Author: Taylor et al., 1983
Field Marks:
One of three species of gigantic filter-feeding sharkss unmistakable with its extremely short but broadly rounded snout, very large and long head, huge terminal mouth that extends behind the eyes, numerous small hooked teeth, moderately long gill slits, internal gill slits lined with dense rows of papillose gillrakers, eyes without nictitating eyelids, two dorsal fins and an anal fin, caudal peduncle without keels, caudal fin asymmetrical, not lunate, but with a short and strong ventral lobe, and no light spots.
Diagnostic Features:
Trunk cylindrical and somewhat compressed, stout. Head very long, about length of trunk; snout extremely short, flattened and broadly rounded, not elongated and bladelike; eyes moderately large; mouth extremely large, terminal on head; gill openings moderately long, not cxtending onto dorsal surface of head, the last two over pectoral fin bases; unique gillrakers of fingerlike dermal papillae with cartilage cores fringing internal gill slits; teeth small, continuously varying and more or less awl-shaped, over 100 rows in either jaws, no anteriors, intermediate teeth or gaps in each upper dental band. First dorsal fin moderately large, semierect and angular; second dorsal less than half size of first but moderately large; pectoral fins large, narrow and elongated, much shorter than head in adults; pelvic fins moderate-sized, smaller than pectoral and first dorsal fins; anal fin smaller than second dorsal and with its base slightly behind second dorsal base, bases of both fins not pivotable; caudal fin not lunate, upper lobe long but less than half as long as rest of shark, lower lobe short but strong; precaudal pits present, caudal peduncle compressed and without lateral keels.
Geographical Distribution:
Known only from the type locality, but likely to be wide-ranging, like other gigantic, plankton-feeding sharks.
Habitat and Biology:
At present known only from the holotype, which was caught in the open ocean, in the epipelagic zone near the Hawaiian Islands at 165 m depth. This shark had been feeding on euphausiid shrimp, Thysanopoda pectinata, that averages 3.1 cm in length. As the shark has greatly reduced teeth, very numerous gillraker papillae, on its internal gill slit, and stomach packed with very small euphasiid prey, it can be properly considered a specialized filter feeder like the basking shark (Cetorhinus maximus) and whale shark (Rhiniodon typus). However, the flabby body, soft fins, asymmetrical caudal fin without keels, and weak calcificiation of the megamouth shark suggest that it is much less active than the whale and basking sharks. The only known prey of the megamouth shark is a common epipelagic and mesopelagic shrimp in the area where the shark was captured. The shrimp has a diel migration pattern with a range of 300 to 1100 m depth during the day; at night it is commonest at 150 to 500 m with a maximum range of 75 to 525 m. When captured, the megamouth shark was apparently at the upper depths where these shrimp are commonest, and possibly might have been feeding on them when it fouled itself by its mouth and teeth on a pair of parachutes being used as sea anchors by a US Navy research vessel. The megamouth shark is the only known selachian victim of the semiparasitc cookiecutter shark, Isistius brasiliensis, and may be especially vulnerable to Isistius attacks because of its soft skin, midwater habitat, and probable sluggishness. The feeding habits and habitat of the megamouth shark suggest that it will be a rare catch in the future, but it may show up in large purse seines operated by tuna boats. The feeding structures of this shark may allow it to feed on other pelagic invertebrates and even small midwater fishes.
Size:
The holotype is an adult male, at 446 cm total length.
Interest to Fisheries:
None at present.
Remarks:
This recently described shark, a giant pelagic filter-feeder, is perhaps the most spectacular discovery of a new shark in the Twentieth Century.
Type material:
Holotype: Bernice P. Bishop Museuom, Honolulu, Hawaii, BPBM-22730, 4460 mm adult male. Type Locality: Off Oahu, Hawaiian Islands, 21°51'N, 157° 46'W, about 42 km northeast of Kahuku Point, at 165 m depth in water about 4600 m deep.