sand tiger shark
- Also called:
- sand tiger, sandtiger, or gray nurse shark
- Related Topics:
- sand shark
- bigeye sand shark
- Galeoidei
- sawtooth sand tiger
sand tiger shark, any of three species of sharks classified in the genera Carcharias and Odontaspis in family Odontaspididae (order Lamniformes), named for their predatory habits and tendency to frequent inshore habitats in tropical and temperate oceans worldwide.
Distribution
The name sand tiger shark typically refers to C. taurus (and C. tricuspidatus, which was once considered a separate species), which inhabits parts of the northwestern and east-central Atlantic Ocean north of the Equator, the Mediterranean Sea, and parts of the Indian and Pacific oceans, especially in the waters surrounding Australia. Two other species, which were once classified with C. taurus in genus Carcharias because of their morphological similarities, are also considered sand tiger sharks. They are the smalltooth sand tiger shark (O. ferox), which is distributed worldwide but is found primarily in the Indian and Pacific oceans and the Mediterranean, Caribbean, and Tasman seas, and the bigeye sand tiger shark (O. noronhai), which is found in open-ocean habitats primarily in the west-central Atlantic, central Pacific, and central Indian oceans.
Natural history
Sand tiger sharks are large with stocky bodies that range in length from about 2.25 meters (7.4 feet) in C. taurus to 3.6 meters (11.8 feet) in smalltooth sand tiger sharks; however, larger smalltooth sand tiger sharks measuring up to 4.5 meters (14.8 feet) in length and weighing 289 kg (637 pounds) have been reported. All three species have pointed snouts and large mouths set at the bottom of the head that extend beyond the back of the eyes. Their five gill slits do not extend to the top of the head. The two dorsal, or top, fins lack the frontal spines found in some other sharks. An upper precaudal pit, or indentation, is located at the top of the body just in front of the tail fin.
The front teeth of sand tiger sharks are generally large and daggerlike, with a single central cusp and tiny cusplets on each side of the main cusp. In the upper jaw, smaller but similarly structured intermediate teeth separate the front teeth from the side teeth. In both jaws, the side teeth gradually decrease in size from the front of the mouth toward the back.
Smalltooth sand tiger sharks are distinguished from sand tiger sharks by their relatively larger size and the size and placement of their dorsal fin. In smalltooth sand tiger sharks, the dorsal fin is larger and set further forward than the dorsal fin of sand tiger sharks. The bigeye sand tiger shark differs noticeably from the others because of its darker coloration and large eyes that lack nictitating membranes. In addition, bigeye sand tiger sharks tend to inhabit the deeper waters of continental slope habitats.
The diets of sand tiger sharks and smalltooth sand tiger sharks consist of fish, crustaceans, and squid; although little is known about the feeding habits of bigeye sand tiger sharks, some sources note that they likely feed on the same kinds of prey. Sand tiger sharks and smalltooth sand tiger sharks are sought after by people for their meat, fins, and liver oil, but, since deeper-diving bigeye sand tiger sharks tend to avoid gill nets and other fishing gear, they are rarely caught.
The sand tiger shark (C. taurus) is notable for two features. Litters, which are produced only once every two years, contain only two pups, one from each uterus, with the surviving pup having engaged in a form of cannibalism in which all of the siblings in each uterus are eaten by the largest developing embryo. This habit is common among members of order Lamniformes, and research suggests that it also occurs in smalltooth and bigeye sand tiger sharks. In addition, the sand tiger shark is capable of swallowing and holding air in its stomach to simulate the swim bladder of bony fishes, which allows it to become neutrally buoyant and hover motionless in the water.
Sand tiger sharks are generally considered nonaggressive toward humans, but they have been implicated in rare attacks on people. However, none of the 36 reported attacks has resulted in a fatality. Most attacks can be attributed to harassment from swimmers, divers, or fishermen. Nevertheless, because of their bulk and sizable teeth, sand tiger sharks should be treated with caution. No attacks involving smalltooth sand tigers or bigeye sand tigers have been reported.
Conservation status
The International Union for Conservation of Nature and Natural Resources (IUCN) considers the sand tiger shark to be the most at risk of the three, listing the species as critically endangered. Sand tiger shark populations have fallen as a result of their slow reproductive rate, and the species is also targeted by some fishing operations and taken as bycatch (that is, captured unintentionally) by commercial fishing fleets. There is also some evidence to suggest that sand tiger sharks may be more sensitive to pollution and human activities, including petroleum exploration and disturbances caused by recreational boating and other activities in its nearshore habitats.
The IUCN lists the smalltooth sand tiger shark as a vulnerable species. Like its smaller-sized relative, it has a slow reproductive rate. Although it tends not to be targeted by commercial fishing, it is a frequent victim of bycatch, and researchers suggest that it might be as sensitive to shallow-water habitat disturbances brought on by human activities as the sand tiger shark. In contrast, the IUCN considers the bigeye sand tiger shark to be a species of least concern.