Megamouth Shark — Megachasma pelagios, the Filter-Feeding Mystery Shark Discovered in 1976
The megamouth shark (Megachasma pelagios) is one of the most surprising vertebrate discoveries of the 20th century. Completely unknown to science until November 15, 1976, when a 4.5-m specimen was hauled up by a US Navy research vessel near Hawaii, the megamouth represents an entirely new family, genus, and species of large shark. It is the third filter-feeding shark (with whale shark and basking shark), reaches 5.5 m and over 1,000 kg, and has been documented at depths to 1,500 m. Fewer than 300 specimens have ever been recorded.
Megamouth Shark
Discovery — one of the most surprising 20th-century vertebrate finds
On November 15, 1976, the US Navy research vessel AFB-14 hauled up its sea anchor near Hawaii and found a 4.5-metre shark tangled in it. The animal belonged to no known family.
The 1976 holotype was so anatomically distinct that ichthyologists Leighton Taylor, Leonard Compagno, and Paul Struhsaker erected an entirely new family (Megachasmidae), genus (Megachasma), and species (pelagios) for it — one of the only times in the 20th century a new shark family was described from a single specimen of this size. The discovery was particularly striking because the animal is large, distinctive (the rubber-rimmed mouth gives the species its English name), and lives in waters that should have been thoroughly explored.
In the decades since, fewer than 300 megamouth specimens have been documented — including only a handful of live observations. The species is presumed pantropical and pelagic, but its rarity in records likely reflects the difficulty of encountering an animal that spends most of its time in mesopelagic darkness.
Identification — the rubber-mouthed deep-water filter feeder
Megamouths look like nothing else. The most distinctive features are the enormous rubber-rimmed mouth at the front of the head, the silvery-white interior of the mouth, and the soft-bodied, dark-skinned profile.
A megamouth's mouth opens to roughly 1.3 m wide. The interior is lined with a silvery membrane that may serve as a lure for the krill and copepods the species feeds on, possibly bioluminescent in life. The body is dark grey-brown above and lighter below, with a soft, flabby texture (compared to the muscular bodies of lamniforme cousins like the great white). The teeth are tiny, with about 50 rows in each jaw, and play no role in feeding — like other filter-feeding sharks, the megamouth feeds entirely through gill-raker filtration.
Range — pelagic and almost certainly circumglobal
Megamouth records cluster in a few locations, but the species is presumed pantropical based on the geographic spread of the available records.
The largest concentration of records is from Japan and Taiwan (which together account for roughly half of all known specimens), followed by the Philippines, the Hawaiian Islands, the Pacific coast of Mexico, and South Africa. Acoustic tagging of one Californian specimen in 1990 documented daily vertical migration: the shark spent daylight hours at 120–160 m and ascended to within 10–25 m of the surface at night, presumably tracking the deep scattering layer of krill and zooplankton. Extreme depth records reach 1,500 m.
Diet — krill and the deep scattering layer
Megamouths are filter feeders specialising in the krill, copepods, and gelatinous zooplankton of the pelagic deep scattering layer.
Stomach analyses of caught megamouths consistently show a diet dominated by euphausiid krill (especially Thysanopoda and Euphausia), with smaller amounts of copepods, jellyfish, and chaetognaths. The species' daily vertical migration tracks the same krill aggregations exploited by sperm whales, mesopelagic squid, and lanternfish — an entire deep-pelagic food web that is more productive than its remoteness suggests. Unlike whale sharks and basking sharks, megamouths are not commonly seen surface-feeding; the bulk of their feeding apparently happens in the dim light below the photic zone.
Reproduction — almost completely unknown
Megamouth reproductive biology is the most mysterious of any large fish. No pregnant female has been examined, no neonate has been documented in the wild, and the species' courtship and birthing locations remain unknown.
The species is presumed ovoviviparous like other lamniformes (the order including great white, sand tiger, and basking shark), but no developing embryo has been described, no pup has been documented, and the gestation period and litter size are unknown. The slow accumulation of records means that even basic life-history parameters — growth rate, lifespan, age at maturity — are estimates at best. The IUCN currently lists the species as Least Concern, but that listing comes with the caveat that the data are too sparse to assess actual population trends.
Where the megamouth sits among the major shark species
Comparing megamouths against the major species at a glance.
The megamouth is one of three filter-feeding shark species (with whale shark and basking shark) and the smallest of the three. It is roughly the size of a large great white but with a fundamentally different ecology — warm-blooded predator vs cold-bodied deep-water filter feeder.
| Species | Max length | Max weight | Bite force | Attacks (unprov.) | Lifespan |
|---|---|---|---|---|---|
| Great white shark | 6.0 m | 2,300 kg | ~4,000 PSI | 354 | 70 yrs |
| Tiger shark | 5.5 m | 900 kg | ~6,000 PSI | 142 | 27–50 yrs |
| Bull shark | 3.5 m | 320 kg | 1,250 PSI | 121 | 16 yrs |
| Oceanic whitetip | 4.0 m | 170 kg | n/a | 15 | 22 yrs |
| Shortfin mako | 4.0 m | 570 kg | n/a | 10 | 30 yrs |
| Great hammerhead | 6.1 m | 580 kg | n/a | 17 | 44 yrs |
| Whale shark | 12.0 m | 19,000 kg | n/a (filter) | 0 | 80–130 yrs |
Related on TheSea
When was the megamouth shark discovered?
The first specimen was hauled up off Hawaii by the US Navy research vessel AFB-14 on November 15, 1976. It was described as a new family, genus, and species in 1983 by ichthyologists Leighton Taylor, Leonard Compagno, and Paul Struhsaker.
How big does a megamouth shark get?
The largest reliably measured megamouth was 5.5 m in length and weighed approximately 1,215 kg. Most documented specimens fall in the 4–5 m range. The species is roughly the size of a great white but with very different proportions.
Are megamouth sharks dangerous?
No — megamouths are filter feeders with no functional teeth. There are zero documented attacks on humans on record. The species lives at depths well below typical recreational diving and is not aggressive even when encountered alive.
How many megamouth sharks have been seen?
Fewer than 300 specimens have ever been documented worldwide, with the largest concentration of records from Japan and Taiwan. Live observations are extremely rare; most specimens were either bycatch or beached.
What does a megamouth eat?
Megamouths are filter feeders specialising in krill, copepods, and gelatinous zooplankton from the deep scattering layer. The silvery interior of the mouth may serve as a lure for prey, possibly bioluminescent in life.
Blane Perun has been exploring Earth’s oceans and marine life for over 25 years. Founder of TheSea.Org in 1999, underwater photographer, coral aquaculture pioneer, and explorer of 80+ countries.
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