Megamouth Shark

fieldmarks: One of three species of gigantic filter-feeding non-batoid sharks, 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 gill rakers, 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. Colour: upper surface of body grey or grey-black above, without light spots, underside white, mouth dusky blackish with dark spotting on lower jaw, and dorsal surfaces of pectoral and pelvic fins blackish with conspicuous light margins. Head very elongated and about length of trunk. Snout extremely short, flattened and broadly rounded, not elongated and blade-like. Eyes moderately large, length 1.6 to 1.8% of precaudal length. Gill openings moderately long, length of first 6.4 to 8.6% of precaudal length, not extending onto dorsal surface of head; last two gill openings over pectoral-fin bases; unique gill rakers of finger-like dermal papillae with cartilage cores fringing internal gill slits. Mouth very large and arcuate, terminal on head; jaws greatly protrusable anteriorly beyond snout tip but not greatly distensible laterally. Teeth small, continuously varying and more or less awl-shaped, in 55 to 115/75 to 121 (130 to 236 total) rows, no differentiation between anterior, intermediate, lateral, or symphysial teeth. Trunk cylindrical and somewhat compressed, stout, and relatively flabby. Caudal peduncle compressed and without lateral keels but with small fossate-shaped upper precaudal pit only. Dermal denticles very small and smooth, with flat crowns, small ridges and cusps and with cusps directed posteriorly on lateral denticles. Pectoral fins large, narrow and elongated, much shorter than head in adults; pectoral skeleton plesodic with radials extending far into fin webs. Pelvic fins moderate-sized, smaller than pectoral and first dorsal fins; fin skeleton probably aplesodic, not extending into fin web. First dorsal fin moderately large, semierect and angular; fin skeleton aplesodic. Second dorsal fin less than half size of first but moderately large. Anal fin smaller than second dorsal fin and with its base slightly behind second dorsal-fin base, bases of both fins not pivotable. Caudal fin not lunate, dorsal lobe long but less than half as long as rest of shark, ventral lobe short but strong. Neurocranium depressed, with short wide rostrum, greatly depressed internasal septum and widespread nasal capsules, small orbits with strong supraorbital crests, small stapedial fenestrae, and with hyomandibular facets not extended outward. Vertebral centra weakly calcified, with rudimentary radii and double cones and no annuli. Total vertebral count 151, precaudal count 64, diplospondylous caudal count 82 to 87. Intestinal valve of ring type with 23 or 24 turns.

Spottily known from 28 specimens (as of February 2005) but probably circumtropical and wide-ranging. Western Atlantic: Brazil. Eastern Atlantic: Senegal. Southeastern Indian Ocean: Australia (Western Australia). Western North Pacific: Japan, Philippines (Macabalan Bay, Cagayan de Oro), Indonesia (Sulawezi, Nain Island, Bunaken Archipelago). Central Pacific: Hawaiian Islands (Oahu). Eastern Pacific: USA (southern California near San Clemente and off Catalina Island and San Diego).

Maximum total length at least 549 cm. Size at birth unknown. A juvenile free-living male from off Brazil was 190 cm long while a possibly smaller specimen from off Senegal was estimated at about 180 cm long. Adult males were 446 to about 549 cm; a late immature or early adolescent female was 471 cm, another female of uncertain maturity was about 5 m, and an adult female was 544 cm.

