The Japanese spider crab (Macrocheira kaempferi) is an immense marine arthropod, holding the distinction of being the largest known arthropod species on Earth. Its massive size is owed almost entirely to its extraordinary appendages. The creature’s appearance is defined by a relatively small, armored body shell supported by ten exceptionally long, spindly legs. This unique structure gives the crab its common name and contributes to its notoriety as a deep-sea giant.
Defining the Maximum Dimensions
The crab’s size is defined by two primary measurements: leg span and carapace size. The maximum length is measured from the tip of one outstretched claw to the other, reaching up to 3.7 meters (12.1 feet). This tremendous leg span establishes the species as the largest arthropod by dimension.
The actual body, or carapace, is substantially smaller than its limb reach. The pear-shaped carapace, which houses the crab’s organs, can grow up to 40 centimeters (16 inches) across and is covered in spiny growths. While the legs grow throughout the crab’s life, the carapace size stabilizes once the animal reaches maturity.
The maximum weight can reach 19 to 20 kilograms (42 to 44 pounds). Males are generally larger, possessing chelipeds (claw-bearing legs) that are significantly longer than those of the female. Commercially captured specimens are smaller, often featuring a leg span of 1.0 to 1.2 meters.
Geographic Home and Deep-Sea Habitat
The Japanese spider crab is primarily found in the cool, deep waters surrounding the Japanese islands. It is endemic to the Pacific side of the country, with notable populations concentrated in areas such as the Sagami, Suruga, and Tosa bays. These environments allow the species to thrive away from the pressures of shallower coastal zones.
Adults inhabit the sandy and rocky bottom of the continental shelf and slope, residing at depths ranging from 50 to 600 meters. Most crabs are consistently found between 150 and 300 meters, where the environment is characterized by high pressure and perpetual darkness. The water temperature in these deep habitats remains constant and cold, often around 10 degrees Celsius.
The crabs venture to shallower depths during the spawning season, migrating to waters as shallow as 50 meters. This temporary move allows for reproduction before they return to the stability of the deeper seafloor.
Unique Biological Traits and Longevity
The Japanese spider crab is known for its exceptional longevity, with estimates suggesting it can live for up to 100 years in its natural environment. This makes it one of the longest-living arthropods known. Its slow growth rate, common in deep-sea species, contributes to its impressive age as it matures over many years.
The species operates as an omnivorous scavenger, playing a role in the deep-sea ecosystem as a cleaner. Their diet consists mainly of dead or decaying organic matter that drifts down to the seafloor, such as detritus and dead fish. They also consume live food, using their large claws to capture small invertebrates, algae, and tear open the shells of mollusks.
Growth requires molting, where the animal sheds its hard outer exoskeleton to increase its size. For a creature of this scale, molting is a perilous event, as the crab is highly vulnerable until its new shell hardens. Observations in captivity show that molting can take over an hour, during which the crab is defenseless against predators.
Human Interaction and Conservation Status
The Japanese spider crab has a long-standing relationship with human activity, particularly in its native range. It is commercially fished, primarily in the waters of Suruga Bay, and its meat is considered a delicacy in Japan. Although valued as a food source, the deep-water fishery is a small-scale operation, typically using small trawling nets.
The species is also a popular exhibit in large public aquariums worldwide due to its striking appearance. These captive environments allow for research and education, but the crabs require specialized care to maintain the cold water temperatures and low-light conditions of their natural habitat.
Although not currently listed as globally threatened by major conservation bodies, the population has faced localized pressure. Overfishing has led to a noticeable decline in catch numbers over recent decades. To protect the species, Japanese law prohibits fishing during the mating season, which occurs from January through April when the crabs are in shallower, more accessible waters.