Creature of the Month: October 2021

Creature of the Month: Japanese Spider Crab
By: Haley Chasin, UHM MOP Student

Many deep sea creatures have inspired the design of monsters or alien creatures in film. The Japanese spider crab, Macrocheira kaempferi, is no exception. Growing up to 3.7 meters in length, this massive arthropod is one of the largest crabs in the world. The Japanese spider crab gets its name from its similarities to a spider, another branch of arthropods. Like a spider, M. kaempferi has no backbone, an external skeleton, and multiple-jointed appendages. Its carapace is shaped like a pear, with females having a wider and shorter abdomen. The longest of the walking, or ambulatory legs are the meri (the fourth leg). The surface of the spider crab’s body is covered with spines mottled in color so the animal can blend in with the seafloor. Two slender spines extend out between the eyes. Their eyestalks are short and stubby with antennae reduced. As such, their sensory system is not as acute as other decapods in the area. These crabs are scavengers whose diet consists of algae, fish, carrion, crustaceans, and detritus.

The Japanese spider crab can be found in depths anywhere from 50 to 600 meters, usually inhabiting waters 200 meters deep and then coming to shallower waters during their breeding season in early spring to spawn and to molt, forming huge congregations.

Females carry the eggs on their back and lower bodies. They can lay as many as 1,500,000 eggs. After these eggs hatch, they go through two zoeal stages, lasting 12-37 days, and one megalopal stage, lasting 30 days. Younger crabs stay in shallower, warmer waters and can camouflage themselves using small objects like sponges and anemones. This becomes less of a concern as they grow, since at their full size, the spider crab has very few predators. These crabs can live for up to 100 years. M. kaempferi are a delicacy in Japan, eaten both raw and cooked. The Japanese do not fish for them during the spring, as this is their reproduction period. Japanese spider crabs are also coveted because their walking legs are so long, and the tendons from the legs or chelipeds are used in research. There is insufficient data on their conservation status, but the catch of the species has declined in the last 40 years. Some researchers are devising a plan for recovery involving restocking juvenile crabs artificially cultured in fisheries.