A coastal and oceanic, epipelagic and neritic species,found in water as shallow as 5 m in a shallow bay and in water 40 m deep on the continental shelf, with at least one washed ashore alive on a sandy beach; also offshore in the epipelagic zone at 8 to 166 m depth in water 348 to 4 600 m deep. The coloration and catch records of the megamouth shark are suggestive of epipelagic rather than deepwater habitat, as is the composition of its liver oil (Itabashi, Yamaguchi and Nakaya, 1997). This is a seldom reported, possibly rare or uncommon shark, with most examples from off Japan and southern California. As the megamouth shark has greatly reduced teeth, very numerous gill-raker papillae on its internal gill openings, and stomach packed with very small prey, it can be properly considered a specialized filter feeder like the basking shark (Cetorhinus maximus), whale shark (Rhincodon typus), and the devil rays (Mobulidae). However, the flabby body, soft fins, asymmetrical caudal fin without keels, and weak calcification of the megamouth shark suggested that it is much less active than the whale and basking sharks (Compagno, 1984, 1990b) and the mobulids. Observations and tracking of a live specimen off southern California confirmed its relative sluggishness (Nelson et al., 1997). The only known prey of the megamouth shark are epipelagic and mesopelagic euphausiid shrimp, copepods, and jellyfish. The first megamouth shark from Hawaii had been feeding on an euphausiid shrimp, Thysanopoda pectinata, that averages 3.1 cm in length. The shrimp has a diel migration pattern with a range of 300 to 1 100 m depth during the day; at night it is commonest at 150 to 500m with a maximum range of 75 to 525 m. When captured during the night the shark was apparently at the upper depths where these shrimp are commonest. It may 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 second megamouth shark from California had euphausiids, copepods and deepwater jellyfish (Atolla vanhoeffeni) in its stomach. Two female sharks from Japan had macerated euphausiids in their stomachs (probably Euphausia nana). The feeding structures of this shark may allow it to feed on other pelagic invertebrates and even small midwater fishes, but so far the limited stomach contents available suggests that this shark primarily targets euphausiid shrimp as prey. There has been considerable speculation on the feeding habits of this shark. Taylor, Compagno and Struhsaker (1983) compared the basking, whale, and megamouth sharks, and suggested that the megamouth shark slowly swam through schools of euphausiid shrimp with jaws widely open, occasionally closing its mouth and contracting its pharynx to concentrate prey before swallowing it. These authors noted the presence of a bright silvery mouth lining, which they suggested was possibly bioluminescent but could not prove it because of inadequate histological evidence. The second specimen had tissue in its lower jaw that may be luminescent (Lavenberg and Seigel, 1985) and could along with the reflective upper jaw tissue serve as a 'light trap' for luring prey to the proximity of its mouth (Diamond, 1985). Compagno (1990b) noted that the protrusability of the jaws in the megamouth shark may allow it to use its mouth as a bellows to suck in prey, and that open-mouthed cruising through prey concentrations was optimal for the basking shark but not the megamouth shark. Taylor, Compagno and Struhsaker (1983) suggested that the soft flabby body and fins, low-flow filter apparatus, and small gill openings of the megamouth shark indicated that it was less active than the whale and basking sharks. Oikawa and Kanda (1997) considered gill histology and filament area in the megamouth shark compared to the shortfin mako and other species, and suggested that the megamouth shark was less active. An adult male megamouth shark of about 4.9 m length was captured in a pelagic gill net off southern California near the surface in water between 300 and 400 m deep. This was still alive when discovered in the net and was towed to Dana Point Harbor where it was tethered by a rope to the fishing boat that caught it (Nelson et al., 1997). The shark was extensively photographed by divers and was still alive after being towed and then being tethered for over a day. Observations suggested that the shark could breathe readily by gill pumping and was not dependent on ram-ventilation and constant swimming unlike more active lamnoid sharks. The shark was then towed out to sea, tagged with an acoustic telemetric tag, and released in water about 20 m deep. The shark immediately descended, outswam the accompanying divers, and headed for deep water beyond the continental shelf.It was tracked over a two-day period, which revealed a pattern of vertical, crepuscular migration in the epipelagic zone. The tagged shark ascended at sunset to a depth of 12 to 25 m at night, then descended at sunrise to a depth of 120 to 166 m during the day with greatest depth achieved at midday but stayed well above the bottom at 700 to 850 m. It stayed on a straight southward course at an estimated speed between 1.5 and 2.1 km/hr during day and night. It was thought that the shark was responding to light levels in choosing its depth range, and as light increased it correspondingly sought an optimal level by diving at dawn and conversely responded to decreasing light levels by ascending to near (but not at) the surface at dusk. It has been suggested that the megamouth shark may also follow vertical migrations of euphausiid prey during diel cycles. The telemetric track suggested that the shark was indeed somewhat less active than makos or basking sharks, but that it could sustain a slow rate of swimming for extended periods. Mode of reproduction is probably aplacental viviparous with uterine cannibalism or cannibal vivipary suspected in the form of oophagy, but no pregnant adult female has been reported to date. A late immature or early adolescent female had two ovaries with many tiny oocytes under 3 mm in diameter, while an adult female had numerous larger oocytes 5 to 10 mm wide. This is similar to the ovaries of several other lamnoids that are oophages. An adult female had numerous bite marks mostly on her flanks and precaudal tail but also on her first dorsal and anal fin and head. These wounds corresponded to the teeth of another megamouth shark (or sharks), and were interpreted as courtship scars inflicted by a male because of the narrow spacing of individual cuts comparable to male rather than female teeth (Yano et al., 1999). The megamouth shark is the only known selachian victim of the semiparasitic cookiecutter shark, Isistius brasiliensis, and may be especially vulnerable to Isistius attacks because of its soft skin, epipelagic habitat in warm seas where Isistius is also found, sluggishness, and relatively slow swimming speed. Three megamouth specimens had 'crater wounds' indicative of cookiecutter attacks. A megamouth shark was seen at the surface off Nain Island, Bunaken Archipelago, North Sulawesi while being possibly harassed or played with by three sperm whales, which left the shark after the observers approached them. The shark was apparently minimally injured by the whales and was photographed at the surface before disappearing.

Interest to fisheries minimal at present. Taken as a rare incidental bycatch in pelagic gill nets, purse seines, pelagic longlines, and fixed shore nets, and so far has been mostly utilized by museums and oceanaria, which prize the few specimens landed as display objects. As with certain other large, rare animals it attracts much attention from the general public and shark fans. A few specimens were released alive from fishing gear, while a recent specimen from Philippines was cut up and utilized by fishermen, but details of its utilization were not recorded. Conservation Status : Conservation status uncertain, but of concern because of the apparent epipelagic and neritic habitat and possible rarity of this shark, which puts it at risk as unrecorded bycatch of oceanic and offshore littoral fisheries.