Anything

With each other.

Heavily armored, they are valued for their fierceness in battle and their ability to traverse the sea floor agilely. They have huge pincer arms and a second set of arms with more human-like hands, but which can double as a second set of legs in a, ahem, pinch.

They call themselves, sometimes, brachyurans. The crabmen are slightly mad, heavily armored crustacean people. They live close to the coasts and are known to mutter to themselves constantly.