Temporal range: 136–0Ma
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell carried around by the hermit crab.
Most species have long, spirally curved abdomens, which are soft, unlike the hard, calcified abdomens seen in related crustaceans. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract. Most frequently, hermit crabs use the shells of sea snails (although the shells of bivalves and scaphopods and even hollow pieces of wood and stone are used by some species). The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell.
As the hermit crab grows in size, it must find a larger shell and abandon the previous one. This habit of living in a second-hand shell gives rise to the popular name "hermit crab", by analogy to a hermit who lives alone. Several hermit crab species, both terrestrial and marine, use vacancy chains to find new shells; when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second-biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on. Hermit crabs often "gang up" on a hermit crab with what they perceive to be a better shell, where they will actually pry its home (shell) away from it and then compete for it, and one will ultimately take it over.
Most species are aquatic and live in varying depths of salt water, from shallow reefs and shorelines to deep sea bottoms. Tropical areas host some terrestrial species, though even those have aquatic larvae and therefore need access to water for reproduction. Most hermit crabs are nocturnal.
Shells and shell competition
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. 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.
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. However, during times of molting, hermit crabs in captivity may temporarily move into a smaller shell, presumably so that less sand comes through the shell opening when they bury themselves. The inverse has also been observed: a hermit crab getting ready to molt may move into a shell that is much too large, especially if the hermit crab is not in a habitat where it can bury itself in sand.
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.
Development and reproduction
Hermit crab species range in size and shape, from species with a carapace only a few millimetres long to Coenobita brevimanus, which can live 30–70 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.
The young develop in stages, with the first two (the nauplius and protozoea) 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.
Hermit crabs are more closely related to squat lobsters and porcelain crabs than they are to true crabs (Brachyura). However, the relationship of king crabs to the rest of Paguroidea is a highly contentious topic. Many studies based on physical characteristics, genetic information, and combined data support the longstanding hypothesis that the king crabs in the family Lithodidae are derived hermit crabs and should be classified as a family within Paguroidea. Other researchers have challenged this, asserting that the Lithodidae (king crabs) should be placed with the Hapalogastridae in a separate superfamily Lithodoidea. Six families are formally recognized in the superfamily Paguroidea, containing around 1100 species in total in 120 genera.
- Coenobitidae Dana, 1851 – two genera: terrestrial hermit crabs and the coconut crab
- Diogenidae Ortmann, 1892 – 20 genera of "left-handed hermit crabs"
- Paguridae Latreille, 1802 – 76 genera
- Parapaguridae Smith, 1882 – 10 genera
- Parapylochelidae Fraaije et al., 2012 – two genera
- Pylochelidae Bate, 1888 – 9 genera of "symmetrical hermit crabs"
- Pylojacquesidae McLaughlin & Lemaitre, 2001 – two genera
The fossil record of in situ hermit crabs using gastropod shells stretches back to the Late Cretaceous. Before that time, at least some hermit crabs used ammonites' shells instead, as shown by a specimen of Palaeopagurus vandenengeli from the Speeton Clay, Yorkshire, UK from the Lower Cretaceous.
Several marine species of hermit crabs are common in the marine aquarium trade. Of the approximately 15 terrestrial species 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 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, and some have lived longer than 32 years.
In general, and despite their moniker, hermit crabs are social animals that do best in groups. In the wild they can be found in colonies of a hundred or more. Therefore, many sellers encourage the purchase of more than one crab. Hermit crab terrarium environments require a temperature around 75-85 °F - and humidity around 70-80% -controlled environment. Hermit crabs need salt and fresh water. Hermit crabs are scavengers, thus the food given to hermit crabs is various. Fruits, vegetables, and cooked meat can be given to hermit crabs, as long as they are not put in preservatives. Substrate needed is sand mixed with coconut mulch or crushed coral substrate that is deep enough to allow them to completely bury themselves while moulting. During molting, hermit crabs should not be dug up. Those that are dug up regularly will create a molt inhibiting hormone called Xanthurenic acid. The exoskeleton that is left behind is eaten by the newly molted hermit crab. The exoskeleton contains much needed nutrients, including calcium. 
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