The Japanese spider crab (Macrocheira kaempferi) is a species of marine crab. It is the largest known living arthropod and has the largest leg span of any arthropod, sometimes measuring up to 12.5 feet from the tip of one front claw to the other. Japanese spider crabs live in the waters around Japan, and get their name from its resemblance to a spider.
The ancestry of spider crabs can be traced all the way back to prehistoric times, but it was first described by Western science in 1836 by Dutch zoologist Coenraad Jacob Temminck. The name Macrocheira kaempferi comes from Engelbert Kaempfer, a German naturalist and physician who studied plants in Japan during the 17th century.
Their Japanese name, Taka-Ashi-Gani, translates to “tall legs crab”.
As its name suggests, the Japanese spider crab resembles a spider, although it has 10 limbs instead of eight. It is the largest arthropod, its maximum size is 12 feet (3.7 m) across, with its body growing about 15 inches (37 cm) wide. It can weigh up to 44 pounds, and males are generally larger than females.
This crab is a striking orange in color and has white spots along the legs. Its carapace is pear-shaped, narrowing towards the head, and females tend to have a wider abdomen, to hold their eggs. While their legs continue to grow, their bodies remain unchanged throughout adulthood.
Their legs hook inward to assist with hooking and grasping, and males have longer chelipeds, with females having much shorter chelipeds, which are shorter than the following pair of legs. Their long legs are considered weak, and these spider crabs are often seen missing one leg.
The Japanese spider crab has the longest lifespan of any crab, and can live to be 100 years old!
Despite the intimidating appearance of these animals, the Japanese spider crab is harmless and is a slow-moving creature. As a decorator crab, it adorns its exoskeleton with algae and sponges to increase its camouflage to hide from predators like octopuses and fish. That being said, there are actually few predators at the depths in which they live.
Because these crabs have a hard outer shell (the exoskeleton) that does not grow, they must shed their shells. This unique molting behavior occurs for 103 minutes, in which the crab loses its mobility and starts molting its carapace rear and ends with molting its walking legs. This can be dangerous for them, as they can get stuck in their existing shells or be preyed on by other crabs during the molting period.
Little is known about the behavior of spider crabs because they live so deep, but it is thought they are not very sociable animals. They usually forage for food alone and there is little communication between the individuals, even when they are kept in captivity.
Mating season is between January and April and, during this time, Japanese spider crabs migrate to the shallower end of their depth range. Fertilization is internal, with the male inserting a spermatophore, or sperm packet, into the female as their abdomens press together.
The female’s abdomen, also called the apron, is where she carries the fertilized eggs, and she can lay up to one million. However, only a few of the million offspring will survive to adulthood. They hatch into tiny planktonic larvae within 10 days.
While Japanese spider crabs are very large, these larvae are very small and undergo four stages of development before they mature into adulthood. The firsts stage lasts only a few minutes, before the molt into two zoeal stages (planktonic) and one megalopa stage, all of which happen rapidly. The speed at which this happens depends on the temperature of the wter, and the temperature can also determine whether these larvae live or die.
At their zoeal stage, spider crabs look nothing like their parents, and small and transparent with a round, legless body and usually drifts as plankton at the surface of the ocean. Once hatched, they will get no parental care.
Japanese spider crabs are omnivorous scavengers, feeding on any matter they can find on the seafloor and occasionally hunting for small invertebrates like crustaceans. Because they are slow-moving, they prefer to eat dead animals or plant matter, but they will also eat live fish.
Location and Habitat
The Japanese Spider crab is found in the Pacific Ocean around parts of Japan. It is found on the seabed, often inhabiting vents and holes on the ocean floor at depths of 160 to 2,000 feet (50-600 m), and their bodies allow them to blend in well with the ocean floor. They thrive in temperatures of about 50 degrees.
Japanese spider crabs have not been evaluated by the International Union for Conservation of Nature. However, their numbers have been declining in recent years, mainly due to fishing. While it is difficult for fishermen to catch the giant Japanese spider crab because of the depth at which it is found, and the species is not widely exploited commercially, it is considered a rare delicacy in Asia. This has led to more and more spider crabs being caught for their meat around the Japanese islands.
To combat this, there has been a ban placed on the amount of fishing that can take place. Fishing for them has been banned in Japan during the spider crab’s breeding season.