To prevent overexploitation from harming the species, conservation efforts have been put in place to protect them and their population from overfishing.

[6] The body may grow to 40 cm (16 in) in carapace width and the whole crab can weigh up to 19 kg (42 lb)[7]—second in mass only to the American lobster among all living arthropod species.

[9] The Japanese spider crab has an armored exoskeleton that helps protect it from larger predators such as octopuses, but also uses camouflage.

Upon picking up the object with the crab's slender chelipeds, the chelae are used to twist and tear off the organism, such as a worm tube or sponge, from the substrate on which it currently resides.

[11] This is most likely because Japanese spider crabs are nocturnally active, so instead of trying to disguise themselves when catching prey, they are actually just trying to avoid predators at night.

[12] The temperature preference of adults is unknown, but the species is regular at a depth of 300 metres (980 ft) in Suruga Bay, where the water generally is about 10 °C (50 °F).

[13] Based on results from public aquaria, Japanese spider crabs tolerate temperatures between 6 and 16 °C (43 and 61 °F), but are typically maintained at 10–13 °C (50–55 °F).

[8][12] Female crabs carry the fertilized eggs attached to their abdominal appendages until they hatch into tiny planktonic larvae.

[15] The Japanese spider crab was originally described by Western science in 1836 by Coenraad Jacob Temminck under the name Maja kaempferi, based on material from Philipp Franz von Siebold collected near the artificial island Dejima.

[17] The specific epithet commemorates Engelbert Kaempfer, a German naturalist who lived in Japan from 1690 to 1692 and wrote about the country's natural history.

The genus Onicinopus has a semi-hardened body, which allows the basal segment of the antennae, which articulates with the head capsule, to move.

This proportionality explains, as in other decapod crustaceans, that spine size decreases as specimens grow older.

As mentioned in the taxonomic section, this genus contains the family's primitive feature of a movable antenna at the basal segment, but "the development of a spine at the posterior angle of the supraocular eave, and the presence of intercalated spine and antennulary septum seem to attribute a rather high position to this genus."

[4] The Japanese spider crab is "occasionally collected for food",[22] and even considered a delicacy in many parts of Japan and other areas in the region.

[23] The population has decreased in number due to overfishing, forcing fishermen into exploring deeper waters to catch them.

[15] Additionally, laws have been put into place in Japan that prohibit fishermen from harvesting spider crabs from January through April, during their typical mating season when they are in shallower waters and more vulnerable to being caught.

[23] This protection method seeks to keep natural populations growing, and enables time for juvenile spider crabs to go through the early stages of their lifecycle.