Hermit crab

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 Parapaguridae Pylochelidae Pylojacquesidae

Hermit crabs are decapod crustaceans of the superfamily Paguroidea , not closely related to true crabs. Most hermit crabs salvage empty seashells to shelter and protect their soft abdomens, from which they derive their name. There are about five hundred known species of hermit crabs in the world, most of which are aquatic, living at a range of depths from shallow coral reefs and shorelines to deep sea bottoms, although some species are terrestrial. A number of 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.

Shell competition

As hermit crabs grow, they must exchange their shell for a larger one. Since intact gastropod shells are a limited resource, there is frequently strong competition among hermit crabs for the available shells. The availability of empty shells depends on the abundance of the gastropods and hermit crabs, but most importantly on the frequency of organisms that prey upon gastropods but leave the shells intact . A hermit crab with a shell which is too tight cannot grow as fast as hermit crabs with well-fitting shells, and is more likely to be eaten .

Also, having sea anemones on shells can be very useful for hermit crabs because they can scare away fish and other marine animals. Sea anemones also have an advantage of eating leftovers when the hermit crab is finished eating the food. Furthermore, sea anemones can be transferred to new shells with the hermit crab.

Development

The various species 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 series of moults, 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.

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 . 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 molting. 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.

Some notable species

• Birgus latro, coconut crab
• Ciliopagurus strigatus, Halloween 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

Hermit crabs as pets

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.

Hermit crabs in an aquarium.

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.