The megamouth shark (Megachasma pelagios) is a species of deepwater shark. Rarely seen by humans, it measures around 5.2 m (17 ft) long 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.
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 caudal keels. The megamouth is considerably less active than the other filter-feeding sharks, the basking shark and the whale shark. The megamouth has a stout body and a long, wide bulbous head.
Megamouths are large sharks, able to grow to 5.49 metres (18 ft 0 in) in length. Mature males average at 4 m (13 ft) and females at 5 m (16 ft). Weights of up to 1,215 kg (2,679 lb) have been reported. A 2019 study suggested that it would have reached 6 m (20 ft) in maximum length. Megamouth sharks can be found as far northward as northern Japan; southern California (LACM 43745-1) and near Punta Eugenia, Baja California, and Hawaii. Megamouth sharks can be found at a depth of up to 1,000 m (3,280 ft). Megamouth sharks are dark blue, brownish-black, or gray above, lighter below; with a white band along the upper jaw; while the posterior margin of its fins are white.
As their name implies, megamouths have a large mouth with small teeth, and a broad, rounded snout, causing observers to occasionally mistake the megamouth for a young orca. The protruding inside of the upper lip is a brilliant silvery-white, which is very visible when the mouth is open. This lip was initially thought to be possibly embedded by luminous photophores when the first shark was examined in the early 1980s, which may act as a lure for plankton, while the team examining the second shark in the mid-1980s instead proposed that the lower lip might glow with the white band used as a reflector of sorts, but neither theory has been proven. In 2020, a study concluded that this species of shark does not in fact produce any light; the white band was found to merely be highly reflective of light. This white band is present in both sexes and could be either a feeding mechanism or possibly be used as a means of identifying other individuals of megamouth sharks. Their mouths can reach up to 1.3 m (4 ft 3 in) in width. Megamouth sharks have up to 50 rows of teeth in their upper-jaw and up to 75 rows of teeth in their lower-jaw.
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 around 120–160 m (390–520 ft), but as the sun set, it would ascend and spend the night at depths between 12 and 25 m (39 and 82 ft). Both day and night, its progress was very slow, around 1.5–2.1 km/h (0.81–1.13 kn). 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. Tissue samples were obtained from twenty-seven megamouths caught in a two-year period off the Hualien coast (eastern Taiwan), and two caught in Baja California, Mexico, and samples taken from GenBank to perform a population genetic analyses of the megamouth shark; the results indicated no genetic diversity between populations found in different geographical locations, which indicates the species forms a single, highly migratory, interbreeding population.
