Mobile Homes & Friendly Neighbours; a Hermit Crab Species Overview

Hermit crabs are a common coastal species found on UK shorelines, infamous for their social nature and distinctive use of snail shells as homes. They belong to the Paguroidea family, which falls under the umbrella term “decapod crustacean”, a collective noun describing the fifteen native species of hermits found in the UK; only 1.36% of the species worldwide. Generally, Hermit crabs will make use of vacant snail shells, often whelk or periwinkle, as their own homes, discarding them and moving into larger ones as they grow. At Ocean Lab, we are currently housing between 30-40 common hermit crabs of varying size, and even though we do get a variety of species in the UK, our aquarium tends to host the most common ones as they are more prolific on the Pembrokeshire coast.  

    Being as widespread as they are, it is unsurprising that there is a large interspecific variation in these animals. Their life span ranges between 1-10 years, and during this period, they can differ in size from around 1-4cm, changing the habitats they occupy as they go. Most peculiarly, there exists a variety of terrestrial hermits that live in tropical areas near water sources so they maintain wet gills, deepwater hermit crabs that live in the far Indian ocean, and even one species of freshwater hermit crab, Clibanarius fonticola, native to the volcanic islands of Vanuatu.

      Nonetheless, the individuality of this family stretches further than habitat type, to the minute details of their home – shells. As an incredibly intelligent species, Hermit crabs have been documented displaying particularly inventive behaviour when it comes to their choice of shell. Firstly, Coenobita, a semi-terrestrial species, have been seen using sections of bamboo stems and broken coconut shells, Xylopargus found in west Indian waters at depths of 180-360m live in hollow cylinders of wood, and other species have been known to make their homes in coral and empty worm tubes. Some hermit crabs even decorate their shells with anemones, such as Pargurus (Eupagurus) bernhardus, a common bright red hermit crab of European and North American coastal waters. This is because they have a symbiotic relationship with these anemones; their stinging cells repel predators and provide camouflage and in return get scraps of food and transported to food sources. Some crabs even transfer their anemones when they go from one shell to another. This process of inhabiting pre-existing shells and swapping as they age saves them a lot of energy metabolically, whilst also making them particularly adaptable to new environments; recently hermit crabs have been seen using discarded rubbish as shells!

These shells are imperative to the hermit crab’s survival, with their abdomen being very soft and making them particularly vulnerable to predation. Within the shell, their body curves to the right and is attached onto the shell using their posterior legs. They have five pairs of legs overall, with the first pair acting as arms and pincers covering the entrance to their shell, the second and third pairs being the ones that they use to move and the fourth and fifth pairs maintaining their position in the shell. Not only do hermit crabs change their shells as they grow, but like other crabs, also moult. Moulting is a common process for this family and occurs as the animal is growing; the new skeleton will grow inside of the old one, the exo-skeleton splits and comes off, and the new one gradually hardens. This is also a period of vulnerability for the crabs as they remain easy prey whilst their skin is soft and exposed.      

Despite their name, hermit crabs are very social animals. They can live in colonies of up to 100 and often sleep piled on top of each other, just like the small hermit crabs in our aquarium do. They are known to swap shells between themselves, often vying for the shell in the best condition and fighting until one crab wins, or knocking loudly on the shell that they want until the current owner is forcibly evicted due to the noise. If in a more peaceful group, the crabs have been seen swapping shells in single file lines, moving from their current shell into the one in front of them.

Written by intern Ruth Clarke, in loving memory of Sea Trust alumni Josh Horn.