Sand tiger shark


The sand tiger shark, grey/gray nurse shark, spotted ragged-tooth shark, or blue-nurse sand tiger, is a species of shark that inhabits subtropical and temperate waters worldwide. It inhabits the continental shelf, from sandy shorelines and submerged reefs to a depth of around. They dwell in the waters of Japan, Australia, South Africa, and the east coasts of North and South America. The sand tiger shark also inhabited the Mediterranean, however it was last seen there in 2003 and is presumed extirpated. Despite its common names, it is not closely related to either the tiger shark or the nurse shark.
Despite its fearsome appearance and strong swimming ability, it is a relatively placid and slow-moving shark with no confirmed human fatalities. This species has a sharp, pointy head, and a bulky body. The sand tiger's length can reach but is normally 2.2–2.5 m in length. They are grey with reddish-brown spots on their backs. Shivers have been observed to hunt large schools of fish. Their diet consists of bony fish, crustaceans, squid, skates and other sharks. Unlike other sharks, the sand tiger can gulp air from the surface, allowing it to be suspended in the water column with minimal effort. During pregnancy, the most developed embryo will feed on its siblings, a reproductive strategy known as intrauterine cannibalism i.e. "embryophagy" or, more colorfully, adelphophagy—literally "eating one's brother". The sand tiger is categorized as critically endangered on the International Union for Conservation of Nature Red List. It is the most widely kept large shark in public aquariums owing to its tolerance for captivity.

Taxonomy

The sand tiger shark's description as Carcharias taurus by Constantine Rafinesque came from a specimen caught off the coast of Sicily. Carcharias taurus means "bull shark". This taxonomic classification has been long disputed. Twenty-seven years after Rafinesque's original description the German biologists Müller and Henle changed the genus name from C. taurus to Triglochis taurus. The following year, Swiss-American naturalist Jean Louis Rodolphe Agassiz reclassified the shark as Odontaspis cuspidata based on examples of fossilized teeth. Agassiz's name was used until 1961 when three palaeontologists and ichthyologists, W. Tucker, E. I. White, and N. B. Marshall, requested that the shark be returned to the genus Carcharias. This request was rejected and Odontaspis was approved by the International Code of Zoological Nomenclature. When experts concluded that taurus belongs after Odontaspis, the name was changed to Odontaspis taurus. In 1977, Compagno and Follet challenged the Odontaspis taurus name and substituted Eugomphodus, a somewhat unknown classification, for Odontaspis. Many taxonomists questioned his change, arguing that there was no significant difference between Odontaspis and Carcharias. After changing the name to Eugomphodus taurus, Compagno successfully advocated in establishing the shark's current scientific name as Carcharias taurus. The ICZN approved this name, and today it is used among biologists.

Common names

Because the sand tiger shark is worldwide in distribution, it has many common names. The term "sand tiger shark" actually refers to four different sand tiger shark species in the family Odontaspididae. Furthermore, the name creates confusion with the unrelated tiger shark Galeocerdo cuvier. The grey nurse shark, the name used in Australia, is the second-most used name for the shark, and in India it is known as blue-nurse sand tiger. However, there are unrelated nurse sharks in the family Ginglymostomatidae. The most unambiguous and descriptive English name is probably the South African one, spotted ragged-tooth shark.

Identification

There are four species referred to as sand tiger sharks:
  1. The sand tiger shark Carcharias taurus
  2. The Indian sand tiger shark Carcharias tricuspidatus. Very little is known about this species which, described before 1900, is probably the same as the sand tiger C. taurus
  3. The small-toothed sand tiger shark Odontaspis ferox. This species has a worldwide distribution, is seldom seen but normally inhabits deeper water than does C. taurus.
  4. The Big-eyed sand tiger shark Odontaspis noronhai, a deep water shark of the Americas, of which little is known.
The most likely problem when identifying the sand tiger shark is when in the presence of either of the two species of Odontaspis. Firstly, the sand tiger is usually spotted, especially on the hind half of the body. However, there are several other differences that are probably more reliable:
  1. The bottom part of the caudal fin of the sand tiger is smaller.
  2. The second dorsal fin of the sand tiger is almost as large as the first dorsal fin.
  3. The first dorsal fin of the sand tiger is relatively non-symmetric.
  4. The first dorsal fin of the sand tiger is closer to the pelvic fin than to the pectoral fin.

