Megamouth shark - Megachasma pelagios

The megamouth shark Megachasma pelagios is a large, softly built filter feeder with a noticeably short, broad head. Its terminal mouth extends behind the small eyes and can be extended far forward. Despite the enormous mouth opening, the numerous teeth are only small and hook-shaped.

The mouth has approximately 50 rows of very small teeth per jaw, of which only a few rows are functional at a time. On the inside of the gill arches there are dense rows of finger-shaped gill trap papillae. Together with the widely expandable mouth and throat, they form the filter apparatus for krill and other zooplankton.

Identifying features

• Very large, rounded head with a wide terminal mouth.
• Dark gray to black-brown upperside and significantly lighter underside.
• Light to silvery areas in the mouth and a light stripe on the snout.
• Two low dorsal fins; the second is significantly smaller than the first.
• Soft body without keels on the caudal peduncle and asymmetrical caudal fin.
• Dense gill trap papillae for retaining small prey from the water.

The Florida Museum states 7.09 meters as the largest documented total length of a female; many well-known animals measured around four to five and a half meters. The squat head, huge mouth and soft, not very strong fins distinguish the species from whale sharks and basking sharks.

The silvery structures in the mouth were previously interpreted as a possible light organ. However, there is no reliable evidence of this. More likely, reflective skin and the bright snout stripe remain visible in low light and may aid in prey capture or detection.

megamouth sharks have been recorded in tropical and warm-temperate marine areas worldwide. Most documented animals come from the West Pacific, especially from Taiwan, the Philippines, Indonesia and Japan. Further Pacific evidence is available China, Vietnam, Australia, the USA, Mexico, Ecuador and Peru before.

In the Indian Ocean The species was introduced, among other things Sri Lanka and South Africa found. From the Atlantic there is evidence Senegal and Brazil known; also from the Caribbean Sea at Puerto Rico there is a find. The scattered reports suggest a highly migratory, probably cross-ocean population.

The global evaluation of 261 records from 2021 examined records of 261 animals from all three major oceans. 214 were from the western Pacific and 35 from the eastern Pacific; The Atlantic and Indian Oceans were significantly less represented, with six specimens each. These numbers also reflect reporting and fishing effort, not just actual frequency.

Open water and daily deep hiking

FishBase mentions finds between around 5 and 600 meters deep, but mostly around 120 to 166 meters. A broader data evaluation extends up to 1,203 meters. The megamouth shark lives epi- to mesopelagically in open water, but can also occur close to islands, continental slopes, bays and coasts.

Acoustic tracking and other capture data show a vertical diurnal migration: In the dark, animals often stay closer to the surface, while in the morning they descend into deeper water layers. They probably follow migrating swarms of krill and other zooplankton.

The map is from 2007 and therefore only shows the image of the find, which was known at the time and was very incomplete. Recent evidence from several additional countries is included in the text; the red dots must not be read as the complete current distribution area.

The megamouth shark is one of only three large plankton-eating sharks. It swims comparatively slowly through areas with concentrated prey, protruding its jaws and expanding its oral cavity. Water flows in through the mouth and is pushed out again through the gill slits.

Krill as the main food

The FAO species catalogue describes krill or euphausiaceae as the most important known food. Copepods and deep-sea jellyfish were also found in the stomachs examined; Small open water fish can supplement the diet. The gill trap papillae retain these small organisms.

The well-known daily migration corresponds to the movement of many krill species: at night they rise to upper layers of water and during the day they sink deeper. An acoustically marked animal stayed at a depth of approximately 12 to 25 meters at night and usually between 120 and 166 meters during the day.

Reproduction largely unknown

To date, no pregnant female has been scientifically examined. The structure of the reproductive organs suggests a placental live birth; The embryos probably also feed on unfertilized eggs, as in several other species of mackerel sharks. Litter size, gestation period and birthplaces are not known for certain.

The smallest free-swimming animal recorded in the global evaluation was around 1.77 meters long. Very small individuals were found mainly in warm areas of the Indian and Atlantic Oceans. This leads to the plausible, but not yet proven, hypothesis that important young animal areas are located there.

Bite marks on adult females are interpreted as possible mating scars. Overall, species biology remains exceptionally incomplete: many findings are based on individual bycatches, strandings and a few short-term observations of live animals.

The IUCN Red List classifies the megamouth shark as not endangered a. This classification does not mean that the species is common or well-researched: there is still a lack of reliable data on population size, age structure, reproductive rate and long-term population development.

Most of the known animals were not specifically caught, but instead ended up as bycatch in drift, fishing or purse seining nets. Further evidence comes from strandings. Wide global distribution and presumably low economic demand limit the risk of targeted overfishing, but do not prevent local losses.

Important risks

• Bycatch in large pelagic nets and offshore gillnets.
• Injury or death when retrieving, landing and measuring very large animals.
• Incomplete reporting or incorrect identification of rare catches.
• Plastic and other pollutants that can be absorbed along with plankton.
• Changes in plankton distribution due to ocean warming and oceanography.

Taiwan now bans fishing and possession; Caught animals must be returned regardless of their condition. Such rules reduce direct use, but at the same time make biological sampling more difficult. Non-lethal documentation, international data sharing and satellite markings are therefore particularly important.

For a more reliable assessment, standardized reports with location, date, depth, fishing gear, body length, gender and photos are needed. Especially in the case of a rarely observed, highly migratory species, a lack of evidence cannot reliably prove either an absence or a population decline.

The Australian Museum describes the megamouth shark as a large, slow plankton eater. There are no confirmed unprovoked attacks on humans. Its huge mouth is specialized for small prey and filtration, not for grabbing large animals.

Live encounters are extremely rare. The following still applies to divers: keep your distance to the sides, leave the swimming path clear and do not touch your mouth, gills or fins. A shark several meters long can cause unintentional injury due to its mass and sudden movement.

Rare observation in Japan

The following video shows a live megamouth shark in Japanese waters. It is classified here as a documented encounter and not as evidence of a specific habitat.

Behavior when caught or sighted

• Do not lift, drag or secure an animal by its tail for photographs.
• If possible, measure in the water, document it and release it without delay.
• Keep gills and mouth clear; Do not place lines through gill gaps or around the head.
• Photographically record the location, time, depth, length, gender and noticeable scars.
• Report sightings to the responsible research or fishing offices.

Every well-documented encounter is valuable for research. Videos and photos can capture body characteristics, scars, behavior and accompanying fauna without killing the animal. However, the priority is always a calm, quick release that is as injury-free as possible.