Hermit crabs are among the most recognizable decapod crustaceans, characterized by their unique habit of borrowing discarded shells for shelter. These animals belong to a large group of invertebrates known as Anomura, sometimes called the “half-crabs” because they share traits with both true crabs and lobsters. The common name “hermit crab” covers a remarkably diverse array of species that occupy habitats from the deepest oceans to tropical forests.
The True Count of Hermit Crab Species
Scientists classify “true” hermit crabs within the superfamily Paguroidea, which contains approximately 1,200 known species globally. This count is distributed across nine distinct families. Three families—Paguridae, Diogenidae, and Coenobitidae—account for the vast majority of commonly recognized hermit crabs. The family Paguridae, often referred to as the right-handed hermit crabs, contributes over 540 species alone, while the family Diogenidae, known for its left-handed enlarged claw, adds more than 420 species to the count.
Defining Divisions: Marine Versus Terrestrial
The most fundamental way to divide hermit crabs is by their primary habitat, separating the immense group into marine and terrestrial divisions. The vast majority of species, over 95%, are marine hermit crabs, belonging mainly to the Paguridae and Diogenidae families. These aquatic species spend their entire lives submerged, inhabiting shallow coral reefs, sandy ocean floors, and intertidal zones around the world.
Marine hermit crabs possess fully functional, delicate gills that require constant contact with saltwater to operate. Terrestrial hermit crabs, a much smaller and specialized group primarily belonging to the family Coenobitidae, have evolved modified gills that require moisture. Although they spend most of their lives on land, these crabs must maintain moisture in their branchial chamber to breathe.
Terrestrial species like those in the genus Coenobita carry a small reservoir of water inside their shell for this purpose. They must return to the ocean to release their eggs, as the larvae develop in saltwater before returning to land. This reproductive cycle limits the terrestrial group to coastal regions.
The Most Recognized Pet Species
While hundreds of species exist, the general public primarily encounters a small handful of terrestrial species that are popular in the pet trade. These pet hermit crabs all belong to the genus Coenobita and are often sold under common names that vary by region.
- The Caribbean Hermit Crab (Coenobita clypeatus) is one of the most common, distinguished by its purple-tipped claws and common nickname, the “Purple Pincher.”
- The Ecuadorian Hermit Crab (Coenobita compressus) is known for its high activity level and varied coloration, ranging from gray to blue.
- The Strawberry Hermit Crab (Coenobita perlatus) is highly valued for its striking red or orange color.
- The Tawny Hermit Crab (Coenobita rugosus), often called a “Ruggie,” is identified by the rough texture of its claws and the small lines on its carapace.
- The Indonesian Hermit Crab (Coenobita brevimanus) is one of the largest terrestrial species, recognizable by its large, rounded body and prominent, short claw.
The Defining Adaptation: Shell Dependency
The feature that unites every type of hermit crab is the physical need for a scavenged gastropod shell. Unlike true crabs, which have a fully hardened, calcified exoskeleton covering their entire body, hermit crabs have a soft, vulnerable abdomen. This abdominal section is asymmetrical and spirally curved, allowing it to fit snugly into the whorls of a snail shell. The crab secures itself inside the shell using specialized, small appendages at the tip of the abdomen called uropods, which grip the central column of the shell. As the crab grows, it must find and switch to a larger shell, a behavior that often involves competition for this limited resource.