to detect electric fields, including the bio-electricity generated by prey. 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 mminute 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. Its long upper tail provides bursts of speed. Tigers normally swim using small body movements. Its high back and dorsal fin act as a pivot, allowing it to spin quickly on its axis. Its 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, and unusually among sharks, its upper and lower teeth have dissimilar shapes. Like most sharks, however, its teeth are continually replaced by rows of new teeth.

Diet

The tiger shark is an apex predator,[12] and has a reputation for eating anything.[4] It also possesses the capability to take on large prey.[12] It commonly preys upon: fish (e.g. Teleost[12]), crustaceans,[12] mollusks,[12] dugongs,[12] seabirds,[12] seasnakes,[12] marine mammals (e.g. bottlenose dolphins,[13] spotted dolphins[14]), and sea turtles (e.g. green turtles[15] and loggerhead turtles[15]). The broad, heavily calcified jaws and nearly terminal mouth, combined with robust, serrated teeth enable the tiger shark to take on large prey like sea turtles and marine mammals.[16] In addition, excellent eyesight and its acute sense of smell enable it to react to faint traces of blood and follow them to the source. Due to high risk of predatory attacks, dolphins often avoid regions inhabited by tiger sharks.[16]

Tiger sharks also attack injured or ailing whales and prey upon them. A group of tiger sharks were once documented attacking and killing an ailing Humpback whale, in 2006, near Hawaii.[17] Tiger sharks also scavenge on dead whales. In one such documented incident, tiger sharks were observed scavenging on a whale carcass alongside great white sharks, Carcharodon carcharias.[18]

The ability to pick up low-frequency pressure waves enables the shark to advance towards an animal with confidence, even in murky water.[19] The shark circles its prey and studies it by prodding it with its snout.[19] When attacking, the shark often eats its prey whole.[19] Because of its aggressive feeding, it often mistakenly eats inedible objects, such as automobile license plates, oil cans, tires, and baseballs. For this reason, the tiger shark is often regarded as the ocean's "Garbage can".[20]

Swimming efficiency and stealth

Tiger sharks generally swim slowly, which, combined with cryptic coloration, may make them difficult for prey to detect in some habitats. Tiger sharks are specially well camouflaged against dark backgrounds.[16] 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.[16]

Reproduction

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).[5] Females mate once every 3 years.[20] 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 is the only species in its family that is ovoviviparous; its eggs hatch internally and the young are born live when fully developed.[4]

The young develop inside the mother's body for up to 16 months. Litters range from 10 to 80 pups.[4] A newborn is generally 51 centimetres (20 in) to 76 centimetres (30 in) long[4]. It is unknown how long tiger sharks live, but they can live longer than 12 years.[20]

Dangers and conservation

Photo of shark hung by its tail on the shore
A tiger shark caught in Kāneʻohe Bay, Oʻahu in 1966

Although shark attacks are a relatively rare phenomenon, the tiger is responsible for a large percentage of fatal attacks, and is regarded as one of the most dangerous shark species.[8][21] Tiger sharks are often found in river estuaries and harbours, as well as shallow water close to shore, where they are bound to encounter humans. Tiger sharks also dwell in river mouths and other runoff-rich water.[4][5]. 3 to 4 shark attacks occur per year on average in Hawaii and most attacks are non-fatal. This attack rate is surprisingly low considering that thousands of people swim, surf and dive in Hawaiian waters every day.[22]

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.[23] South African shark scientist Mark Addison demonstrated that they could be tamed somewhat in a 2007 Discovery Channel special.[24]

The tiger shark is captured and killed for its fins, flesh, liver. It is caught regularly in target and non-target fisheries. There is evidence of declines for several populations where they have been heavily fished, but in general they do not face a high risk of extinction. However, continued demand, especially 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 (IUCN).[25]

While shark fin has very little 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.[4]

In 2010, Greenpeace International has added the tiger shark to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."[26]

Mythology

Tiger sharks are considered to be sacred nā ʻaumākua (ancestor spirits) by some native Hawaiians, who think their eyeballs have special seeing powers. This aligns with the general known facts about sharks and their highly developed senses.

See also

For a topical guide to this subject, see Outline of sharks.

