Hermit crabs belong to the infraorder Anomura, a group of decapod crustaceans that currently includes more than 2,500 described species. Although their name suggests they live alone, many species are quite social and play an essential role in the marine ecosystems they inhabit. What makes them so recognizable is their peculiar habit of living inside empty molluscs shells, which they use for shelter and protection.
The infraorder Anomura is not composed only of hermit crabs. It also includes other groups such as the yeti crabs (Kiwaidae), the king crabs (Lithodidae), and the porcelain crabs (Porcellanidae). However, the true hermit crabs mostly belong to the families Paguridae and Diogenidae, with representative genera like Pagurus or Clibanarius.
Morphologically, one of the most distinctive characteristics of hermit crabs is their soft and asymmetrical abdomen, which contrasts with the hard carapace of the rest of the body. This vulnerability has led to their famous strategy of taking refuge in empty shells, usually those of sea snails. The curved shape of their abdomen allows them to fit perfectly inside these shells, which they change as they grow.
These crustaceans have ten legs like all decapods, but not all serve the same function. Two are modified into large claws, two serve for walking, and the smaller rear legs are used to hold onto the shell. Some species even decorate their shells with sea anemones or other organisms, which provide additional protection in exchange for mobility and food scraps.
Hermit crabs are found in all the world’s oceans, from tropical coasts to cold deep seas. Some species, like Coenobita clypeatus, have colonized terrestrial environments and live on beaches or rocky areas, returning to the sea only to reproduce.
Regarding their diet, they are omnivores and scavengers. They feed on dead animal remains, algae, small invertebrates, and detritus (decomposing organic matter), making them important recyclers in the marine ecosystem. Their varied diet and adaptability have allowed them to colonize a wide diversity of habitats.
The reproduction of hermit crabs is sexual, with separate males and females. The male transfers sperm to the female through copulation, and she deposits the fertilized eggs under her abdomen, protected by the shell. After an incubation period, the eggs hatch and release microscopic larvae that go through several planktonic stages before settling on the seabed and seeking their first shell.
An interesting aspect of their life is the competition for shells. Often, several individuals fight or exchange shells in a cooperative behavior known as ‘vacancy chains,’ where several hermit crabs line up by size and simultaneously switch shells.
In summary, hermit crabs of the infraorder Anomura stand out not only for their peculiar lifestyle but also for their ecological role as marine recyclers and their adaptability to diverse environments. They are a fascinating example of how evolution finds creative solutions to nature’s challenges.
