Last week, a fisherman off the coast of Shizouka, Japan, caught a female Megamouth shark (Megachasma pelagios), which was subsequently dissected in a public presentation. There's a lot we don't know about this seldom seen shark, but here are a few things we do know.

1. The Megamouth shark was discovered in 1976, when a U.S. Navy research vessel hauled up an adult male specimen off the coast of Hawaii near Oahu.

The paper on the new species, published in 1983, described its discovery, preservation, and dissection:

The ship had deployed two large parachutes as sea anchors at a depth of about 165 m in water with a bottom depth of approximately 4600 m. When the parachutes were hauled to the surface … one of them had entangled in it a large adult male shark 4.46 m (14.6 ft) long and 750 kg (1653 lbs). Crew members of the AFB-14 realized that the shark was unusual and brought it aboard with much difficulty. The shark was shipped to the Kaneohe Bay facility of the Naval Undersea Center and tied alongside the dock overnight. … Preliminary examination indicated that it represented a very distinct, undescribed species, and it was decided that it should be preserved intact. Accordingly, the shark was winched out of the water by the tail using a Navy crane, but the caudal fin broke off and the shark fell into the water and had to be retrieved by divers. The shark was quick-frozen at Hawaiian Tuna Packers, Honolulu, while a large preservation tank was constructed. On 29 November 1976, the shark was transported frozen to the Kewalo dock site of the National Marine Fisheries Service for thawing and injection with formalin. Subsequent examination of the shark … indicated that it is a lamniform shark that is not assignable to any known genus or family and is herein described as Megachasma pelagios, new genus and species, and placed in the new family Megachasmidae.

2. Since then, there have been only 55 confirmed sightings (and a few other rumored sightings), including the animal that was pulled up last week (which you can see below).

3. According to the paper that accompanied the announcement of the species, newspapers actually came up with the name: “It was dubbed the ‘Megamouth shark’ in reference to its unusually large oral cavity. This common name has since been adopted by several authors, and we suggest that it be considered as the accepted common name for the species."

4. It’s one of three known filter feeding sharks, including the basking shark and the whale shark. Scientists believe that the Megamouth cruises through groups of krill with its big mouth open, pushing out its jaw (which has "extreme protrusibility," according to the 1983 paper) and expanding its baccal cavity to suck prey inside, then expelling water through its gills.

Schematic showing the Megamouth's protruding jaw; via Archive.org.

5. The Megamouth is a wide-ranging species: It can be found in the Indian, Atlantic, and Pacific Oceans.

6. While most sharks clock in at around 6.5 feet long, the Megamouth gets much bigger. Maximum size is at least 17 feet, and a number of the discovered specimens go beyond that: One carcass found off the coast of Taiwan measured more than 23 feet long, and another, caught and released off the California coast, might have been as long as 25 feet. Not even Great Whites get that big. According to the 1983 paper on the Megamouth,

The new shark joins the company of the few giant sharks commonly reaching total lengths over 4 m, including the broadnose sixgill shark (Hexanchus griseus), Pacific sleeper shark (Somniosus pacificus), Greenland shark (S. microcephalus), whale shark (Rhiniodon typus), great white shark (Carchawdon carcharias), tiger shark (Galeocerdo cuvier), and great hammerhead (Sphyrna mokarran). The common thresher (Alopias vulpinus) and bigeye thresher (A. superciliosus) also reach total lengths over 4 m, but these sharks have greatly elongated caudal fins and hence are relatively small-bodied in comparison to the giant species. Although new species of small sharks are discovered fairly frequently, giant sharks are not, and almost all of the great species were described in the 18th and 19th centuries.

7. It’s not a great swimmer. According to the Florida Museum of Natural History, “the megamouth is considered to be less active and a poorer swimmer than the basking or whale sharks. Poor mobility likely is a reflection of its flabby body, soft fins, asymmetrical tail, lack of keels and weak calcification.”

8. The Megamouth has 50 rows of tiny teeth on each jaw, but only the first three rows are functional.

9. We call it Megamouth, but in Dutch, it's grootbekhaai; in French, it's requin grande gueule; and in Spanish, it's tiburón bocudo.

10. Scientists put radio tags on a male Megamouth that was caught in a net in 1990 and tracked it for two days, revealing that the sharks undergo vertical migration. Tom Haight, who swam with the shark and photographed it underwater as the animal was tagged and released, wrote that "From dawn to sunset he swam slowly at 450 to 500 feet into the prevailing current, apparently feeding on krill that were at that depth during the daytime. From sunset to sunrise he ascended to 39 to 46 feet below the surface to feed on the krill as they also ascended. The extreme daylight depth could explain why the megamouth shark is so rarely spotted."

11. Researchers observed a Megamouth shark being attacked by sperm whales off the coast of Italy. "The base of the dorsal fin and the gills showed signs of the whales' attack and the shark was swimming slowly and still rather bewildered at the surface," the scientists wrote. Other specimens have had marks that scientists believe were made by cookiecutter sharks, which, according to the Florida Museum of Natural History, "attaches itself to its prey with its sucking lips and sharp pointy upper teeth. Once it is attached, the small shark spins its body removing a cookie-shaped plug from the flesh of its prey with its larger serrated bottom teeth. The prey is left with a perfectly round cookie cutter-shaped hole in the side of its body."

For more on the Megamouth, visit the Florida Museum of Natural History.