Hermit crabs might be accused of stealing their homes. Maybe “stealing” is a bit strong. Perhaps we should say they are “scavenging” or “recycling.” There are over 800 species of of these crabs, and most of them find an abandoned shell of a gastropod (snail) and move in.
The fact that hermit crabs (superfamily Paguroidea) live alone in shells is why we call them “hermits.” For a gastropod such as a snail, the shell is part of its body—an exoskeleton that offers protection. When the owner dies, the hermit takes over the abandoned shell. You might call it a mobile home because hermit crabs, like the original owners, carry the shells with them as they move.
Outside the shell, a hermit crab is vulnerable to predators because of its soft abdomen. Inside the shell, the crab is protected and can retract its entire body if needed. It has a curved abdomen to fit the shell, and the tip is designed to grip the shell tightly. Because they depend on shells for protection, sometimes two of them will fight over one they both like.
Marine hermit crabs spend most of their time underwater, breathing through gills. As long as their gills stay wet, they can stay on land briefly. There are about 15 species of land hermit crabs, but they still need access to water. People sometimes keep them as pets.
As a hermit crab grows, it needs larger shells, so they compete to find new homes. Some have observed them lining up in a queue from largest to smallest. When the largest moves to a bigger shell, each of the others moves up to the next size. Having shells available requires a balance between the number and size of the crabs and the gastropods that die. Sometimes, they are forced to find shelter in hollow pieces of wood or rock. Sadly, they may even take refuge in plastic trash from which they cannot escape.
Some larger hermit crabs support sea anemones on their shells. The venomous anemones protect the crabs from predators, and they benefit by eating food fragments that escape the crab’s grasp. Once again, we see how the web of life is designed to work together for survival. We believe this is no accident but the work of a Master Designer.
— Roland Earnst © 2025
