Megamouth Shark

The megamouth lives in the deep scattering layer of the ocean. It seems to enjoy warmer climates, though it has been discovered in temperate waters.

Aquatic Biomes: benthic ; coastal

The megamouth Shark is five meters long and weighs approximately 750 kilograms. Its body is cylindrical and flabby, its eyes small, and it swims in stiff, slow movements.

Average mass: 750 kg.

Other Physical Features: bilateral symmetry

The Megamouth Shark was recently discovered in deep water off of the Hawaiian Islands and near the shores of California, Japan and Western Australia.

Biogeographic Regions: pacific ocean (Native )

The Megamouth is a filter feeder. It uses its enormous mouth to draw in water and filter out small planktonic animals such as crustaceans and shrimp.

Though its large size and huge mouth can create a sinister appearance, this shark is particularly timid.

Despite its vast size, this particular species of shark was only discovered in 1976. Less than ten have been studied or even sighted, and as a result, knowledge is limited.

IUCN Red List of Threatened Species: data deficient

No negative economic effects by the megamouth shark have been reported.

Little is known about the economic importance of the megamouth shark, though sharks in general have proven to be an essential element in the aquatic ecosystem. In areas where sharks were killed because of the belief that they threatened fisheries by preying on certain fish, a significant disruption of the ecosystem (such as overpopulation of small fish and planktonic animals) occurred.

The megamouth reproduces sexually through internal fertilization. There are separate sexes, and the offspring are miniature versions of the adult at birth.

Since its discovery in 1976, only 17 sightings have been recorded of this elusive fish (3) and the majority of these have been of dead specimens; either accidentally caught or stranded individuals (5). Consequently, very little is known of their natural ecology and behavioural observations have only been possible from the tagging of one individual for a brief two-day period in 1990 (3). This shark was observed to undergo vertical migrations, spending the day in deep water and ascending to midwater at night; it is likely that this migration is undertaken in response to the movements of prey species such as krill (3). Megamouth sharks are thought to feed by swimming through groups of small prey items with their mouths open; however, no direct observations have yet been achieved (3).

A greater understanding of this mysterious giant of the deep is urgently required before the necessity for any conservation measures can be properly realised or implemented.

The megamouth shark is one of the most mysterious and least understood of all the sharks. It was first recorded in 1976 and is so different from other shark lineages that it has been placed in its own family: Megachasmidae (3). Megamouth sharks can reach over 5 metres in length (4); the head is large with a short snout and, as the name would suggest, an extremely large mouth (5). The mouth contains over 50 rows of very small, hooked teeth. The body is tapered with a fleshy appearance; it is a blackish-brown colour above and white below (3). There are two unequal sized dorsal fins and the tail has a longer upper lobe (3).

Found in the open ocean and believed to occur at depths of 150 to 1,000 metres (4).

Megamouth sharks have been reported from the Indian, Pacific and Atlantic Oceans (3).

Classified as Data Deficient (DD) on the IUCN Red List (1).

The lack of data concerning either the distribution or behaviour of megamouth sharks makes it particularly difficult to assess the severity and types of threats faced by this species. A number of specimens have been caught accidentally as by-catch of deepwater fishing methods and it is likely that this practice will increasingly affect population numbers (4).

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.

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.

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.

Exhibit ovoviparity (aplacental viviparity), with embryos feeding on other ova produced by the mother (oophagy) after the yolk sac is absorbed (Ref. 50449). Distinct pairing with embrace (Ref. 205). Size at birth

Oceanodromous. Migrating within oceans typically between spawning and different feeding areas, as tunas do. Migrations should be cyclical and predictable and cover more than 100 km.

Dorsal spines (total): 0; Vertebrae: 125

Oceanic, possibly occurring in depths between 150 and 1,000 m (Ref. 6871). Feeds on planktivorous prey such as euphausiid shrimps, copepods and jellyfish (Ref. 6871). May also eat small midwater fishes. Possibly less active than the basking and whale sharks (Ref. 6871). Its feeding habits and habitat suggest that it may be a rare catch in the future.

