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Hermit crab
Coenobita compressus
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 Prapaguridae Pylochelidae Pylojacquesidae

Hermits are decapod crustaceans of the superfamily Paguroidea ^[1]. They are not closely related to true crabs.

Most species of hermit crabs have long soft abdomens, and to protect themselves, they live inside salvaged empty seashells. Most often they use the shells of sea snails, marine gastropod mollusks. The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell.

This habit of living in a second hand shell is what gave rise to the popular name "hermit crab", which is a reference to the idea of a hermit living alone in a small cave.

There are about five hundred known species of hermit crabs in the world, most of which are aquatic, living in saltwater, at a range of depths from shallow coral reefs and shorelines to deep sea bottoms. However in the tropics, a number of species are terrestrial, and some of these are quite large, for example, Coenobita clypeatus.

A number of other species, most notably king crabs, have abandoned seashells for a free-living life; these species have forms similar to true crabs and are known as carcinised hermit crabs.

Hermit crabs live in the wild in colonies of 100 or more, and do not thrive in smaller numbers.

Contents 1 Shell competition 2 Development 3 Fossil record 4 Some notable species 5 Hermit crabs as pets 6 References

As hermit crab grow they require larger shells. Since intact gastropod shells are a limited resource, there is frequently strong competition among hermit crabs for the best available shells. The availability of empty snail shells at any given place depends primarily on the relative abundance of gastropods in the right range of sizes, compared to the demographics of the population of hermit crabs.

An equally important issue is the frequency of organisms which prey upon gastropods but leave the shells intact ^[2]. A hermit crab with a shell that is too small cannot grow as fast as hermit crabs with well fitting shells, and is more likely to be eaten ^[3].

For some larger marine hermit crab species, having one or more sea anemones growing on the shell can be very useful, because the anemones tend to scare away fish and other marine predators that might otherwise attack the crab. The sea anemone also benefits because it is well positioned to consume loose fragments of the hermit crab's meals. Furthering this mutualism, sea anemones can transfer to a new shell when the hermit crab changes shells.

The various types range in size from the Pacific hermit crab, which rarely grows larger than a peach, 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. Terrestrial hermit crabs begin their lives in the sea, but through a process of moulting develop the ability to breathe air. After the last developmental moult, the young hermit crab will drown if left in water for an indefinite period of time. Their link with the sea is never entirely broken however, as hermit crabs carry a small amount of water in their shells at all times to keep their abdomen moist and their modified gills hydrated. It is believed that C. brevimanus is the species of Coenobita best adapted to life on land.

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 ^[4]. The reproductive organs of hermit crabs are located near and just below the crab’s heart and open to the outside at the base of the last pair of walking legs in the male. In the female, they’re located at the base of the middle pair of walking legs. Female hermit crabs usually lay their eggs shortly after copulating, however they can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. The eggs are carried and hatched in a mass attached to the abdomen inside the shell. The number of eggs is usually large, but depends on the crab’s size. The developing crabs go through four stages, two of which (the baupilus and protozoea) occur while still in the egg. Most crabs hatch at the third stage, the zoea. This is a larvae stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. The fourth stage of development is the magelops. Hermit crabs are usually born in the ocean, near the shore. Because of this, hermit crabs cannot reproduce in captivity. After the crabs are born, they move inland away from the water, where they search for abandoned shells to inhabit. Hermit crabs then begin growing and developing through a process called moulting. In this process, the crabs shed their exoskeleton. During this, the crabs are extremely vulnerable and inactive, and usually find protection by burrowing in the ground. It takes around 10 days for their new exoskeleton to harden, and during this period the crab is able to regenerate any lost or broken claws or legs. A hermit crab can molt as often as every other month when young, or every 18 months when they are older. The life span of the hermit crab in the wild is up to 30 years.

• Birgus latro, coconut crab
• Ciliopagurus strigatus, Halloween hermit crab
• Clibanarius tricolor, Blue legged hermit crab
• Coenobita cavipes
• Coenobita clypeatus, Caribbean hermit crab
• Coenobita compressus, Ecuadorian hermit crab
• Coenobita perlatus, strawberry land hermit crab
• Coenobita variabilis, Australian land hermit crab
• Pagurus bernhardus
• Pagurus pollicaris, flat-clawed hermit crab

There are several species of hermit crabs that are common in the marine aquarium trade. These omnivorous or herbivorous species are useful in the household aquarium as scavengers, eating algae and other debris. They also eat many fruits and vegetables.

The scarlet hermit crab, or red reef hermit crab, Paguristes cadenati, is a handsome and interesting species with a bright red body and yellow eyestalks, and stays rather small (about 2–5 cm / 1–2 inches across). Smaller species of a similar passive nature include the zebra hermit crab (brown legs with white bands), the red-tip crab and blue-legged crab.

In Europe, the common hermit crab, Pagurus bernhardus, is popular.

While most species available in pet stores are small like those listed above, and are simply scavengers, others may grow quite large (some on the Pacific coast can grow to 30 cm / 12 inches) and may eat coral, clams and other crustaceans.

Most marine hermit crabs will appreciate a salinity of between 1.023 and 1.025 (measured in specific gravity), and temperatures between 4–14°C (temperate species) and 24–27°C (tropical species), with a good bed, algae to graze on and a variety of shells to change into. They will happily switch shells frequently if given the opportunity - an interesting display to watch.

There are approximately 15 terrestrial species in the world, and, of those, 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.