The World's Smallest Types of Crabs

The pea crab (Pinnothera faba) is the smallest crab in the world. Female pea crabs measure half an inch across at their largest, and male pea crabs are significantly smaller at less than a third of an inch wide. This small size, and a circular exoskeleton, earn the pea crab her common name. Pea crabs are parasitic, and spend their lives infesting mollusks, using oysters, mussels and clams to provide safety, oxygen and food. These tiny crustaceans do not feed on the mollusks themselves, but eat zooplankton and food elements the bivalves ingest.

The tiny coral gall crab (Hapalocarcinus marsupialis) grows to only half an inch across her carapace; this size allows the female coral gall crab to slip into small depressions in live coral reefs. The coral grows around the gall crab, trapping her inside. Water and nutrients flow into the gall, and the coral provides security from predators. The female gall crab is an obligate symbiont, meaning that in order to survive she must successfully locate a coral pit and form a gall.

The flattop crab (Petrolisthes eriomerus) stands in line with the world's smallest crabs at half an inch across his shell. A bottom filter feeder, this crab is commonly found along the shores and coastal waters of the eastern Pacific ocean, venturing as far as 300 feet below the surface to feed on the ocean floor. Ranging from California to Alaska, the flattop crab prefers strong currents and plenty of places to hide; rocks and kelp beds provide the most common nesting grounds.

The northwestern shores of the Atlantic Ocean are home to the marsh fiddler crab (Uca pugnax), the most common species of fiddler crab in the world. This minuscule crustacean ranges from half an inch to just under 1 inch across, and can be found on beaches from Florida to Cape Cod. Fiddler crabs are easily distinguished by their single large claw, or chelea. Either one of the male fiddler crab's chelea will grow until it is half the crab's body weight.