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Characteristics, distribution and ecology
Taxonomische classification
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Author: (Matsubara, 1936)

Field Marks:
A small, very distinctive oceanic shark, with huge eyes lacking nictitating eyelids, long gill slits, slender, spindle-shaped body, long-cusped, prominent teeth in a long, angular mouth with highly protrusible jaws, small pectoral fins, two small, spineless dorsal fins and an anal fin, weak keels and precaudal pits on the caudal peduncle, and an asymmetrical caudal fin with a long ventral lobe.

Diagnostic Features:
Trunk cylindrical and slender. Head short, much shorter than trunk; snout moderately long, pointed and bulbously conical, not greatly elongated, flattened and bladelike; eyes very large; mouth large, ventral on head; gill openings long, extending onto dorsal surface of head, all in front of pectoral fin bases; no gillrakers on internal gill slits; teeth large, the anteriors narrow and awl-like, the laterals more compressed and blade-like, with less than 30 rows in either jaw; two rows of anterior teeth on each side of upper and lower jaws, the uppers separated form the upper lateral teeth by a row of small intermediate teeth. First dorsal fin small, low, and angular, second dorsal smaller than first but larger than anal fin; second dorsal with a broad, nonpivoting base but anal fin pivotable; pectoral fins small, short and broad, muchshorter than head in adults; pelvic fins large, somewhat smaller than pectoral and first dorsal fins; caudal fin not lunate, upper lobe moderately long but less than half as long as rest of shark, lower lobe, short but strong; precaudal pits present, caudal peduncle slightly depressed and with low lateral keels.

Geographical Distribution:
Nearly circumtropical. Eastern Atlantic: South east of Cape Verde Islands, between them and Guinea Bissau, Guinea, Angola, and South Africa (southwest Cape seas). Western Indian Ocean: Mozambique Channel southwest of southern Madagascar. ? Eastern Indian Ocean: Bay of Bengal (possibly erroneous). Western North Pacific: Off Japan, Taiwan Island and the Koreas. Central Pacific: Marquesas Islands, Hawaiian Islands, open ocean between Marquesas and Hawaiian islands, open ocean between Hawaiian Islands and Baja California. Eastern Pacific: Off Costa Rica and Panama.

Habitat and Biology:
A rare to locally abundant oceanic, epipelagic and possibly mesopelagic shark, usually found offshore and far from land butsometimes occurring inshore and near the bottom, at depths from the surface to at least 300 m. The long body cavity, large liver, and small fins of this shark give it a superficial resemblance to Isistius, Squaliolus, Euprotomicrus, and other oceanic squaloids, and like these squaloids its extremely oily liver is probably important in maintaining neutral buoyancy. Its habits are little-known, but its firm body musculature, small precaudal fins, and large caudal fin suggests that it is relatively active but perhaps sporadically so. The large eyes of the crocodile shark suggest nocturnal or deepwater activity, and possibly a diel pattern of movement toward the surface at night and away from it in the day.

The crocodile shark is ovoviviparous and a uterine cannibal, with the young having yolk sacs at 3 or 4 cm long but resorbing them and subsisting on fertilized eggs and probably other embryos beyond this size. Number of young in a litter 4, 2 per uterus; egg cases formed in the oviducts have 2 to 9 fertilized eggs, but apparently only 2 of these survive, possibly through elimination of extra rivals. An interesting question is why do two young survive in each uterus in this shark and some other lamnoids, while in Eugomphodus taurus only one fetus per uterus is normally produced.

Feeding habits of this shark are little known, but its long, flexed teeth suggests small to moderately large, active oceanic prey. Of 5 specimens examined by the writer, the stomachs of 4 were empty and the fifth (a subadult male) had a number of small bristlemouths (gonostomatids) and possibly lanternfishes (myctophids) as well as small shrimps and squid beaks. The jaws of the crocodile shark can be protruded a considerable distance from its head.

The crocodile shark has never been involved in attacks on people and is regarded as not dangerous to people, particularly because of its small size and slender, non-cutting teeth; but its powerful jaws and jaw muscles and the size of its teeth should invite respect. It snaps strongly and vigorously when captured (S. Kato, pers.comm.) and can bite very hard.

Size:
This is the smallest living lamnoid, with maximum total length at least 110 cm; males adult at 74 to 110 cm, females adult at 89 to 102 cm, size at birth about 41 cm.

Interest to Fisheries:
This shark is primarily caught by the Japanese pelagic longline fisheries, but details are sketchy. Abe et al. (1969) noted that the species is often caught on tuna longlines, but discarded because of its small size and meat that is apparently unsuitable for the Japanese market. The liver of this species is very large and very high in squalene, and hence is of potential value.

Remarks:
The common name is derived from one of its Japanese common names, Mizu-wani (water crocodile or alligator). The synonymy of this shark follows Bass, d'Aubrey and Kistnasamy (1975b).

Type material:
Holotype: Imperial Fisheries Institute, Japan, Fish Spec. 1823, 735 mm male. Type Locality: Koti Fish Market, Japan.

Crocodile shark (Pseudocarcharias kamoharai)