References

Footnotes

  1. ↑ Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Chondrichthyes entry)". Bulletins of American Paleontology 364: 560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=575&rank=class. Retrieved 2008-01-09. 
  2. ↑ Simpfendorfer (2000). Galeocerdo cuvier. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006. Database entry includes justification for why this species is near threatened
  3. ↑ 3.0 3.1 "Summary of Large Tiger Sharks Galeocerdo cuvier (Peron & LeSueur, 1822)". http://homepage.mac.com/mollet/Gc/Gc_large.html. Retrieved May 3, 2010. 
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 "Tiger Shark Biological Profile". Florida Museum of Natural History Icthyology Department. http://www.flmnh.ufl.edu/fish/Gallery/Descript/Tigershark/tigershark.htm. Retrieved 2005-01-22. 
  5. ↑ 5.0 5.1 5.2 "Galeocerdo cuvier Tiger Shark". Marine Bio. http://marinebio.org/species.asp?id=37. Retrieved 2006-10-14. 
  6. ↑ http://www2.hawaii.edu/~carlm/
  7. ↑ 7.0 7.1 Knickle, Craig. "Florida Museum of Natural History Ichthyology Department". www.flmnh.ufl.edu. http://www.flmnh.ufl.edu/fish/Gallery/Descript/Tigershark/tigershark.htm. Retrieved 2009-07-02. 
  8. ↑ 8.0 8.1 "ISAF Statistics on Attacking Species of Shark". Florida Museum of Natural History University of Florida. http://www.flmnh.ufl.edu/fish/sharks/Statistics/species2.htm. Retrieved 2008-05-04. 
  9. ↑ Daley, Audrey (1994). Shark. Hodder & Stroughton. ISBN 0-340-61654-7. 
  10. ↑ 10.0 10.1 "ITIS report, Galeocerdo cuvier". Taxonomic Information System. http://www.itis.usda.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=160189. Retrieved 2006-09-22. 
  11. ↑ "Galeocerdo cuvier". Fishbase. http://www.fishbase.org/Summary/SpeciesSummary.php?id=886. Retrieved 2006-09-28. 
  12. ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 Heithaus, Michael R. (2001). "Copyright: The biology of tiger sharks, Galeocerdo cuvier, in Shark Bay, Western Australia: sex ratio, size distribution, diet, and seasonal changes in catch rates". Environmental Biology of Fishes 61: 25–36. doi:10.1023/A:1011021210685 
  13. ↑ HEITHAUS, MICHAEL; DILL, LAWRENCE (2002). "FOOD AVAILABILITY AND TIGER SHARK PREDATION RISK INFLUENCE BOTTLENOSE DOLPHIN HABITAT USE". Ecological Society of America 83 (2): 480–491. doi:10.1890/0012-9658(2002)083[0480:FAATSP]2.0.CO;2. http://www.monkeymiadolphins.org/Pdf/Heithaus%20and%20Dill%202002.pdf 
  14. ↑ Maldini, Daniela (2003). "Evidence of predation by a tiger shark (Galeocerdo cuvier) on a spotted dolphin (Stenella attenuata) off Oahu, Hawaii". Aquatic Mammals (Hawaii: European Association for Aquatic Mammals) 29 (1): 84–87. doi:10.1578/016754203101023915. http://www.alaskasealife.org/New/Contribute/pdf/Maldini_2003.pdf. Retrieved 11 May 2010. 
  15. ↑ 15.0 15.1 Heithaus, Michael R.; A. Frid and L. M. Dill (2002). "Shark-inflicted injury frequencies, escape ability, and habitat use of green and loggerhead turtles". Marine Biology (Australia: Springer) 140: 229–236. doi:10.1007/s00227-001-0712-6. http://www.monkeymiadolphins.org/Pdf/Heithaus%20et%20al%202002.pdf. Retrieved 4 May 2010. 
  16. ↑ 16.0 16.1 16.2 16.3 Heithaus, Michael R. (2001). "Copyright: Predator–prey and competitive interactions between sharks (order Selachii) and dolphins (suborder Odontoceti): a review". Journal of Zoology 253: 53–68. doi:10.1017/S0952836901000061 
  17. ↑ "Humpback Whale Shark Attack: A Natural Phenomenon Caught on Camera". http://sanctuaries.noaa.gov/news/features/1106_sharkattack.html. Retrieved May 4, 2010. 
  18. ↑ Dudley, Sheldon F. J.; Michael D. Anderson-Reade, Greg S. Thompson, and Paul B. McMullen (2000). "Concurrent scavenging off a whale carcass by great white sharks, Carcharodon carcharias, and tiger sharks, Galeocerdo cuvier" (PDF). Marine Biology. Fishery Bulletin. http://fishbull.noaa.gov/983/13.pdf. Retrieved 4 May 2010. 
  19. ↑ 19.0 19.1 19.2 "Tiger Shark". ladywildlife.com. http://ladywildlife.com/animals/tigershark.html. Retrieved 2006-12-21. 
  20. ↑ 20.0 20.1 20.2 Ritter, Erich K. (15 December 1999). "Fact Sheet: Tiger Sharks". Shark Info. http://www.sharkinfo.ch/SI4_99e/gcuvier.html. Retrieved 2009-05-15. 
  21. ↑ Ritter, Erich K.. "Which shark species are really dangerous?". Shark Info. http://www.sharkinfo.ch/SI1_99e/attacks2.html. Retrieved 2009-05-15. 
  22. ↑ http://www2.hawaii.edu/~carlm/
  23. ↑ "Federal Fishery Managers Vote To Prohibit Shark Feeding". Western Pacific Regional Fishery Management Council. http://www.wpcouncil.org/press/2006Oct23_PRESSRELEASE_135CM.pdf. Retrieved 2007-06-15. 
  24. ↑ "Shark Week: 'Deadly Stripes: Tiger Sharks'". LA Times. 30 July 2007. http://latimesblogs.latimes.com/showtracker/2007/07/shark-week-dead.html. Retrieved 2007-09-28. 
  25. ↑ http://www.iucnredlist.org/apps/redlist/details/39378/0
  26. ↑ Greenpeace International Seafood Red list

Notations

External links