Japanese Spider Crab

The Japanese spider crab "tall-footed crab", Macrocheira kaempferi, is a species of marine crab that lives in the waters around Japan. It has the largest leg span of any arthropod, reaching up to 3.8 metres (12 ft) and weighing up to 19 kilograms (42 lb). It is the subject of small-scale fishery which has led to a few conservation measures.

Description

The Japanese spider crab has the greatest leg span of any arthropod, reaching 3.8 metres (12 ft) from claw to claw. The body may grow to a size of 40 cm or 16 in and the whole crab can weigh up to 19 kilograms (42 lb). The males have the longer chelipeds (claws); females have much shorter chelipeds, which are shorter than the following pair of legs. Apart from its outstanding size, the Japanese spider crab differs from other crabs in a number of ways. The first pleopods (limbs) of males are unusually twisted, and its larvae appear primitive. The crab is orange, with white spots along the legs. It is reported to have a gentle disposition "in spite of its ferocious appearance".

Habitat

Japanese spider crabs are mostly found off the southern coasts of the Japanese island of Honshū, from Tokyo Bay to Kagoshima Prefecture. Outlying populations have been found in Iwate Prefecture and off Su-ao in Taiwan. Adults can be found at depths of up to 600 m (2,000 ft), or as shallow as 50 m (160 ft). They like to inhabit vents and holes in the deeper parts of the ocean.

Life Cycle

Female crabs carry the fertilized eggs attached to her abdominal appendages until they hatch into tiny planktonic larvae. Development of the planktonic larvae is temperature-dependent and takes between 54 and 72 days at 12–15 °C (54–59 °F). During the larval stage the young crab looks nothing like its parents. It is small and transparent with a round, legless body and usually drifts as plankton at the surface of the ocean The Japanese spider crab is an omnivore, consuming both plant matter and animals. It also sometimes acts as a scavenger consuming dead animals. Some have been known to scrape the ocean floor for plants and algae while others pry open the shells of mollusks. They live at depths of 150 metres (490 ft)-300 metres (980 ft) or more. The giant spider crabs migrate up to a depth of around 50 metres (160 ft) during breeding season.

Taxonomic History

The Japanese spider crab was originally described in 1836 by Coenraad Jacob Temminck under the name Maja kaempferi, based on material from Philipp Franz von Siebold collected near the artificial island Dejima. The specific epithet commemorates Engelbert Kaempfer, a German naturalist who lived in Japan from 1690 to 1692 and wrote about the country's natural history. It was moved to the genus Inachus by Wilhem de Haan in 1839, but placed in a new subgenus, Macrocheira. That subgenus was raised to the rank of genus in 1886 by Edward J. Miers. Although currently placed in the family Inachidae, M. kaempferi does not fit cleanly into that group, and it may be necessary to erect a new family just for the genus Macrocheira. As well as the single extant species, four species belonging to the genus Macrocheira are known from fossils.

Fishery

Temminck, in his original description, noted that the crab was known to the Japanese for the serious injuries it can cause with its strong claws. The Japanese spider crab is "occasionally collected for food," and even considered a delicacy in many parts of Japan and other areas in the region. A total of 24.7 tonnes (54,000 lb) were collected in 1976, but only 3.2 tonnes (7,100 lb) in 1985. The fishery is centred around Suruga Bay and they are caught using small trawling nets. The population has decreased in number, forcing fishermen into exploring deeper waters to catch them. Harvesting of the crab is forbidden during the spring, when crabs move to shallower water to reproduce. Populations of this species of crab have diminished over recent years and there are many efforts to protect them. The average size caught by fishermen is a leg span of 1.0–1.2 m (3 ft 3 in–3 ft 11 in).