Hermit crabs are a species of crustacean that has lost its natural shell, so it has to seek discarded shells from other species, notably gastropods. The crab roams until it finds a spare shell and then enters it. The crab will discard the shell as it grows in size and seek a new one.
Biology of crabs
The facts about hermit crab There are 1100 species of hermit crab in the world. They belong to the super family (group of biological families). All of these are characterised by an asymmetrical abdomen, around which no shell grows. This is not normal for members of the crustacea, the biological class to which they belong. However, they have shell on their front parts, which they shed as they grow, just as other sea crabs do. How they adapt The absence of shell makes them vulnerable, but they have adapted by being able to take up discarded shells. They do not use crab shells, but gastropod (sea snail) shells, especially whelk, which are quite large and give them plenty of room. The crab's rear legs have been adapted by evolution to grab onto the acquired shell and hold it so that they can drag it with them as they forage. They move on their front legs. A small number of hermit crab species do not take movable shells, but find a home in small holes in coral reefs, but none of these are found in British waters. The two species found in British waters are Pangurus bernhardus and the smaller Pangurus prideauxi.
Crabs like whelk shells because they provide space to retreat if the crab is attacked. If a foraging gull tries to take the crab, it retreats deep into the shell and unless the gull can break the shell, the crab is safe. Whelks live in deeper water, where the hermit crabs live, so they are more likely to find whelk shells there. As hermit crabs age and grow, they periodically discard the small shell that they are carrying to seek a larger one. This gives rise to vacancy chains, which are sometimes seen in action. Hermit crabs queue up around a shell to be discarded. As the crab leaves it to choose a bigger shell, the next largest crab takes over, often discarding its own shell in the process which is then taken up by the next largest crab and so on.