Crabs are crustaceans, like shrimp and lobsters, but their bodies are more rounded. Crabs tuck their abdomens and tails up under their bellies. Most move by crawling along the ocean floor, although there are some species that swim. They have large front pincers that help them find and catch prey like clams, small fish, snails, and other crabs. Crabs do not stalk or pursue their meals; they usually wait for prey to swim by. They also scavenge dead and decaying organisms and look for food by sifting through sand or silt with their legs and antennae.
The spotted porcelain crab (Porcellana saya) is about 1 inch (2.5 cm) long. Its exoskeleton is orange red with white and violet spots that are ringed with red. Some species of spotted porcelain crabs are free living, but others live in symbiotic relationships with various types of hermit crabs and queen conchs (Strombus gigas).
Hermit crabs are not classified as true crabs. True crabs have five pairs of legs, including the claws, and hermit crabs have only four pairs. Like other crustaceans, a hermit crab has a crusty exoskeleton over most of its body; however, this protective armor does not extend over its abdomen, which is soft and vulnerable to predators. For this reason, hermit crabs have to find suitable shells to cover their abdomens. Without this protection, they could become a meal for some other animal.
To forage, hermit crabs travel across the reef floor, dragging their homes with them. Their shells are heavy, so the crabs cannot move very fast. To avoid some of their predators, they usually look for food at night. If frightened, hermit crabs retreat into their shells and cover the entrances with their large right claw. Their smaller left claw is for eating. The largest species in this group is the red hermit crab (Petrochirus diogenes), a rusty-red animal that can grow to 10 inches (25 cm). The red hermit crab is an active predator, but can scavenge food when necessary.
On the reef, true crabs include the lesser sponge crab (Dromidia antillensis), the purse crab (Persephona punctata), and the green reef crab (Mithrax sculptus). The lesser sponge crab is only about 3 inches (7.5 cm) long with the last pair of legs bent over the back to hold a sponge. The purse crab is even smaller, at 2 inches (5 cm), with a white to gray shell that is marked with large red-brown spots. The extremely common tiny green reef crab is only 1 inch (2.5 cm) long and has hairy-looking legs.