The Japanese spider crab, Macrocheira kaempferi, is the largest known arthropod; fully grown it can reach a leg span of almost 4 m (13 ft), a body size of up to 37 cm (15 inches) and a weight of up to 20 kg (44 lb). The crab's natural habitat is on the bottom of the Pacific Ocean (some 300 to 400 m deep) around Japan, where it feeds on dead animals and shellfish. It is believed to have a life expectancy of up to 100 years.
Currently, this is the only living species of the genus Macrocheira, but there have been two reports of other fossil specimens. M. kaempferi is named in honor of Engelbert Kaempfer, the Japan traveller and naturalist from Lemgo, Germany.
Its compound eyes are situated on the front, and two thorns stick out between them. Younger specimens feature hair and thorns on the shell, and their frontal horns are longer, but these gradually atrophy as the crab ages.
The Japanese spider crab is caught using small trawling nets, and is often eaten salted and steamed. It is caught in the Sagami, Suruga, and Tosa bays and also around the Izu Islands. Catching of the crab is forbidden during the spring, when it lays its eggs.
It is considered a specialty around Suruga Bay, but numbers of the crab have diminished over recent years, and there are efforts to protect them. In Wakayama Prefecture, the crabs are caught when they move to shallower waters in the spring.
The Japanese spider crab is also used for research and ornamental purposes. It has a gentle disposition and is often reared in aquaria.