Japanese spider crab

The Japanese giant spider crab, known scientifically as Macrocheira kaempferi, is a massive marine crustacean. It holds the title for the largest leg span of any known arthropod. An arthropod is an animal with an exoskeleton and jointed legs. This crab can reach a leg span of 3.75 meters, or about 12 feet.

While its carapace, or main shell, reaches a width of 350 mm, its legs continue to grow even after it reaches maturity. This growth pattern causes the spines on its shell to appear smaller as the crab gets older. It is a heavy creature, ranking second only to the American lobster in mass among living arthropods.

To survive in the deep ocean, this crab uses a sophisticated method of camouflage.

It possesses a bumpy carapace covered in small, spike-like tubercles. The crab actively decorates its shell with sponges and other marine organisms to disguise its shape. It uses its slender chelipeds, or claws, to tear objects like worm tubes from the sea floor. The crab then brings these items to its mouthparts to orient and shape them. Finally, it attaches them to its exoskeleton using secretions and mechanical adhesion. This allows new organisms to colonize and grow directly on the crab's shell.

Because these crabs are nocturnally active, they focus on hiding their entire structure rather than matching specific colors. This deception helps them avoid predators like octopuses during the night. The crab's armored exoskeleton also provides physical protection. The species is an omnivore, meaning it eats both plant and animal matter. It may scrape algae from the ocean floor or pry open the shells of small mollusks. It also acts as a scavenger by consuming decaying fish.

The lifecycle of the Japanese giant spider crab begins with a massive reproductive effort.

A single female can lay up to 1.5 million eggs in one season. These eggs typically hatch within 10 days into tiny planktonic larvae. The larvae must navigate four distinct developmental stages to reach adulthood. The first is the prezoeal stage, which is transparent and lasts only a few minutes. This is followed by two zoeal stages and a final megalopa stage. Temperature is a critical factor for survival during these phases. For example, optimal survival occurs between 15 and 18 °C, but survival rates drop significantly in other conditions.

Most adult crabs live in the deep waters off the southern coasts of Honshū, Japan.

They are typically found at depths between 250 and 750 meters, often inhabiting vents or holes in the ocean floor. While they can tolerate temperatures between 10 and 20 °C in aquariums, they prefer slightly different conditions in the wild. The larvae drift in much warmer surface waters, ranging from 12 to 15 °C, during the hatching season from January to March. This temperature difference separates the drifting young from the deep-dwelling adults.

Humans have interacted with this species for a long time, often viewing it as a delicacy.

However, this has led to significant population declines due to overfishing. In 1976, 4,500 crabs were collected, but that number fell to only 1,000 by 1985. To prevent overexploitation, Japan has implemented conservation laws. One rule prohibits harvesting from January through April. This period is the crab's typical mating season when they move into shallower waters. Conservationists also use restocking methods, where they release artificially cultured juvenile crabs into the wild to help the population recover.

Taxonomically, the Japanese giant spider crab is quite unique.

It is the only living member of the genus Macrocheira. Scientists have identified three fossil species belonging to this genus: M. ginzanensis and M. yabei from the Miocene, and M. teglandi from the Oligocene. The species was first described in 1836 by Coenraad Jacob Temminck. It was later moved through different classifications before being placed in its own genus in 1886 by Edward J. Miers. Some scientists believe the genus is so distinct that it may eventually require its own family, separate from Inachidae.