Japanese Spider Crab (Macrocheira kaempferi): Facts and Size

What does a Japanese spider crab look like?

The body is a rounded, pear-shaped shell called the carapace, wide at the back and narrowing toward the eyes. Its surface is covered with small bumps, called tubercles, and short spines. Young crabs have long, straight spines for their size; as the crab grows, the spines become shorter and curve outward at the tip.

Like all crabs, it has ten legs. The front pair end in small pincers and are called chelipeds; the other four pairs are long, thin walking legs. The legs are many times longer than the body, which gives the crab its spidery outline. In adult males the claw-bearing front legs are very long, while in females they are shorter than the walking legs behind them.

The shell and legs are orange to reddish brown, with white spots and patches along the legs. In its Japanese homeland it is called taka-ashi-gani, which means “tall-legged crab.”

How big is a Japanese spider crab?

Most of the Japanese spider crab’s famous size is leg. Measured with the legs stretched out, the largest crabs span about 3.7 m (12 ft), enough to reach across a small car. The shell itself is only up to about 40 cm (16 in) across.

These measurements make it the largest crab and the arthropod with the greatest leg span. It is not quite the heaviest arthropod, though: that record belongs to an American lobster weighing 20.1 kg (44.4 lb), described in how big do lobsters get? For a ranking of other giant crabs, see what is the biggest crab in the world?

Where do Japanese spider crabs live?

Japanese spider crabs live only in the northwestern Pacific. Most are found off the Pacific coast of southern Honshu, Japan’s main island, from Tokyo Bay to Kagoshima Prefecture in the far south. Outlying records come from Iwate Prefecture in northern Honshu and from waters off northeastern Taiwan.

Adults live on sandy and rocky bottoms of the continental shelf and upper slope, usually 50–600 m (160–1,970 ft) deep. They often shelter near holes, crevices and rocky outcrops. Suruga Bay, a very deep bay in Shizuoka Prefecture, is the best-known place for them; they are common there at about 300 m (980 ft), where the water stays close to 10 °C (50 °F) all year. Public aquariums keep them in water of roughly 6–16 °C (43–61 °F).

This cold, dark world is home to many other unusual creatures, featured in deep-sea animals.

What do Japanese spider crabs eat?

The Japanese spider crab is an omnivore and a scavenger. It eats dead fish and other carrion that sink to the seafloor, along with mollusks, worms and other small invertebrates. Some crabs scrape algae and other plant material from rocks.

It is not a fast hunter. The crab picks its way slowly across the bottom, using its slender claws to pick up food, tear it apart and pass it to the mouthparts. With shells such as clams and snails, it pries and picks rather than crushing them with huge claws as some other crabs do. By cleaning up dead animals, spider crabs help recycle nutrients on the seafloor.

Camouflage and decorating

The bumpy, spiny shell blends with the rocky seafloor, and the crab adds to the disguise. Like many of its relatives, which are often called decorator crabs, it attaches living things to its body, such as sponges, algae and the tubes of marine worms. It tears a piece from the seafloor with its claws, shapes it with its mouthparts and then fixes it to its shell, where tiny hooked bristles help hold it in place. Some decorations keep growing after they are attached.

Unlike some smaller crabs that choose decorations to match their background color, the Japanese spider crab seems simply to break up its outline. It is most active at night, so camouflage mainly helps it avoid being noticed by predators rather than sneaking up on prey.

Behavior and molting

Japanese spider crabs are slow walkers and cannot swim. Despite their alarming appearance, they are usually described as calm and unaggressive. They are not harmless to handle, though: the Dutch naturalist Coenraad Jacob Temminck, who described the species in 1836, noted that Japanese fishers knew it for the injuries its strong claws could cause.

Like all crustaceans, this crab grows by molting, shedding its hard outer skeleton and replacing it with a larger one. Aquariums that have watched molts describe the crab freeing its body first and its long legs last, a process that can take more than an hour. Until the new shell hardens, the crab is soft and vulnerable. A molt also lets a young crab replace a lost leg, as explained in can crabs regrow legs?

Reproduction and life cycle

Japanese spider crabs breed in late winter and spring, roughly January to April, when adults move up into shallower water, sometimes only about 50 m (160 ft) deep. After mating, the female carries her fertilized eggs under her folded abdomen until they hatch. A female can produce up to about 1.5 million eggs in a season.

The young go through several larval stages before they look like crabs:

• Prezoea: a tiny, transparent larva that molts again within minutes of hatching.
• Two zoea stages: drifting plankton near the surface, each lasting about 7–18 days depending on temperature.
• Megalopa: a final larval stage of about 25–45 days, which then settles on the bottom as a tiny crab.

Hatching happens from about January to March, when surface water is around 12–15 °C (54–59 °F), warmer than the deep water where adults live. Laboratory studies found that larvae survive best at about 15–18 °C (59–64 °F), and that colder water slows every stage. Few larvae survive to settle.

How long do Japanese spider crabs live?

Many websites and aquarium signs say Japanese spider crabs can live for 50 or even 100 years. These figures are estimates, not measurements. Crabs shed every hard body part when they molt, so they keep no growth rings, and no one has followed wild Japanese spider crabs throughout their lives. The evidence is explained in how long do Japanese spider crabs live?

Predators, threats and conservation

Large adults have few natural enemies thanks to their size, spines and armor; large octopuses are among the few predators thought to tackle them. Larvae and young crabs are far more vulnerable and are eaten by fish and other plankton feeders.

The main threat is fishing. The crabs are caught with small trawl nets, especially in Suruga Bay. Japanese catches fell from about 24.7 tonnes in 1976 to only 3.2 tonnes in 1985, and fishers moved into deeper water to find them. Japan responded with seasonal closures during the spring breeding period, when crabs in shallow water are easiest to catch, and by releasing young crabs raised in hatcheries.

The IUCN Red List has not assessed the species, so its status is Not Evaluated. Its population trend is not well known.

Japanese spider crab vs European spider crab

European visitors often recognize the shape, because the coasts of Europe have their own spider crabs. The spiny spider crab of the northeastern Atlantic (Maja brachydactyla) and its Mediterranean twin (Maja squinado) have the same pear-shaped, spiny shell and also decorate themselves with seaweed. They are much smaller, though, with a shell usually no more than about 20 cm (8 in) long and far shorter legs, and they live in shallow coastal water rather than the deep sea.

The resemblance is partly a family likeness. All of these crabs belong to the spider crab superfamily, Majoidea, but the European species are placed in the family Majidae, while the Japanese giant has its own family, Macrocheiridae. Scientists once grouped it with the long-legged crabs of the family Inachidae, but its unusual larvae and body structure set it apart. No crab in European or North American waters comes close to its leg span.

Japanese spider crabs and people

Western science first described the species in 1836 from specimens collected by Philipp Franz von Siebold near Dejima, the Dutch trading post at Nagasaki. Its species name honors Engelbert Kaempfer, a German naturalist who lived in Japan from 1690 to 1692. Today it is the only living species in its genus. Fossils of related Macrocheira crabs come from rocks around 30 million years old in Washington State, USA, and from younger rocks in Japan, which is why it is sometimes called a living fossil.

In Japan the crab is eaten as a seasonal delicacy, usually steamed or boiled, and towns around Suruga Bay are known for serving it. Elsewhere it is best known as an aquarium star. Its long legs and slow, deliberate movements make it one of the most striking animals in public aquariums in Japan, Europe and North America.