Megamouth Shark

The megamouth lives in the open ocean, mostly in deep, dark mid-water far from land, it swims slowly and does not come to the surface much, and it is genuinely rare. Big animals in that mesopelagic zone are simply not encountered often, because people rarely go there and the shark does not interact with fisheries or coastlines the way most large sharks do. The first one was found by accident — on 15 November 1976, the US Navy research ship AFB-14, operating off Oahu, Hawaii, hauled in its parachute-type sea anchor from about 165 metres and found a 4.5-metre shark tangled in it, having apparently tried to swallow it. Scientists at the time immediately recognised it was not just a new species but so different in anatomy that it required its own new genus and family, and it became one of the most significant fish discoveries of the 20th century.

It is completely harmless to humans. Like the whale shark and basking shark, the megamouth is a filter feeder, and its hundreds of rows of tiny hooked teeth are useless as weapons; it eats krill, small crustaceans, and jellyfish. Its feeding style differs from the other two filter-feeding sharks, though. The whale shark actively sucks and gulps, and the basking shark rams forward with its mouth wide open, straining large volumes of water through its gill rakers. The megamouth appears to be a slower, more delicate feeder that swims gently through plankton layers with its huge, soft, protrusible mouth open, engulfing mouthfuls of plankton-rich water and expelling the water through its gills. Its flabby body and weak muscles suggest it is not built for the sustained fast cruising of a basking shark.

The evidence is suggestive but not proven. Around the upper jaw of the megamouth is a distinct band of silvery-white tissue and modified skin denticles that stands out against the dark body, and early researchers proposed that this "lip" could be luminescent — either producing its own light through bioluminescence or, more likely, reflecting the faint downwelling light or the glow of bioluminescent plankton — and that a shark cruising with its mouth open would present a glowing ring that draws phototactic plankton and small crustaceans inward, straight into the mouth. No one has yet directly observed the lip glowing on a live shark in the deep, and some scientists think the pale tissue is just reflective rather than a true lure, so it remains an appealing but unconfirmed idea.

Very few. From the 1976 discovery through the mid-2020s, the total number of megamouth sharks recorded worldwide is only around 250 to 300, and the great majority of those are dead animals — stranded on beaches, caught accidentally in nets set for tuna and other fish, or found floating. Live megamouths in their natural habitat have been filmed only a small number of times, including a famous 1990 case off California where a tagged individual was tracked for two days, and occasional encounters by divers off Indonesia and the Philippines. New records now often come from Taiwan and Japan, where they turn up as fisheries bycatch. Each sighting is still notable enough to make the news, more than 45 years after the species was first described.