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Author: Garrick and Springer, 1964

Field Marks:
Small size, cigar-shaped body, small, spineless, nearly equal-sized dorsal fins far posterior on back, no anal fin, huge, triangular-cusped lower teeth without blades and in 19 rows, suctorial lips, very short, bulbous snout, asymmetrical caudal fin with short ventral lobe.

Diagnostic Features:
Snout very short, about length of eye; eyes set well forward on head, with extensive anterior binocular field; teeth in 29/19 rows, lower teeth enormous. Interdorsal space subequal to first dorsal base, space between second dorsal insertion and upper caudal origin about equal to second dorsal base; second dorsal height about 1.3 times first; pectoral fins rounded, pelvic fins smaller than dorsal fins; caudal fin small and noticeably asymmetrical, with a short ventral caudal lobe less than half length of dorsal caudal margin. Colour: no dark collar-marking over branchial region.

Geographical Distribution:
Western North Atlantic: off Alabama (USA), Gulf of Mexico. Western North Pacific: off Okinawa (Japan).

Habitat and Biology:
A little-known epipelagic and possibly bathypelagic shark, known from only two localities. Its infrequency of capture compared to I. brasiliensis suggests that it is either very localized in its distribution, or, perhaps more likely, is usually found in much deeper water than its congener. The small dorsal fins and caudal of this shark suggests that it is a weaker, less active swimmer than I. brasiliensis, that also takes advantage of a 'hepatic float'. However, its larger, more powerful jaws, bigger mouth and gigantic lower teeth (proportionately the largest in any living shark) equip it for taking larger bites out of its prey, something shown by a

This was as high and wide as the diameter of its mouth but over twice as long, and indicates with its suctorial lips and feeding apparatus that this shark is probably at least a facultative ectoparasite like its congener. Although this shark is probably capable of cutting circular plugs like I. brasiliensis, it apparently can readily gouge elongate plugs out of its victims, perhaps with a different action that the twistinq motion required to cut out cones.

A curious feature of this shark is its extremely short snout and anteriorly positioned eyes. When viewed frontally the largetooth cookiecutter apparently has a broad anterior field of vision, unlike I. brasiliensis with its longer snout and more posterolaterally positioned eyes; this apparently allows for binocular vision, and may be helpful in precisely locating its victims prior to attacking them.

Size:
Maximum total length at least 42 cm.

Interest to Fisheries:
None, except as a possible causative agent of "crater wounds" on fishes such as scombrids, billfishes, etc., and hence of negative interest to fisheries.

Type material:
Holotype: US National Museum of Natural History, USNM 188386, 423 mm adult female. Type Locality: Gulf of Mexico off Alabama, 28°58'N, 88°18'W, over water from 814 to 997 m deep.

Largetooth cookiecutter shark (Isistius plutodus)