Hermit crab

Hermit Crab Temporal range:
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. 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 of hermit crab 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. As the hermit crab grows in size, it has to find a larger shell and abandon the previous one.

Two possible etymologies exist for the name of "hermit crab." One is that their habit of living in a second hand shell gave rise to the name, which is analogous to a hermit living alone in a small cave. The second is that it is a translation of the scientific name of the Caribbean hermit crab, Coenobita clypeatus, which translates as "shield-bearing monk"; this species is one of a few known to climb trees.

There are about five hundred known species of hermit crab in the world, most of which are aquatic and live in varying depths of saltwater, 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, such as Petrochirus diogenes.

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

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 the best shells. The availability of empty snail shells at any given place depends on the relative abundance of gastropods in the right range of sizes compared to the population of hermit crabs. An equally important issue is the frequency of organisms which prey upon gastropods but leave the shells intact .

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 as it cannot withdraw completely into the shell .

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 be transferred to a new shell when the hermit crab changes shells.

Development

The various types range in size, 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.

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 water and 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 .

Reproduction

The reproductive organs of hermit crabs are located near and just below the animal's heart and open to the outside at the base of the last pair of walking legs in the male. In the female, they are located at the base of the middle pair of walking legs. Female hermit crabs usually lay their eggs shortly after copulating, but they can also store sperm for many months. The eggs are fertilised 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 animal's size.

The developing hermit crabs go through four stages, two of which (the nauplius and protozoea) occur while still in the egg. Most crabs hatch at the third stage, the zoea. This is a larva stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. The fourth stage of development is the megalopa.

Land hermit crabs will 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 moult as often as every other month when young, or every 18 months when they are older.

Hermit crabs release their eggs in the ocean, near the shore. Because of this, hermit crabs cannot reproduce in captivity without special equipment to simulate a shoreline .

Little is known about reproducing hermit crabs in captivity. It is not yet understood whether seasonal changes contribute to the animals' mating patterns. Some studies, however, suggest that males in captivity become more sexually interested when they are provided with brightly colored shells that increase self-confidence. To have a Hermit Crab breed, the conditions must be near perfect, and even then, after the eggs are hatched in the pool or water dish. They die as they need salt water, plankton for food etc.

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 crabs also require certain specifications to be able to thrive well as pets. The list is as follows:

• Humidity gauges (humidity: 75-85% relative)
• Temperature gauges (temperature: 70-75°F)
• Substrate: sand, coconut fiber (must be deep and diggable for moulting)
• Minimum of a 5 gallon tank for one crab; and 10 gallon tank for about 20 crabs
• Shells (shells are changed during growth)
• Separate moulting tank
• Fresh water dish
• Salt water pool for submerging (aquarium salt)

These omnivorous or herbivorous species can be seen as 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 thought to be "throwaway pet" that would only live a few months, but with proper care can thrive for many years. For example, Coenobita clypeatus is commonly listed as having a 23 year lifespan if properly cared for , and some have lived longer than 32 years .  .

In general, and despite their moniker, hermit crabs are social animals that do best when kept in groups . They also require a temperature and humidity controlled environment, and an adequate depth of substrate to allow them to dig while moulting.

References

hermit krabs