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Hermit crabs are decapod crustaceans of the superfamily Paguroidea. 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 . 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 living 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. Some of these are quite large, such as Petrochirus diogenes.

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 .

Shells and shell composition

Hermit crabs fighting over a shell
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 .

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 .

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 .

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.

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 .

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.

Hermit crabs release their eggs in the ocean, near the shore. Because of this, it is very hard to breed hermit crabs in captivity.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.

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 Claymarker, Yorkshiremarker, UKmarker from the Lower Cretaceous .

Hermit crabs 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, 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:
*Humidity gauges (humidity: 75-85% relative)
*Temperature gauges (temperature: 70-80°F)
*Substrate: sand, coconut fiber (must be deep and diggable for moulting)
*Minimum of a 5 gallon tank for one crab; 10 gallon tank for about 2 crabs
*Separate moulting tank
*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 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  and some have lived longer than 32 years.

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


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