Hermit crab

Hermit crabs can be divided into two groups:[6]

Four hermit crabs in an aquarium

• The first group is the marine hermit crabs (with a single species, Clibanarius fonticola, in freshwater). These crabs spend most of their life underwater as aquatic animals, live in varying depths of saltwater from shallow reefs and shorelines to deep sea bottoms and rarely leave for land. As pets, several marine species of hermit crabs are common in the marine aquarium trade. They are commonly kept in reef fish tanks. They breathe through gills but they don't have to carry around their water to do so. Most can survive briefly out of water as long as their gills are damp. However, this ability is not as developed as it is in land hermit crabs. A few species do not use a "mobile home" and inhabit immobile structures left by polychaete worms, vermetid gastropods, corals, and sponges.[4]
• The second group, the land hermit crabs, spend most of their life on land as terrestrial species in tropical areas, though even they require access to both freshwater and saltwater to keep their gills damp or wet to survive and to reproduce. They belong to the family Coenobitidae. Of the approximately 15 terrestrial species of genus Coenobita in the world, the following are commonly kept as pets: Caribbean hermit crab (Coenobita clypeatus), Australian land hermit crab (Coenobita variabilis), and the Ecuadorian hermit crab (Coenobita compressus). Other species, such as Coenobita brevimanus, Coenobita rugosus, Coenobita perlatus or Coenobita cavipes, are less common but growing in availability and popularity as pets.

Hermit crabs fighting over a shell

A hermit crab retracted into a shell of Acanthina punctulata and using its claws to block the entrance

As hermit crabs grow, they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, vigorous competition often occurs among hermit crabs for shells. The availability of empty shells at any given place depends on the relative abundance of gastropods and hermit crabs, matched for size. An equally important issue is the population of organisms that prey upon gastropods and leave the shells intact.[7] Hermit crabs kept together may fight or kill a competitor to gain access to the shell they favour. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain nonexistent.[8] Hermit crabs with too-small shells cannot grow as fast as those with well-fitting shells, and are more likely to be eaten if they cannot retract completely into the shell.[9]

As the hermit crab grows in size, it must find a larger shell and abandon the previous one. Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells.[8] When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for up to 8 hours. As new crabs arrive they also inspect the shell and, if it is too big, wait with the others, forming a group of up to 20 individuals, holding onto each other in a line from the largest to the smallest crab. As soon as a crab arrives that is the right size for the vacant shell and claims it, leaving its old shell vacant, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size.[10] Hermit crabs often "gang up" on one of their species with what they perceive to be a better shell, and pry its shell away from it before competing for it until one takes it over.[11] There are cases when sea shells are not available and hermit crabs will use alternatives such as tin cans or any other types of debris or even custom-made shells.

For some larger marine species, supporting one or more sea anemones on the shell can scare away predators. The sea anemone benefits, because it is in position to consume fragments of the hermit crab's meals. Other very close symbiotic relationships are known from encrusting bryozoans and hermit crabs forming bryoliths.[12]

Hermit crab species range in size and shape, from species with a carapace only a few millimetres long to Coenobita brevimanus, which can live 12–70 years and can approach the size of a coconut. The shell-less hermit crab Birgus latro (coconut crab) is the world's largest terrestrial invertebrate.[13]

The young develop in stages, with the first two (the protozoa and nauplius) occurring inside the egg. Most hermit crab larvae hatch at the third stage, the zoea. In this larval stage, the crab has several long spines, a long, narrow abdomen, and large fringed antennae. Several zoeal moults are followed by the final larval stage, the megalopa.[14]

Hermit crabs are often seen as a "throwaway pet" that would live only a few months, but species such as Coenobita clypeatus have a 23-year lifespan if properly treated,[15] and some have lived longer than 32 years.[16][17]