Body stout, tapering posteriorly (Ref. 6871), tadpole-like with larger head and tapering trunk and tail (Ref. 47786). Snout extremely short but broadly rounded (Ref 43278, 47786). Head huge, blubbery (Ref. 6871). Mouth very broad and terminal on head, with corner extending behind the eyes (Ref. 6871, 43278). Jaws huge, protrusible anteriorly but not greatly distensible laterally (Ref. 43278, 47786), lower jaw extending to snout tip (Ref. 6871). Teeth very small, numerous, hooked (Ref. 43278, 6871, 47787). Gill slits moderately long, not reaching dorsal surface of head (Ref. 6871; 43278), internal gill slits lined with dense rows of papillose gill rakers (Ref. 43278). Eyes semicircular (Ref. 47786), with no nictitating membrane (Ref. 43278, 6871, 47786). Two dorsal fins, relatively low and angular; small anal fin; long, narrow pectoral fins; moderate-sized pelvic fin; caudal fin asymmetrical, non-lunate, with a short and strong ventral lobe; upper pre-caudal pit only; caudal peduncle without keels or ridges (Ref, 6871, 43278).

Oceanic, possibly occurring in depths between 150 and 1,000 m (Ref. 6871). Epi- and mesopelagic (Ref. 58302). Feeds on planktivorous prey such as euphausiid shrimps, copepods and jellyfish (Ref. 6871). May also eat small midwater fishes. Ovoviviparous (Ref. 50449). Possibly less active than the basking and whale sharks (Ref. 6871). Its feeding habits and habitat suggest that it may be a rare catch in the future. Ovoviviparous, embryos feeding on yolk sac and other ova produced by the mother (Ref. 50449). Males mature by 400 cm (Ref. 6871). A bright white band on the snout just above the upper jaw may play a role in feeding behavior or with recognition of individuals (Ref. 47761). Preyed on by the semi-parasitic cookiecutter shark, Isistius brasiliensis.

fisheries: of no interest

主要發現區域包括太平洋； 日本、臺灣、印尼、菲律賓、夏威夷與美國加州等。 大西洋； 巴西與塞內加爾等。臺灣發現於東部之花蓮外海。

非常罕見魚種。

體延長而呈圓柱形，稍側扁，但粗壯。頭很長，約略等於軀幹長。吻頗短，平扁而圓廣。眼中大。口非常大，端位，口角延伸至眼後；齒小，錐狀，兩頜各具逾100列以上。鰓孔後兩孔位於胸鰭基部之上方；具指狀鰓耙。背鰭低而小，第二背鰭尤甚；臀鰭極小；胸鰭大型，狹長；尾鰭帚形。尾柄側扁而無側脊，但具上下凹槽。脊椎總數為125。全世界計1屬1種(Nelson, 1994)，即為巨口鲨(/Megachasma pelagios/)。台灣發現於東部之花蓮外海。

罕見之大洋洄游的濾食性鯊魚，棲息深度可能介於 5 與 1000公尺 之間。生性不活躍，可能更甚於象鲨與鯨鲨等之鯊類。主要以濾食浮游動物為生，例如磷蝦，橈腳類的動物與水母等，也可能捕食小型中層水域之魚類。其上頜正上方的吻上有一個亮白色的條紋，在攝取食物的行為上可能扮演的一個重要角色。卵胎生，胚胎以卵黃囊及同胎之其他的卵為食。雄性成熟體長約400公分左右。

The megamouth shark (Megachasma pelagios) is a species of deepwater shark. It is rarely seen by humans and is the smallest of the three extant filter-feeding sharks alongside the relatively larger whale shark and basking shark. Since its discovery in 1976, fewer than 100 specimens have been observed or caught. Like the other two planktivorous sharks, it swims with its mouth wide open, filtering water for plankton and jellyfish. It is recognizable from its large head with rubbery lips. The megamouth is so unlike any other type of shark that it is usually considered to be the sole extant species in the family Megachasmidae, though some scientists have suggested it may belong in the family Cetorhinidae.