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Characteristics, distribution and ecology
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Author: (Quoy and Gaimard, 1824)

Field Marks:
Small size, cigar-shaped body, small, spineless dorsal fins far posterior on back, no anal fin, triangular-cusped lower teeth without blades and in 25 to 31 rows, suctorial lips, short, bulbous snout, nearly symmetrical caudal fin with long ventral lobe.

Diagnostic Features:
Snout moderately short, about length of eye; eyes anterior on head but sufficiently far back to lack an extensive anterior binocular field; teeth in 31 to 37/25 to 31 rows, lowers moderately large. Interdorsal space over twice first dorsal base, space between second dorsal insertion and upper caudal origin over twice second dorsal base; second dorsal height about equal to first; pectoral fins subquadrate, pelvic fins larger than dorsal fins; caudal fin large and nearly symmetrical, with a long ventral caudal lobe over 2/3 length of dorsal caudal margin. Colour: a prominent dark collar-marking over branchial region.

Geographical Distribution:
Oceanic and circumtropical. Atlantic: Bahamas and southern Brazil to Cape Verde Island, Guinea to Sierra Leone, southern Angola and South Africa, including Ascension Island. Southern Indian Ocean: Mauritius to New Guinea and Western Australia. Pacific: Japan and Lord Howe Island to Hawaiian and Galapagos Islands.

Habitat and Biology:
A wide-ranging tropical oceanic shark, epipelagic to bathypelagic in distribution. It is caught at night, sometimes at the surface but usually below it at depths between 85 and 3500 m, but its preferred depth range and maximum depth are uncertain. Apart from those captured at the surface specimens are generally taken in midwater nets fished over a wide depth range, and it is difficult to tell at what depth these sharks were captured. This shark is thought to be a vertical migrator on a diel cycle, coming to the surface and to the level of midwater trawl hauls at night and presumably dropping below this during the daytime. This implies a long vertical distance travelled, in excess of 2000 to 3000 m up and down in the ocean basins. These sharks are often caught near islands; this may imply an inshore pupping ground or merely the distribution of large potential victims. The cookiecutter shark may be capable of living in water of lower oxygen content than Euprotomicrus bispinatus or Squaliolus laticaudus, but this is hypothetical.

The small paired fins, long body cavity and enormous, oily liver of this shark point to its being neutrally buoyant and not dependent on forward motion and its fins for dynamic lift. The liver and body cavity is proportionately much larger than in Euprotomicrus bispinatus or Squaliolus laticaudus, and much more oil is present in its body cavity and gut. This may be an adaptation for greater depths than those attained by the other species, but may also compensate for its more highly calcified skeleton, which in turn may be necessary for supporting its activities in taking larger prey and gouging flesh from large animals. It can be quite quick and active when caught and can bite its captors if they are unwary.

This shark has luminous organs that cover the entire lower surface of its trunk with the exception of its fins and the dark collar marking. It is reported as glowing a bright, ghostly green.

Reproduction presumably ovoviviparous, but with embryos and litter size uncertain; 6 or 7 large eggs have been found in ovaries.

This shark has very powerful jaws and large teeth. It feeds on freeliving deepwater prey, including squid with bodies almost as large as itself, gonostomatids, and crustaceans, but is also a facultative ectoparasite on larger marine organisms. It has highly specialized suctorial lips and a strongly modified pharynx that allow it to attach to the sides of large bony fishes such as marlin, tuna, albacore, wahoo, and dolphinfishes, as well as dolphins and other cetaceans and even the megamouth shark (Megachasma). The shark then drives its razor-sharp sawlike lower dentition into the skin and flesh of its victim, twists about to cut out a conical plug of flesh, then pulls free with the plug cradled by its scooplike lower jaw and held by the hooklike upper teeth. This method of feeding leaves 'crater wounds' on victims which were long thought to be caused by bacteria or invertebrate parasites until Jones (1971) connected them to the cookiecutter shark. It has been hypothesized that the strong luminescense shown by this shark may serve to lure in other predators to attack it, with the result that the shark parasitizes them instead; incomplete crater wounds often show that the cookiecutter shark attacked its victims head on, perhaps after they attacked it. Unusual non-edible victims of this shark include nuclear submarines of the US Navy, which have had rubber sonardomes bitten by I. brasiliensis. Despite its rather vampire-like mode of feeding, it is not dangerous to people because of its small size and habitat preferences; the chances of it attacking a swimmer or diver are remote though possible.

An unusual habit of this shark, perhaps related to maintaining sufficient calcium levels in its body, is swallowing and possibly digesting its own lower teeth as they are replaced and become loose in entire series.

Size:
Maximum total length about 50 cm; males maturing at about 31 to 37 cm and reaching at least 39 cm, females maturing between 38 and 44 cm and reaching at least 50 cm.

Interest to Fisheries:
Of little interest to fisheries because of its small size and low abundance, but reportedly captured by bottom trawls and used for fishmeal in the eastern Atlantic. I. brasiliensis might be of slight negative interest to fisheries because the species gouges plugs of flesh from commercially important fishes, which may increase their mortality rate, but this is uncertain.

Type material:
Holotype: Muséum National d'Histoire Naturelle, Paris, MNHN A. 7787, 140 mm female. Type Locality: Brazil.

Cookiecutter shark (Isistius brasiliensis)