The megamouth shark (Megachasma pelagios) is an extremely rare species of deepwater shark, and the smallest of the three planktivorous sharks besides the whale shark and basking shark. Since its discovery in 1976, few megamouth sharks have been seen, with 58 specimens known to have been caught or sighted as of May 2014, including three recordings on film. Like the other two filter feeders, it swims with its enormous mouth wide open, filtering water for plankton and jellyfish. It is distinctive for its large head with rubbery lips. It is so unlike any other type of shark that it is usually considered to be the sole extant species in the distinct family Megachasmidae, though suggestion has been made that it may belong in the family Cetorhinidae, of which the basking shark is currently the sole extant member. In addition to the living M. pelagios, however, two extinct megamouth species — the Cretaceous M. comanchensis and the Oligocene-Miocene M. applegatei — have also recently been proposed on the basis of fossilised tooth remains.
The appearance of the megamouth is distinctive, but little else is known about it. It has a brownish-black colour on top, is white underneath, and has an asymmetrical tail with a long upper lobe, similar to that of the thresher shark. The interior of its gill slits are lined with finger-like gill rakers that capture its food. A relatively poor swimmer, the megamouth has a soft, flabby body and lacks keels.
Megamouths are large sharks, able to grow to 5.5 metres (18 ft) in length. Males mature by 4 metres (13 ft) and females by 5 metres (16 ft). Weights of up to 1,215 kg (2,679 lb) have been reported. The average size of their mouth is about 1.3 meters.
As their name implies, megamouths have a large mouth with small teeth, and a broad, rounded snout, causing observers to occasionally mistake megamouth for a young orca. The mouth is surrounded by luminous photophores, which may act as a lure for plankton or small fish. Their mouths can reach up to 1.3 metres wide.
In 1990, a 4.9-m (16-foot) male megamouth shark was caught near the surface off Dana Point, California. This individual was eventually released with a small radio tag attached to its soft body. The tag relayed depth and time information over a two-day period. During the day, the shark swam at a depth of around 120–160 m (400–525 ft), but as the sun set, it would ascend and spend the night at depths of between 12 and 25 m (39–80 ft). Both day and night, its progress was very slow at around 1.5–2.1 km/h (1–1.3 mph). This pattern of vertical migration is seen in many marine animals as they track the movement of plankton in the water column. The shark captured in March 2009 was reportedly netted at a depth of 200 m (660 ft).
Reproduction is ovoviviparous, meaning that the young sharks develop in eggs that remain within the mother's body until they hatch.
The first megamouth was captured on November 15, 1976 about 25 miles off the coast from Kāneʻohe, Hawaiʻi when it became entangled in the sea anchor of United States Navy ship AFB-14. Examination of the 4.5 m (14.7 ft), 750 kg (1,650 lb) specimen by Leighton Taylor showed it to be an entirely unknown type of shark, making it one of the more sensational discoveries in 20th-century ichthyology (see also coelacanth).
As of May 2014, only 58 megamouth specimens have been caught or sighted, the most recent being one caught by Japanese fishermen. They have been found in the Pacific, Atlantic and Indian oceans. Japan, the Philippines, and Taiwan have each yielded at least 10 specimens, the most of any single area, amounting to more than half the worldwide total. Specimens have also been sighted in or come out of the waters near Hawaii, California, Mexico, Indonesia, Australia, Brazil, Senegal, South Africa, and Ecuador.
On March 30, 2009 off Burias Island in the Philippines, an 880- to 1,100-pound (400- to 500-kilogram) 4-metre (13-foot) megamouth shark died while struggling in a fisherman's net and was subsequently taken to nearby Donsol in Sorsogon province, where it was examined by scientists, before being butchered and sold.
On 12 June 2011, a 3 m (10 ft) dead juvenile male was found by fishermen near the western Baja California peninsula coast, in a region called Bahía de Vizcaíno. It was picked up by the same fishing vessel that in 2006 captured another megamouth specimen in Vizcaino bay, which has led Mexican scientists to believe that the megamouth could be a seasonal visitor to the Baja Peninsula. The new specimen was taken to Ensenada, Mexico, where it was photographed and sliced in order for Scripps Institution of Oceanography and Mexican researchers to study the structure of its muscles and gills.
On May 7, 2014 a 3.96 m (13 ft) 1,500 lbs female shark was captured at a depth of 2,600 ft off the coast of Shizuoka, Japan. The body was dissected in front of the public, by staff at the Marine Science Museum in Shizuoka City, Japan.
- For a topical guide to this subject, see Outline of sharks.
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- Florida Museum of Natural History Megamouth Shark # 38: The First Megamouth Shark, Megachasma pelagios, Found in Mexican Waters