Invertebrates
Japanese Spider Crab
Macrocheira kaempferi · also called giant spider crab, taka-ashi-gani
The Japanese spider crab is the largest crab in the world by reach — a deep-sea scavenger from the cold water off Japan whose spindly legs can span nearly four metres, and which may live for a century on the dark, muddy slope of the continental shelf.
Overview
Japanese spider crabs live on the deep seabed around Japan, mainly along the Pacific coast of Honshu, at depths of roughly 150 to 300 metres, though they are found shallower in spring when they move up to breed and deeper on occasion. The bottom there is soft mud, sand, and rock, cold and dim, and the crabs move slowly across it on their long, jointed legs, feeding on whatever the seabed offers.
The biggest reach in the animal kingdom
No living arthropod spans as wide. A large male’s legs, fully extended, reach about 3.7 metres from claw to claw, while the body — a roughly triangular, spiny carapace — is only around 30–40 cm across. The front pair of legs end in claws (longer in males), and the crab uses them to pick apart carrion and prise open shells. The long legs make the animal look enormous, but they are thin and relatively fragile, and old crabs are often missing one or more.
Diet and decoration
The Japanese spider crab is an omnivorous scavenger, eating dead fish and marine mammals, molluscs, kelp, algae, and slow invertebrates. Like other spider crabs it is a decorator: it fixes sponges, anemones, and seaweed onto the hooked hairs of its shell for camouflage and chemical defence, tending the garden and transferring it to each new carapace after moulting.
Life cycle
In spring, spider crabs gather in shallower water to mate, and a female may carry over a million tiny eggs beneath her body until they hatch into planktonic larvae. The larvae drift for weeks before settling and beginning the long series of moults toward adult size. Growth is rapid at first and then very slow, and the largest crabs are likely many decades old.
Threats and status
The Japanese spider crab is listed as Least Concern but is declining. It has been fished commercially off Japan for well over a century, taken in trawls and pots and sold as a delicacy, and catches have fallen substantially, prompting seasonal closures during the breeding period and other restrictions. Deep-sea habitat disturbance from trawling is an additional pressure. Its slow growth and late maturity make it slow to recover from overfishing.
Japanese Spider Crab: common questions
How big is a Japanese spider crab?
It has the greatest leg span of any arthropod alive — up to about 3.7 metres, or 12 feet, measured from the tip of one front claw to the tip of the other on the largest males. The body itself is much smaller, a carapace around 30–40 cm across, and the animal weighs up to roughly 19 kg. Only the American lobster is a heavier crustacean; nothing has a wider reach.
How long do Japanese spider crabs live?
Estimates run up to around 100 years, which would make it one of the longest-lived of all crustaceans. Growth is fast when young and then slows dramatically, and very large individuals may have stopped growing altogether decades earlier. Precise ageing is difficult because crabs moult their skeleton and leave no permanent growth record, so the century figure is an informed estimate rather than a measured maximum.
Is the Japanese spider crab dangerous?
No. Despite the size and the alarming appearance, it is a gentle, slow-moving scavenger. Its claws can pinch and the largest could injure a careless hand, but the animal is not aggressive and poses no threat to divers, who occasionally encounter them on deep dives. In aquariums they are handled routinely by staff.
Why does it stick things to its shell?
It is a "decorator crab." The Japanese spider crab attaches living sponges, sea anemones, kelp, and other bits of seabed life to the hooked bristles on its carapace and legs. This camouflages it against the bottom, and some of the attached organisms — stinging anemones, foul-tasting sponges — actively deter predators such as octopuses and large fish. The crab tends and rearranges its decorations, moving them to a fresh shell after each moult.