The Japanese spider crab (Macrocheira kaempferi) is a species of marine crab and is the biggest one that lives in the waters around Japan. It has the largest known leg-span of any arthropod around 3.7 meters. The Japanese name for this species is taka-ashi-gani, (Japanese: たかあしがに), literally translating to “tall legs crab”. It goes through three main larval stages along with a prezoeal stage to grow to its great size.
The genus Macrocheira contains multiple species. Two fossil species of this genus have been found, M. ginzanensis and M. yabei, both from the Miocene of Japan. Its diverse taxonomic history is an important part of what these creatures are and how they evolved to be what they are today. They are sought by crab fisheries, and are considered a delicacy in Japan. To prevent overexploitation from harming the species, conservation efforts have been put in place to protect these creatures and their population from overfishing.
The Japanese spider crab is similar in appearance to the much smaller European spider crab (Maja squinado), though the latter, while within the same superfamily, belongs to a different family, the Majidae.
The Japanese spider crab has the greatest leg span of any known arthropod, reaching up to 3.7 m (12.1 ft) from claw to claw. The body may grow to 40 cm (16 in) in carapace width and the whole crab can weigh up to 19 kg (42 lb)—second in mass 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 the 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 Spider Crab also has a unique molting behavior that occurs for about 100 minutes, in which the crab loses its mobility and starts molting its carapace rear and ends with molting its walking legs.
The Japanese Spider Crab has an armored exoskeleton that helps protect it from larger predators such as octopodes, but also uses camouflage. The crab's bumpy carapace blends into the rocky ocean floor. To further the deception, a spider crab adorns its shell with sponges and other animals. The way in which a spider crab is able to pick up and cover itself with such organisms is by following a specific routine behavior. 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. Unlike other species of crab, such as the Chilean crab Acanthonyx petiveri, the Japanese spider crab does not specifically look for matching colors to blend into its environment; it simply camouflages in a way that disguises its entire structure. 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. Once the material is picked up, it is brought to the crab's mouthparts to specifically orient and shape it before it is attached to the exoskeleton. Then, through mechanical adhesion and secretions, the materials attach to the crab, and are able to regenerate, and colonize on the crab.
Japanese spider crabs are mostly found off the southern coasts of the Japanese island of Honshū, from Tokyo Bay to Kagoshima Prefecture. Outlying populations have been found in Iwate Prefecture and off Su-ao in Taiwan. Adults are found at depths between 50 and 600 m (160 and 1,970 ft). They like to inhabit vents and holes in the deeper parts of the ocean. 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). 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).
The Japanese spider crab is an omnivore, consuming both plant-matter and animals. It also sometimes acts as a scavenger, consuming dead animals. Some have been known to scrape the ocean floor for plants and algae, while others pry open the shells of mollusks.
Female crabs carry the fertilized eggs attached to their abdominal appendages until they hatch into tiny planktonic larvae. They can lay up to 1.5 million eggs per season, and these eggs hatch in 10 days on average. Once hatched, these larvae undergo four stages of development before they mature into adulthood.
The first, or prezoeal, stage lasts only a matter of minutes, with most molting within 15 minutes to enter the first zoeal stage. It looks very different from its parents at this stage, with a small, transparent body.
M. kaempferi undergoes two zoeal stages and a megalopa stage before it reaches adulthood. Each of these stages is influenced greatly by temperature, both in terms of survival and stage length. The optimum rearing temperature for all larval stages is thought to be between 15 and 18 °C, with survival temperatures ranging from 11 to 20 °C. At these temperatures, the zoeal stages can last 7 to 18 days, with the megalopa stage lasting 25 to 45 days. Colder water is associated with longer durations in each stage.
During the larval stages, M. kaempferi is found near the surface, as the planktonic forms drift with ocean currents. This surface water ranges between 12 and 15 °C during the hatching season (January to March). This is much warmer than the waters at depths below 200 m, where adults are found, with waters steadily around 10 °C. Optimal temperatures have a 70% survival through the first zoeal stage, which is greatly reduced to a 30% survival in the second zoeal and megalopa stages.
Temminck, in his original description, noted that the crab was known to the Japanese for the serious injuries it can cause with its strong claws. The Japanese spider crab is "occasionally collected for food", and even considered a delicacy in many parts of Japan and other areas in the region. In total, 24.7 tonnes (54,000 lb) were collected in 1976, but fell to only 3.2 tonnes (7,100 lb) in 1985. The fishery is centred on Suruga Bay. The crabs are typically caught using small trawling nets. The population has decreased in number due to overfishing, forcing fishermen into exploring deeper waters to catch them. The average size caught by fishermen is a legspan of 1.0–1.2 m (3 ft 3 in – 3 ft 11 in). Populations of this species of crab have diminished over recent years and many efforts are being made to protect them. One of the primary methods of recovery of the species being used is restocking artificially cultured juvenile crabs in fisheries. 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. 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.
