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Description
The Japanese Spider Crab is known for having the largest known leg-span of any arthropod. This large species can be found in waters near Japan and is seen as a delicacy in the country. The animal on average is 12 inches across while its legs can grow up to being 12 feet long. They weigh around 40 pounds. There is no known number how long they live for, but the estimate is around 100 years. The difference between both genders is that adult males have longer chelipeds while females have shorter chelipeds.
Evolution
The Japanese Spider Crab was first described in 1836 by Coenraad Jacob Temminck while researching material from the artificial island Dejima. It was first named the Macrochaeira kaemferi because of its resemblance to a spider. Three ancestors to the Japanese Spider Crab are the European Spider Crab, Snow Crab, and Great Spider Crab. The closest relative to the species is the Arrow Crab.
Taxonomy
Kingdom - Animalia
Phylum - Anthropoda
Class - Malacostraca
Family - Inachidae
Order - Decapoda
Genus - Macrochaeira
Species - Kaempferi
Habitat
The Japanese Spider Crab lives on the seafloor along the pacific coast of Japan. They're found around 160-1970 ft deep in the ocean in temperatures of 50 degrees. On the seafloor, the Japanese Spider Crab live along the rocky and sandy slopes. Along the seafloor the crabs live along with mollusks, algae, and small fish.
Distribution
Japanese Spider Crabs are only found along Japan's Pacific coast. They are not invasive and actually benefit marine ecosystems because of them feeding on dead material.
Diet
The Japanese Spider Crab eats decaying fish, invertebrates, and algae. Sometimes they do eat live mollusks and algae by tearing them with their pincers. They get their food by moving quickly with their long legs to catch their prey. The crab eats at any moment that prey comes around.
Social Structure
Japanese Spider Crabs are solitary detritivores. When they forage for food, they like to do it alone. But when they travel, they prefer to do it in groups. They blanket the ocean floor and stack on top of each other. They also like to make groups for mass molting rituals. Where they would leave their old shells behind in one spot as they could come out looking brand new.
Reproduction
Mating season for the crabs is between January and April. For mating season, they travel to the shallower sides of the ocean to meet up with other crabs. They mate through the male crab inserting a spermatophore into the female crab. In a season Japanese Spider Crabs can lay up to 1.5 million eggs, but only a little amount would survive.
Development
The stages of a Japanese Spider Crab are: The zoeal larvae moults which happens 12 days after hatching, zoeae moults which happens two weeks after first moulting, then the megalopa stage. The process of reaching sexual maturity can take several years due to Japanese Spider Crabs usually being around the ages of 7-10. Throughout development their carapace stays the same when they're an adult, but their legs continue to grow. For male crabs their first pleopods develop in a twisted way and their larvae appear primitive. They also have much larger claws while female have small claws with little to no meat in them.
IUCN Rating
Due to overfishing, the Japanese Spider Crab population has slowly been decreasing, but it has not yet been rated in the IUCN. Technically they are not listed as endangered, but Japan has banned harvesting the crab during the mating season. The population estimate is around 60,000.
Causes of death
The biggest predators towards the Japanese Spider Crab are large fish and invertebrates such as octopus and stingrays. Many crabs lose a limb or two trying to flee from these predators. Another big predator on the Japanese Spider Crabs are humans, who usually kill the crabs for hunting. They do not have any known diseases or sexual competition.
Competition
One of the Japanese Spider Crabs rivals is the green crab (also known as the Asian Shore Crab in Asia). The green crab is an invasive species that has competed with Japanese spider crabs for food and supply. This crab has been competing with other native species and their prey for multiple decades.
Impact of Humans
In asia, the crab is seen as a delicacy and is quite significant to japanese culture. Many overfish the species and use the crab for special meals. It can be eaten both raw and cooked. In some parts of japan its popular to use the carapace of the crab and decorate it. For researchers, they usually use the tendons of the long legs or the chelipeds for their studies. The overfishing of the species can decrease the population over time which could lead to the endangerment of the crab. Due to the problem, Japan decided to create a ban that would go against harvesting Japanese Spider Crabs in the spring mating season.
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