This giant crab belongs to the infra-order Brachyura and super-family Majoidea.
The giant Japanese spider crab has the biggest leg span of any other arthropod. The largest crab has the width of 12 feet 6 inches, from one claw to the other and 16 inch wide body. Weight of such a crab can be somewhere around 20 kg. Male spider crabs have longer leg span than the females. Apart from the leg span, there are numerous things in which this crab differs from the other crab species. It has orange body with thin legs which are covered with white spots. Their claws are usually longer than the legs and when open, they can reach to a width of about 3 meters! It has oval shaped shell and a pair of compound eyes. Young crabs bear small thorns on their shell and frontal horns, which they molt as they get older.
Its natural habitat is the ocean beds, especially of the Pacific Ocean. They are mostly found off the south eastern coasts of Honshu (a Japanese island), Tokyo Bay and Kagoshima Prefecture. Their noticeable population is also found off the coasts of Taiwan. Adult crabs can be found from deep water areas of 2000 feet to the shallow areas of 160 feet as well. But their most prominent habitat is the depth of 800 to 1000 feet. Sea goers have found their eggs at the depths of 300 feet, during springtime. Another interesting fact is that, they can live up to 100 years when they are in their natural habitat.
Giant Japanese spider crabs have separate sexes, similar to most of the crustaceans. Male crabs are generally larger than the females, with bigger claws. The spermatophore (a sac containing sperm) of males crabs, is transferred to the female crabs during mating, with help of their two frontal appendages. Once the fertilization is over, the female crabs carry the egg sac in their abdominal appendages. They create a cement like material to secure it to their abdomens. The newborn crabs look totally dissimilar to their parents. They bear translucent body with no legs. They molt their skin for a few times, until they get their permanent skin.
They are omnivorous arthropods and hence they can survive on a large food menu! They sometimes have habit to act like the scavengers and sometimes like vegetarians; while most of the time, they are the fearsome 'Japanese spider crab predators'! Diet mainly includes deep water algae, plants, mollusks, starfish, shellfishes and sometimes the animal and human carcasses. Though they are giant, the spider crabs need protection from the larger sea animals and hence they attach sponges and other such smaller animals to their shells for camouflaging.
Though their claws can be pretty painful, they are occasionally caught for food. Due to their delicious meat, their number has noticeably decreased from the shallow waters. The largest Japanese spider crab that was ever caught by the fishers had the leg span of around 3 feet. All in all, this crab is nature's marvel, and saving it from extinguishing is our responsibility.