What is the largest invertebrate in the world by mass?

The colossal squid is the largest invertebrate in the world by mass. Its scientific name is Mesonychoteuthis hamiltoni, and it lives in the cold, deep waters of the Southern Ocean around Antarctica.

The largest complete specimen ever recorded was a mature female caught in the Ross Sea in 2007. After thawing, it weighed 495 kilograms and measured about 4.2 metres long. Scientists believe even larger individuals exist: enormous beaks recovered from sperm-whale stomachs suggest possible masses of 600 kilograms or more.

This answer is often confused with the giant squid, Architeuthis dux. Giant squid may be longer because of their exceptionally long feeding tentacles, but the colossal squid is heavier and has a larger mantle. It also has swivelling hooks on its arms and tentacles, adaptations that help it seize fish and other prey in the deep ocean.

The species is remarkably elusive. Much of what scientists know comes from damaged specimens, beaks, and remains found in predators, although the 2007 catch provided an unusually complete look at this deep-sea giant.