Macrocheira kaempferi holds the arthropod record for leg span, with the largest documented males reaching roughly 3.7 metres across the outstretched claws. The carapace itself tops out near 40 centimetres and the whole animal weighs on the order of 15 to 19 kilograms, which is a great deal less than the span implies. The legs are thin, lightly muscled and comparatively fragile; specimens are frequently found with one or more missing, regrown over successive moults.
The proportions suit the habitat. On soft mud at depths of a few hundred metres, a wide stance spreads weight and lets the crab pick over carrion and detritus without stirring the sediment. Movement is slow, and the animal relies on camouflage rather than speed, with juveniles in particular decorating their shells with sponges and anemones.
The crab is often described as the largest crustacean, which depends on the measure used. By mass, the American lobster is heavier; by leg span, the Japanese spider crab is unmatched among all living arthropods.