    Description

Adult sand tigers range from to in length with most specimens reaching a length of around 2.2–2.5 m and to in weight. The head is pointy, as opposed to round, while the snout is flattened with a conical shape. Its body is stout and bulky and its mouth extends beyond the eyes. The eyes of the sand tiger shark are small, lacking eyelids. A sand tiger usually swims with its mouth open displaying three rows of protruding, smooth-edged, sharp-pointed teeth. The males have grey claspers with white tips located on the underside of their body. The caudal fin is elongated with a long upper lobe. They have two large, broad-based grey dorsal fins set back beyond the pectoral fins. The sand tiger shark has a grey-brown back and pale underside. Adults tend to have reddish-brown spots scattered, mostly on the hind part of the body. In August 2007, an albino specimen was photographed off South West Rocks, Australia. The teeth of these sharks have no transverse serrations but they have a large, smooth main cusp with a tiny cusplet on each side of the main cusp. The upper front teeth are separated from the teeth on the side of the mouth by small intermediate teeth.

Habitat and range

Geographical range

Sand tiger sharks roam the epipelagic and mesopelagic regions of the ocean, sandy coastal waters, estuaries, shallow bays, and rocky or tropical reefs, at depths of up to.
The sand tiger shark can be found in the Atlantic, Pacific and Indian Oceans, and in the Adriatic Seas. In the Western Atlantic Ocean, it is found in coastal waters around from the Gulf of Maine to Florida, in the northern Gulf of Mexico, around the Bahamas and Bermuda, and from southern Brazil to northern Argentina. It is also found in the eastern Atlantic Ocean from the Mediterranean Sea to the Canary Islands, at the Cape Verde islands, along the coasts of Senegal and Ghana, and from southern Nigeria to Cameroon. In the western Indian Ocean, the shark ranges from South Africa to southern Mozambique, but excluding Madagascar. The sand tiger shark has also been sighted in the Red Sea and may be found as far east as India. In the western Pacific, it has been sighted in the waters around the coasts of Japan and Australia, but not around New Zealand.

Annual migration

Sand tigers in South Africa and Australia undertake an annual migration that may cover more than. They pup during the summer in relatively cold water. After parturition, they swim northwards toward sites where there are suitable rocks or caves, often at a water depth ca., where they mate during and just after the winter. Mating normally takes place at night. After mating, they swim further north to even warmer water where gestation takes place. In autumn they return southwards to give birth in cooler water. This round trip may encompass as much as. The young sharks do not take part in this migration, but they are absent from the normal birth grounds during winter: it is thought that they move deeper into the ocean. At Cape Cod, juveniles move away from coastal areas when water temperatures fall below 16 °C and day length decreases to less than 12 h. Juveniles, however, return to their usual summer haunts and as they become mature they start larger migratory movements.

Behavior

Hunting

The sand tiger shark is a nocturnal feeder. During the day, they take shelter near rocks, overhangs, caves and reefs often at relatively shallow depths. This is the typical environment where divers encounter sand tigers, hovering just above the bottom in large sandy gutters and caves. However, at night they leave the shelter and hunt over the ocean bottom, often ranging far from their shelter. Sand tigers hunt by stealth. It is the only shark known to gulp air and store it in the stomach, allowing the shark to maintain near-neutral buoyancy which helps it to hunt motionlessly and quietly. Aquarium observations indicate that when it comes close enough to a prey item, it grabs with a quick sideways snap of the prey. The sand tiger shark has been observed to gather in hunting groups when preying upon large schools of fish.

Diet

The majority of prey items of sand tigers are demersal, suggesting that they hunt extensively on the sea bottom as far out as the continental shelf. Their diet consists mainly of teleosts and other elasmobranchs. Bony fish form about 60% of sand tigers' food, the remaining prey being sharks, skates, other rays, lobsters, crabs and squid. In Argentina, the prey includes mostly demersal fishes, e.g. the striped weakfish and whitemouth croaker. The most important elasmobranch prey is the bottom-living narrownose smooth-hound shark. Benthic rays and skates are also taken, including fanskates, eagle rays and the angular angel shark, with larger individuals feeding on a higher number of benthic elasmobranchs than smaller individuals. Stomach content analysis indicates that smaller sand tigers mainly focus on the sea bottom and as they grow larger they start to take more pelagic prey. This perspective of the diet of sand tigers is consistent with similar observations in the north west Atlantic and in South Africa where large sand tigers capture a wider range of shark and skate species as prey, from the surf zone to the continental shelf, indicating the opportunistic nature of sand tiger feeding. Off South Africa, sand tigers less than in length prey on fish about a quarter of their own length; however, large sand tigers capture prey up to about half of their own length. The prey items are usually swallowed as three or four chunks.