TOKYO: The world's largest and extremely rare "megamouth" shark drifted ashore this week in Tokyo Bay, a senior researcher at a museum in Chiba province said yesterday.

A 5.63m shark, weighing a tonne, washed ashore on Monday at a beach in Ichihara city.

The shark was identified as a rare female Megachasma pelagios, commonly known as "megamouth", by Dr Masaki Miya, curator of fishes at the National History Museum and Institute in Chiba.

He said it was the largest "megamouth" found so far. The shark's lower jaw measured 114cm. It was the 21st "megamouth" found in the world and the seventh in Japan.

"It is believed to have entered Tokyo Bay by mistake while chasing planktons," Dr Miya said.

He said it had been alive until a day ago, but died of stress.

Dr Miya said the rare sea creature lived in temperate and tropical zones in the Pacific, Atlantic and Indian oceans.

He said during the day the shark stayed about 120m to 170m underwater but came up at night to feed on planktons.

In April 1988, a "megamouth" was found in Fremantle, Western Australia.

Dr Miya said it was the second time he had identified a "megamouth", following the one he had found in 1993 in Suruga Bay southwest of Tokyo.