Hermit crab

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Hermit crab

Hermit crabs Fossil range: 136–0 Ma PreЄ Є O S D C P T J K Pg N
Pagurus armatus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Decapoda
Suborder:	Pleocyemata
Infraorder:	Anomura
Superfamily:	Paguroidea Latreille, 1802
Families
Coenobitidae Diogenidae Paguridae Parapaguridae Pylochelidae Pylojacquesidae

Hermit crabs are decapod crustaceans of the superfamily Paguroidea.^[1] They are not closely related to true crabs. Hermit crabs are quite commonly seen in the intertidal zone: for example, in tide pools.

Most species have long, soft abdomens which are protected from predators by a salvaged empty seashell carried on the crab's back, into which the crab's whole body can retract. Most frequently hermit crabs use the shells of sea snails; the tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell ^[2]. As the hermit crab grows in size, it has to 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.

Of about five hundred known species, most are aquatic and live in varying depths of saltwater, from shallow reefs and shorelines to deep sea bottoms. However, tropical areas host some terrestrials.

The king crabs in the family Lithodidae were formerly considered to be derived hermit crabs, but are now placed in a separate superfamily ^[3].

Shells and shell composition

Hermit crabs fighting over a shell

As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, there is often vigorous competition 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 ^[4].

A hermit crab with a too-small shell cannot grow as fast as those with well-fitting shells, and is more likely to be eaten if it cannot retract completely into the shell ^[5].

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.

Development and reproduction

Hermit crab species range in size, shape, from species with a carapace only a few millimetres long to Coenobita brevimanus which can approach the size of a coconut. The shell-less hermit crab Birgus latro is the world's largest terrestrial invertebrate ^[6].

The male's reproductive organs are located near and just below the heart and open to the outside at the base of the hindmost pair of walking legs. In the female, they are located at the base of the middle pair of walking legs. Females usually lay their eggs shortly after copulating, but they can also store sperm for many months. The eggs become fertilised as they passing through the chamber holding the sperm just before laying. The mother carries the eggs in a mass attached to her abdomen inside the shell. The number of eggs is usually large, but depends on the animal's size.^{[citation needed]}

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. This is a larval stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. After several zoeal moults, this is followed by the final larval stage, the megalopa stage ^[7].

Terrestrial hermit crabs begin life in the sea, but become air-breathers via moulting. After the final developmental moult, the youngster must live mostly on land. Its link with the sea is never entirely broken however, because it always carries a small amount of water in its shell to moisten its abdomen and hydrate its modified gills.

Juvenile land hermit crabs move away from the water. They then grow and develop through moulting in which they shed their exoskeleton. During moulting, they are extremely vulnerable and inactive, and usually burrow in the ground for protection. The new exoskeleton hardens and the crab regenerates lost or broken claws or legs in around 10 days. A hermit crab can moult as often as every other month when young, and every 18 months when they are older.^{[citation needed]}

Hermit crabs release their eggs in the ocean, near the shore. Little is known about reproduction in captivity, for example whether seasonal changes contribute to the animals' mating patterns. Some studies suggest that captive males become more sexually interested if they have brightly colored shells. Breeding hermit crabs in captivity requires near-perfect conditions and even then, after the eggs hatch, they may die from inappropriate food and water.^{[citation needed]}

Fossil record

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 ^[8].

As pets

Four hermit crabs in an aquarium

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.

These omnivorous or herbivorous species can be useful in the household aquarium as scavengers, because they eat algae and debris.

Hermit crabs were once seen as a "throwaway pet" that would live only a few months, but species such as Coenobita clypeatus has a 23 year lifespan if properly treated ^[9] and some have lived longer than 32 years.^[10][11].

In general, and despite their moniker, hermit crabs are social animals that do best in groups ^[12]. They also require a temperature and humidity-controlled environment, and adequate substrate to allow them to bury themselves while moulting.

The Wikibook Animal Care has a page on the topic of Land hermit crab

References

Wikimedia Commons has media related to: Paguroidea

Wikispecies has information related to: Paguroidea

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Articles with unsourced statements from October 2009

Anomura

Hermit crabs

Pet crustaceans