Photograph by Joel Sartore, National Geographic Photo Ark
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Japanese spider crab, Macrocheira kaempferi, at the Aquarium of the Pacific.

Photograph by Joel Sartore, National Geographic Photo Ark

Japanese spider crab


What is a Japanese spider crab?

They may look like something from a 1950s sci-fi film, but Japanese spider crabs are gentle giants.

And giants they are. Of the 60,000 species of crustaceans on Earth, Japanese spider crabs are the largest, spanning up to 12.5 feet from the tip of one front claw to the other. They’re also one of the world’s largest arthropods, animals with no backbone, external skeletons, and multiple-jointed appendages. In this crab’s case, those appendages are its 10 legs.

Geographic range and appearance

Japanese spider crabs live on the Pacific side of Japan as far south as Taiwan and at chilly depths ranging from 164 feet to as low as 1,640 feet. (They spawn at the shallower end of that spectrum.) They thrive in temperatures of about 50 degrees.

In these waters, their mottled orange-and-white bodies, cream-colored undersides, and spiny, oval carapaces blend in with the rocks on the ocean floor. Those round shells and long legs give Japanese spider crabs an arachnid-like look, hence their common name. These animals also have spines behind and in front of their short eye stalks. Males are larger than females and have larger chelipeds, the legs that hold their claws, though females have wider abdomens to hold their eggs.

Diet and behavior

These slow-moving crabs don’t hunt, preferring to scavenge for dead animal or plant matter, though they may also eat live fish or invertebrates such as other crustaceans.

This species is part of a group known as decorator crabs which adorn their shells with sponges or anemones for camouflage. Juvenile Japanese spider crabs will do this, but with few predators at the depths in which they live, adults of this species don’t have to dress to impress.

Reproductive behavior

Japanese spider crabs migrate to the shallower end of their depth range during mating season, which lasts from January until April. Fertilization is internal, with the male inserting a spermatophore, or sperm packet, into the female as their abdomens press together. The female’s abdomen, also called the apron, is where she carries the fertilized eggs.

These enormous animals start out tiny—females produce more than a million eggs that are about .03 inches each. Not many will survive to hatch, but those that do will emerge after about 10 days and get no parental care. They molt for the first time about nine to 12 days after they hatch.

Researchers have observed that a molt of a single Japanese spider crab in captivity took 103 minutes, and the crab’s growth rate was nearly 22 percent.

Conservation

Japanese spider crabs have not been evaluated by the International Union for Conservation of Nature, but the catch of the species has declined in recent decades. They aren’t subject to large-scale commercial fishing because they live at such depths. When they are fished, however, they're caught in small trawl nets. They are considered a delicacy in Japan, but fishing for them is prohibited in Japan during their breeding season to allow their numbers to increase.