hermit crab

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Hermit Crab 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 Lithodidae 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 utilize 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.^{[citation needed]} As the hermit crab grows in size, it has to find a larger shell and abandon the previous one.

Two etymologies exist for the name "hermit crab." One is from their habit of living in a second hand shell, which is analogous to a hermit living alone in a cave. The other is from a translation of the Caribbean hermit crab's scientific name, Coenobita clypeatus, "shield-bearing monk"^[2]; this species is one of a few that climb trees.

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. Some of these are quite large, such as Petrochirus diogenes.^{[citation needed]}

A number of other species, most notably king crabs, have abandoned seashells; these species are more similar in form to true crabs, and are known as carcinised hermit crabs.^{[citation needed]}

Shells and shell composition

Hermit crabs fighting over a shell in a Central California tide pool

As hermit crabs grow they require larger shells. Since suitable intact gastropod shells are a limited resource, there is frequently strong 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 ^[3].

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 hide in the shell ^[4].

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. Furthering this mutualism, the anemone can migrate with the hermit crab to a new shell.^{[citation needed]}

Development

Hermit crab species range in size,shape, from species with a carapace only a few millimeters 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.^{[citation needed]}

Terrestrial hermit crabs begin life in the sea, but become air-breathers via moulting. After the final developmental molt, 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. It is believed that C. brevimanus is the species of Coenobita best adapted to life on water and land.^{[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 ^[5].

Reproduction

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 fertilized 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 four stages, two of which (the nauplius and protozoea) occur while still in the egg. Most 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. The fourth stage of development is the megalopa.^{[citation needed]}

Juvenile land hermit crabs move away from the water. They then grow and develop through moulting in which they shed their exoskeleton. During molting, 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 molt 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. Because of this, they cannot reproduce in captivity without a simulated shoreline ^[6].

Little is known about reproduction in captivity. E.g., 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, possibly increasing their self-confidence. Breeding requires near-perfect conditions and even then, after the eggs hatch, they may die from inappropriate food and water.^{[citation needed]}

Hermit crabs as pets

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, and the Ecuadorian hermit crab, Coenobita compressus. Other species such as Coenobita brevamanus, Coenobita rugosus, Coenobita perlatus or Coenobita cavipes are less common but growing in availability and popularity as pets. Hermit crab pets also require certain habitat to be able to thrive. The list is as follows:^{[citation needed]}

• Humidity gauges (humidity: 75-85% relative)
• Temperature gauges (temperature: 70-80°F)
• Substrate: sand, coconut fiber (must be deep and diggable for molting)
• Minimum of a 5 gallon tank for one crab; 10 gallon tank for about 2 crabs
• Shells (shells are changed during growth)
• Separate molting tank^{[dubious – discuss]}
• Fresh water dish
• Salt water pool for submerging (aquarium salt)

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

Hermit crabs in an aquarium

The Caribbean hermit crab is one species commonly sold as pets in the United States.

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 ^[7] and some have lived longer than 32 years.^[8][9].

In general, and despite their moniker, hermit crabs are social animals that do best in groups ^[10]. 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