The Japanese spider crab (Macrocheira kaempferi) is species of spider crab found in coastal waters around Japan. It has the longest leg span of all arthropods. It is the subject of fishery and is considered a delicacy. Two fossil species belonging to the same genus have been found, Macrocheira ginzanensis and Macrocheira yabei, both from the Miocene of Japan.
The Japanese spider crab has the greatest leg span of any arthropod, reaching up to 5.5 meters from claw to claw. The body may grow to a size of 40 cm in carapace width and the whole crab can weigh up to 19 kilograms, second only to the American lobster among all living arthropod species. The males have the longer chelipeds. 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 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, despite its ferocious appearance.
The Japanese name for this species is taka-ashi-gani literally translating to “tall legs crab.” Their armored exoskeletons help protect them from larger predators such as octopuses, but giant spider crabs also use camouflage. The crab’s bumpy carapace blends into the rocky ocean floor. To further the illusion, a spider crab will adorn its shell with sponges and other animals.