Anyone who has ever seen crabs fight for the meat off a fish carcass or discarded chicken bone knows how terrifying it would be to face a swarm of the creatures…let alone a giant one.
In the mangrove swamps of Skull Key, a traveler might assume the crabs are plentiful. She would be right—to her horror. The crabs spend much of their life in the trees, but come down at night in swarms, feasting on anything between them and the shore.
Sahuagin hate the Huge shark-eating crabs. They sponsor great hunts to wipe them out, feasting on crab flesh and constructing shields and bucklers from the crabs’ armored shells. By tradition, no one but an aquatic elf who volunteers for the hunt may be refused—including any land-dwelling humanoids, who will be grudgingly accepted provided they can breathe water and keep up with the shark-men.
The proconsul of Epheme prides himself on the aquarium on his estate, one of the tanks of which holds several giant crabs. This tank is also a last-ditch security measure. Should he be threatened, pulling any one of three levers both empties the tank and frees the crabs. A devotee of tattoo magic, the proconsul plans to use a stored spell to turn gaseous and escape while the crabs turn on any intruders.
—Pathfinder Bestiary 50