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Japanese giant spider crab, Macrocheira kaempferi (introduction – habitats – biology – utilization)

Introduction

This photo of the Japanese giant spider crab (Macrocheira kaempferi) was filmed by Samart Detsathit (Thailand)  (shown in the photo) at Kamogawa Sea World, Japan.

The name of this marine crab goes to its existence in the water around Japan. The crab is known in Japan as “Taka-ashi-gani” which means “tall legs crab”.

The Japanese Spider Crab has the largest leg span of any arthropod in the world reaching in adult specimens up to nearly four meters with an average length of about 3 meters as measured from the tip of one claw to the other when stretched apart. The average leg span of 1.0–1.2 meters when caught by fishermen. This longest known arthropod is not the heaviest arthropod as an animal of this size weighs only 16-20 kg keeping in mind that the well-calcified carapace of the crab may grow to only around 37- 40 centimeters as carapace width in adult specimens. It may worth to mention that while the walking legs and claws lengthen as the crab ages, the carapace tends to stay the same size throughout adulthood.

The Japanese spider crab has eight long and weak legs which could be lost in some incidences. Reports show that the Japanese Giant spider crabs can survive with up to 3 walking legs missing. However, walking legs often grow back during the successive molts

The Japanese Spider Crab is one of the longest living creatures in the world and is often reported that it may live up to 100 years in its natural habitat. However, other reports indicate that the species generally lives for more than 50 years.

 

Distribution and habitats

Distribution: The Japanese Spider Crab, Macrocheira kaempferi can be found off the southern coasts of the Japanese island of Honshu, from Tokyo Bay to Kagoshima Prefecture.

Habitats: Japanese spider crabs most often inhabit the sandy and rocky bottom of the continental shelf and slope. Adults can be found at depths of up to 600 m, or as shallow as 50 meters. Younger crabs tend to live in shallower areas with warmer temperatures. The spider crab prefers the vents and holes in the deeper waters that are big enough to take it.

 

Feeding habits

The giant spider crab is an omnivorous and scavenges for food. They consume animal materials (e.g. small fish, carrion, aquatic crustaceans and other marine invertebrates) and plant matter such as algae and macroalgae. In nature, they normally do not hunt. Instead, they crawl along the seafloor and pick at dead and decaying matter. Having such sharp teeth enables the giant spider crab to shred and crush food materials.

On the other hand, their slow movement of giant crab on the sea floor makes them an easy target for possible predators even though this is not common. In order to deal with possible predation, crab juveniles may decorate their carapaces with sponges, kelp, or other objects as a means of camouflage as well as protection. Most use their large claws against predators especially the small ones.

 

Reproduction

Like most of the crustaceans, the giant Japanese spider crabs have separate sexes. The mature male and female of the crabs migrate up during the breeding season to a depth of around 50 meters.  During mating, the male crab transfers the spermatophore (a sac containing sperm) into the female’s abdomen using his first two chelipeds. Once the fertilization is done, the female crabs carry the fertilized eggs tight against their bottom portions of their bodies and backs during the incubation period. Mother crabs use their back limbs to shift around the surrounding water in order to provide their embryos with vital oxygen.  Hatching takes place about 10 days of incubation where tiny planktonic larvae emerge. The newly hatched larvae drift as plankton at the surface of sea waters. Those larvae are clear and legless and so they are totally dissimilar to their crab parents. They develop and grow through molting until they get their permanent skin. There is no parental care provided by any of the parents after hatching. A female Japanese spider crab deposits more than 1.5 million eggs annually; however, the vast majority of them do not survive to maturity.

 

Utilization of the giant crab

The Japanese giant spider crab, Macrocheira kaempferi has its importance to the Japanese culture. Also, the economic importance of the spider fishery comes from its utilization as food, aquariums, and even for taking and decorating its carapace.

Fishery of the giant spider crab: The crab fishery is centered around Segami, Tosa and Suruga Bays and they are caught using small trawling nets. The decrease of the crab population has forced fishermen to explore deeper waters to catch them.

Utilization of the giant crab as food: This crab is occasionally collected for food even in small quantities. The crabs are often served as a delicacy during the appropriate crab-fishing seasons and are often eaten raw, salted and steamed.

Giant spider crab in Aquariums: The very unusual shape and large size of the crab is attracting hobbyists who look for one of the most unusual aquatic pets while enjoying gentle nature. In such case, a huge aquarium will be required.

Conservation measures: This crab is not subject to commercial fishery because of difficulties in capturing them in large quantities at their deep habitats.  Hence, crabs are only caught by small-scale fishery implying less damaging actions to their stocks. However, the observed decline in the catch of the crab has led to a few conservation measures that centered on the banning of crab harvesting during the spring, when crabs move to shallower water to reproduce.

References: Wikipedia, Tennesse Aquarium, Encyclopedia of Life, Natural World, Georgian Aquaculture, Pawnation

Japanese giant spider crab

 

 

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