Porcellanidae - Porcelain Crabs
Porcelain crabs are not "true crabs". They actually evolved from a relative of the squat lobster. The differences between these crabs and true crabs are as follows: porcelain crabs have only 3 pair of walking legs (4th pair small and rudimentary) where true crabs have 4; porcelain crabs lack a wrist segment (carpus) on its claw appendage; and porcelain crabs have very long antennae vs. very short in true crabs. Their abdomens fold under their bodies and move freely. When the crab is alarmed, it sometimes flaps its abdomen to aid in swimming. Their bodies and claws are flat which allow them to hide under rocks and other tight spaces. These crabs easily shed their limbs when caught by a predator or tight squeeze, only to regrow the limb later. A dropped claw may continue to move to distract a predator while the crab makes a getaway.
Porcelain crabs have large claws but do not use them for feeding. They feed by filtering the water for plankton using their feather-like mouthparts. Their claws are used in territorial struggles.