The Megamouth Shark (Megachasma pelagios) is an extremely rare and unusual species of shark, discovered in 1976, with 36 specimens known to be caught or sighted as of 2006. Like the basking shark and whale shark, it is a filter feeder, consuming plankton and jellyfish, and is distinctive for its large head with rubbery lips. It is so unlike any other type of shark that it is classified in its own family Megachasmidae, though it has been suggested that it may belong in the family Cetorhinidae of which the Basking shark is currently the sole member.

The appearance of the Megamouth is distinctive. It has a large mouth with small teeth, a broad rounded snout (observers have mistaken it for a young orca), a generally brownish-blackish color on top and white underneath, and an asymmetrical tail with a long upper lobe, similar to 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 keels.

The first Megamouth was captured on November 15, 1976 about 25 miles off the coast from Kaneohe, Hawaii when it became entangled in the sea anchor of a United States Navy ship. Examination of the 4.5 m (14.6 ft), 750 kg (1,650 lb) specimen by Leighton Taylor showed it to be an entirely unknown type of shark, rivaling the coelacanth as the most sensational discovery in ichthyology during the 20th century.

The long delay between initial discovery (1976) and the scientific description (1983), became the focus of an elaborate practical joke by two friends of Leighton Taylor, Richard Ellis of the American Museum of Natural History and John McCosker, director of San Francisco's Steinhart Aquarium. Ellis and McCosker photocopied random articles from Japanese scientific journals and inserted photographs of the megamouth shark and a map of the type location and an English abstract, making it appear as if a Japanese team under guidance of John E. Randall of the Bishop Museum was to snatch the scientific merits of the description right from under Taylor's nose. An accomplice in Japan then mailed the "preprints" to Taylor, who was naturally dumbstruck. He then had his Japanese-American secretary translate the "paper", only to be told that it contained things like musings about the cat in Japanese art, and rhinoceroses in Ueno Zoo, but nothing about the megamouth shark. Hidden on the last page were the names of Ellis and McCosker, put there deliberately for Taylor to find them. Realizing he had been had, Taylor finally wrote up the description. The remark on its last page, "Particular thanks go to Richard Ellis and John McCosker for preparation of a preliminary manuscript which was of great help in the production of this final paper," is in reference to this incident.

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