megamouth shark (Megachasma pelagios)Schoolchildren looking into the tooth-lined mouth of a 4.2-meter- (13.8-foot-) long preserved megamouth shark (Megachasma pelagios) specimen at Tokai University Marine Science Museum in Shizuoka, Japan.(more)megamouth shark, (Megachasma pelagios), species of large filter-feeding sharks inhabiting the continental shelf and open-ocean regions of tropical and temperate oceans worldwide. The species is found between latitudes 40° N and 40° S from just below the surface to depths of as much as 1,500 meters (about 4,920 feet). The megamouth shark is relatively new to science; it was discovered near Kaneohe, Hawaii, in 1976, and it is the only living species in family Megachasmidae (order Lamniformes).

　　The megamouth shark has a distinctive appearance characterized by a bulky body and a bulbous head. Together, these features contribute to its teardrop-shaped form, which tapers and connects to a long, asymmetrical tail fin. The top and sides of the body are colored gray, and its underside is white. The species balances itself with two narrow pectoral fins. Both dorsal fins are small, the first dorsal fin being nearly twice as large as the second. The shark’s pelvic fins are also relatively small. The shark’s unusually wide mouth is another defining feature; the mouth contains as many as 50 rows of tiny teeth on both the upper part and lower part of the jaw. Some of these are pointed, whereas others are hook-shaped or flattened. These robust sharks can grow quite large, the biggest individuals weighing as much as 1,215 kg (about 1.3 tons). Males are roughly three-quarters the size of females (see also sexual dimorphism): the largest males can grow as long as 5.5 meters (18 feet), whereas the largest females may reach lengths of as much as 7.1 meters (23.3 feet).

　　Megamouth sharks are slow swimmers that migrate up to the surface at night and downward in the water column during the day; they follow the movements of plankton such as copepods, krill, and other small shrimp. Although the shark is primarily a filter feeder, it is capable of capturing larger prey, and researchers speculate that the shark’s diet might also include jellyfish and small bony fishes. The species is prey for cookiecutter sharks, and there is at least one documented case of megamouth sharks being attacked by sperm whales. Megamouth sharks also appear as bycatch as part of commercial fishing hauls.

　　Little is known about how the species reproduces. It is ovoviviparous (that is, the eggs are fertilized, grow, and hatch within the body of the female and young are born alive), and there is evidence that the sharks engage in oophagy, a type of cannibalism wherein early developing young attack and consume their developing siblings as well as fertilized eggs in their mother’s uterus. It is thought that, under such circumstances, megamouth sharks give birth to fewer young but that these young are large. Sexual maturity appears to be a function of body length rather than age, with males and females becoming sexually mature when they measure about 4 meters (13.1 feet) and 5 meters (16.4 feet) long, respectively. Solid figures of the shark’s life span remain elusive, because the species has been sighted fewer than 300 times and far fewer specimens have been collected. Researchers speculate, however, that they might live as long as basking sharks do (that is, about 50 years) in the wild. No megamouth sharks are kept in captivity.

　　The International Union for Conservation of Nature and Natural Resources (IUCN) lists the megamouth shark as a species of least concern. However, the species is incredibly rare and has, thus far, not appeared often in commercial catches. The IUCN’s first and only population assessment of megamouth sharks did not occur until 2018, and there is evidence that megamouth shark captures are increasing; the species occurred more frequently in commercial hauls during the 2010s. Because megamouth sharks and sharptail molas (Masturus lanceolatus)—a species that has been subjected to increased fishing pressure during the 21st century near Asia—travel at similar depths in the water column and have been captured together in fishing nets, the IUCN suggests that megamouth sharks may be vulnerable to the same threats and should be tracked and assessed more regularly.