Japanese Spider Crab: Enormous and Harmless to Humans

These massive crustaceans are native to Japan. xia yuan / Getty Images

The Japanese spider crab is a fascinating and impressive animal. In fact, it is the largest arthropod in the ocean!

With its long, spindly legs and armored exoskeleton, this giant crustacean looks like something out of a sci-fi movie. Despite its alien appearance, it's an important part of marine ecosystems.

Living along the rocky ocean floor of Japan’s coastal waters, the Japanese spider crab is known for its massive size and impressive camouflage skills. Unlike other crabs, this species spends much of its life in deeper waters, only venturing into shallower waters during breeding season.

Contents

Biggest Crabs in the World
A Nonthreatening Giant
Life Cycle: From Tiny Larvae to Giant Crustacean
How Japanese Spider Crabs Feed
Visiting Japanese Spider Crabs in Aquariums

Biggest Crabs in the World

The Japanese giant spider crab (Macrocheira kaempferi) is famous for its size.

The chelipeds (the legs with claws) on the males of the species can stretch over 12 feet (3.7 meters) from claw to claw, and the walking legs are relatively smaller (although still big by crab standards).

Interestingly, on female specimens, the chelipeds are shorter than the rest of the legs [source: Animal Diversity Web].

A Nonthreatening Giant

Japanese spider crab. Yiming Chen / Getty Images

Despite its intimidating size, the Japanese giant spider crab is not an aggressive predator; it primarily feeds on dead animals, plant matter, mollusks and other crustaceans.

Its large size helps it avoid many predators, but it still relies on camouflage to blend into the ocean floor. By covering itself with sponges and shells, this crab reduces the chance of being spotted by larger ocean predators.

Life Cycle: From Tiny Larvae to Giant Crustacean

Japanese spider crabs start life as fertilized eggs attached to their mother's rear legs (abdominal appendages, if we're being biologically precise).

After they hatch, they become tiny planktonic larvae drifting in the ocean. These larvae are extremely vulnerable to predators and ocean currents. As they grow, they undergo multiple molting stages, shedding their old carapaces (their hard protective shells) to make room for their expanding bodies.

Juvenile spider crabs look quite different from their giant adult forms. Younger crabs have shorter legs and a more compact body, making them resemble other crabs more closely. It’s only when the crab reaches adulthood that it develops its iconic long legs and massive size.

How Japanese Spider Crabs Feed

These animals are often scavengers, meaning they help clean the ocean by consuming dead animals and aquatic plants.

They also use their strong claws to pry open shells and catch shrimp, mollusks, and other small ocean creatures. This diet plays an important role in maintaining the health of marine ecosystems.

Their feeding habits make them valuable in ocean services, but they are also considered a delicacy food in Japan, leading to conservation concerns. Overfishing could threaten their populations, which is why aquariums work to protect them by raising awareness.

Visiting Japanese Spider Crabs in Aquariums

If you’re interested in seeing a giant Japanese spider crab up close, you don’t have to dive into the ocean; many aquariums house these incredible crustaceans.

The Georgia Aquarium, along with other aquariums around the world, features these species in exhibits where visitors can learn more about their biology, behavior and role in the ocean ecosystem [source: The Georgia Aquarium].

We created this article in conjunction with AI technology, then made sure it was fact-checked and edited by a HowStuffWorks editor.