What is the largest crustacean by leg span?

The largest crustacean by leg span is the Japanese spider crab.

The Japanese spider crab (Macrocheira kaempferi) can reach about 3.7 metres from the tip of one extended leg to the other. That is the greatest known leg span of any arthropod, the animal group that includes crabs, insects, spiders, and lobsters. Its body is much smaller by comparison, with a carapace up to about 40 centimetres wide.

This giant crab lives in marine waters around Japan, often on the seafloor at considerable depth. Its long, slender legs give it a spider-like appearance, while its orange body and white-spotted legs provide a distinctive pattern. Despite its intimidating size, it is generally described as relatively gentle and is caught as a food species in Japan.

The wording “by leg span” matters. The Japanese spider crab is not the heaviest living arthropod; the American lobster exceeds it in mass. Nor is every large crab a rival: king crabs and Tasmanian giant crabs may be heavy, but they do not match its maximum span.