Japanese spider crab

The Japanese giant spider crab (Macrocheira kaempferi) is a species of marine crab and is the largest crab found in the waters around Japan. With a leg-span of around 3.75 metres (12.3 ft), it holds the record for the widest reach of any arthropod. In Japan, it is called taka-ashi-gani, which means "tall-legged crab".

This amazing crab is the only living member of the genus Macrocheira. Scientists have found fossils of three older relatives from millions of years ago in Miocene and Oligocene rock layers in Japan. People enjoy eating these crabs, so special rules have been made to make sure there are still enough crabs in the ocean.

The Japanese giant spider crab looks similar to the smaller European spider crab (Maja squinado), but they are from different families even though they are distant cousins in the animal world.

Description

The Japanese spider crab has the largest leg span of any known animal, stretching up to 3.7 metres (12.1 feet) from claw to claw. Its body can grow to 40 centimetres (16 inches) wide and it can weigh as much as 19 kilograms (42 pounds). Males have longer claws than females.

This crab is orange with white spots on its legs and is known to be gentle despite its scary look. It has a special way of changing its shell that takes about 100 minutes. During this time, it cannot move while it sheds its old shell.

The crab’s hard shell helps protect it from bigger animals like octopuses. It also blends into the ocean floor by covering its shell with sponges and other small sea creatures. The crab carefully picks up these creatures, shapes them, and attaches them to its shell to hide better. Unlike some other crabs, it doesn’t try to match specific colours but instead focuses on hiding its shape, especially at night when it is active.

Distribution and habitat

Japanese spider crabs live mostly in the waters near the southern coast of Honshū, from Tokyo Bay to Kagoshima Prefecture. Some smaller groups have been seen in Iwate Prefecture and near Su-ao in Taiwan. These crabs are usually found deep in the ocean, between 50 and 600 metres below the surface. They like to hide in vents and holes in the ocean floor.

These crabs eat both plants and small sea animals. Sometimes they eat dead fish they find on the ocean floor. Some crabs scrape the bottom for plants and algae, while others open shells to get to the creatures inside.

Lifecycle

Female Japanese spider crabs carry their eggs until they hatch into tiny larvae. They can lay up to 1.5 million eggs each season, and the eggs take about 10 days to hatch. After hatching, the larvae go through four stages before becoming adults.

The first stage only lasts a few minutes. The larvae then go through two more stages, and each stage is affected by water temperature. The best temperature for the larvae is between 15 and 18 °C, and they survive best in water between 11 and 20 °C. During these early stages, the larvae stay near the ocean surface where the water is warmer.

Taxonomic history

The Japanese spider crab was first described by scientists in 1836. A naturalist named Engelbert Kaempfer, who lived in Japan long ago, wrote about this crab in his notes. Over time, scientists moved this crab to different groups as they learned more about it. Today, it is placed in a group called Inachidae, though some think it might need its own special group because of its unique features. Scientists have also found fossils of similar crabs from millions of years ago in Japan and even in the United States.

Anatomy

The Japanese giant spider crab, M. kaempferi, is a very large crab with a pear-shaped shell that measures about 350 mm (13.7 in) along its middle. Its shell has many small, spike-like bumps. Young crabs have longer spines on their shells than older crabs, but as they grow, their legs keep getting longer while their shells stay the same size.

Male crabs have very long front legs with claws, but even these are not as long as the back legs of female crabs. This makes the crabs look quite different depending on whether they are male or female.

Fishery and conservation

The Japanese spider crab is sometimes caught for food and is considered a special treat in Japan and nearby areas. In the past, more of these crabs were caught, but now there are fewer because they have been caught too quickly. Fishermen now have to go deeper into the water to find them.

To help protect the crabs, special rules have been made. For example, fishermen cannot catch these crabs during certain months when they are preparing to have babies. This gives the crabs a chance to grow and have their young in safety. Scientists are also working to raise baby crabs in safe places and then put them back into the sea to help the population grow.