Animal Spotlight: Japanese Spider Crab

Taxonomy:

japanese_spider_crab_15340536895_750_500_70
A giant spider crab at Kaiyukan Aquarium in Osaka, Japan. Photo By: Takashi Hososhima
  • Kingdom: Animalia
  • Phylum: Arthropoda
  • Class: Crustacea
  • Order: Decapoda
  • Family: Majidae
  • Genus: Macrocheira
  • Species: Kaempferi
  • Average Body Length: 15 inches (38 cm)
  • Average Leg Span: 13-15 feet (4-4.5 m)
  • Average Weight: 44 lbs (20 kg)
  • Lifespan: Estimated up to 100 years

The Japanese spider crab, a huge and fearsome creature, is found only in the Pacific Ocean near the Japanese islands Honshu and Kyushu. They are found most often in Sagami, Suruga, and Tosa bays as well as off the coast of the Kii peninsula. However, one crab was found as far south as Su-ao, in eastern Taiwan. It is considered an anomaly, caused by a fishing trawler or extreme weather that carried this individual much further than its species’ home range.

These crabs call the vents and holes of the deep, dark depths of the ocean their home, found at depths between 500 to 1,000 ft (152 to 304 m) or more. During breeding season, they will migrate up to 160 ft (49) depth. They will rarely come up to the surface, therefore very little is known about this species.

Physical Characteristics:

Like the name suggests, the Japanese spider crab gets its name from its resemblance to a spider. It has a rounded carapace body, the hard upper shell, covered with stubby projections and long, slim, and spindly legs. The rostrum, an extension of the carapace above the head, is shaped into two slender spines, the antennae, that jut out from between the eyes. The base of the well-developed antennae is fused with the epistome, the area above the mouth. Their sensory systems are not as acute as those of other decapods in the area due to the lack of predators and is not a hunter. Like most species, the male is larger than the female and typically has larger claws.

japanese-spider-crab

All arthropods have exoskeletons that help protect them from larger predators. However, while the shell does play such a role, the Japanese spider crab relies more on its camouflage abilities for protection. It does not possess cryptic coloration that a chameleon has, therefore unable to change color.

The crab’s bumpy carapace, the hard upper shell, blends into the rocky ocean floor. These spiny and stubby tubercles (growths) cover the carapace, ranging from dark orange to light tan in color. To further the illusion, a spider crab will adorn its shell with sponges and other animals. This is especially true for smaller and younger crabs who had a higher risk of being eaten by predators. Adults are typically safe due to their massive size. All spider crab follow this behavior, but the Japanese spider crab does not “decorate” itself as much as other species do.

It sheds its exoskeleton as it grows, just like a snake shedding its skin. A new soft skeleton develops beneath the old. And once it sheds, the new skeleton is expanded by pumping in seawater before it hardens. This allows for extra room for the crab to grow while still providing protection.

The carapace tends to stay the same size throughout adulthood, but the legs and claws (chelipeds) will continue to length as the crab ages.

jgsc1The legs are colored similar to the shell, mottled with orange and white. Its walking legs have inwardly-curving dactyls, the movable part at the tip of the leg, assisting the creature in climbing and hooking onto rock while preventing it from picking up or grasping objects. Their long legs are weak and poorly-jointed to the body of the organisms so they tend to come off due to predators and nets. One study found that three-quarters of surveyed crabs were missing at least one limb. Spider crabs can usually survive with up to three of their walking legs missing. Over time, the walking legs will often regrow during the successive molts.

Its claws are known to be strong, able to grab items quickly and apply enough force to cause serious injury.

Diet:

The Japanese spider crab is an omnivore, consuming both plant matter and animals, but is also also act as a scavenger, eating any corpse it finds.

Generally, these crustaceans do not hunt, picking at any dead and decaying matter along the seabed. However, they will occasionally eat living kept and algae. And although they move slowly, these giant crabs will occasionally hunt for small marine invertebrates that they can catch easily. It is also known to prey open the shells of mollusks.

Behavior:

Not much is known about communication in the Japanese spider crab. They often scavenge alone and remain isolated, even in aquariums. The only time the crustaceans interact is during the breeding season. They mate once a year, seasonally between January and April. Typically, the females will lay 1.5 million eggs, but the vast majority will not make it to adulthood.

The female will carry its fertilized eggs on her abdominal appendages until they hatch. In this way, the mother can stir the water with her back legs to oxygenate the eggs. After the eggs hatch into tiny planktonic larvae, there is no parental investment.

During the larval stage, the young crab looks nothing like its parents. It is small and transparent with a round, legless body. It takes up to 72 days, depending on the water temperature, molting its exoskeleton until it grows legs and begins to look like their parents. As it matures, the spider crab will continue to grow into its giant size.

Conservation:japanese_spider_crab1

The Japanese spider crab is listed as “not evaluated” on the IUCN Red List due to the insufficient data concerning the conservation status of the species.

Because of its preference for the ocean floor, not only is it difficult to gather information, but it also means the species is not widely exploited commercially. However, it is considered a rare delicacy in Asia, caught using small trawling nets.

Japanese fishermen have reported that they are not catching as much as they used to and scientists fear that its population might have decreased significantly in the past 40 years.

Some researchers have recommended a recovery plan which involves restocking with juvenile crabs artificially cultured in fisheries. Otherwise, there are no conservation measures set in place, only a Japanese law prohibiting the harvesting of these spider crabs during the spring, to give the species a chance to spawn.

Interesting Facts:

  • It is the largest known species of crab in the world.
  • Its scientific name, Macrocheira kaempferi, comes from two Greek words: ‘makros’ meaning ‘big’ or ‘long’ and ‘cheir’ meaning ‘arms’. Together, they mean ‘long-armed’. In japanese, the crab is called taka-ashi-gani, which translates to ‘tall-legs-crab’.
  • Mariners used to tell tales of the spider crab dragging sailors underwater and feasting on their flesh. It is regarded as untrue, although it is certainly plausible that if one would feast upon a dead body of a sailor who had drowned to the bottom of the ocean.
  • The crab’s influence extends into the English literature. In “War of the Worlds” by H. G. Wells, the robotic stalk-legged machines that invaded the earth were designed based off of the Japanese spider crab.

References + For More Reading

Tennessee Aquarium: Giant Japanese Spider Crab

Georgia Aquarium: Japanese Spider Crab

Giant Japanese Spider Crab Facts

Macrocheira kaempferi

The shinin-gani – Macrocheira kaempheri

Japanese Spider Crab Carer Manual

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