Hermit crab

Hermit crab
Pagurus bernhardus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Mandibula
Class:	Crustacea
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.

Most species of hermit crabs have long soft abdomens which are protected from predators by the adaptation of carrying around a salvaged empty seashell, into which the whole crab's body can retract. Most frequently hermit crabs utilize 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. 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 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 and live in saltwater at depths ranging 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 which are more 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

Hermit Crabs picking at the underside of a dead Gumboot chiton, Cryptochiton stelleri in a California tide pool

As hermit crabs 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.

Hermit crabs fighting over a shell in a Central California tidepool

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 transfer to a new shell when the hermit crab changes shells.

Development

File:Anemone hermit crab 2 frames in one.jpg

anemone hermit crab

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 water and land

Reproduction

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.

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 molt 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.

Some notable species

• 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
• Pagurus bernhardus

Hermit crabs as pets

Several marine species of hermit crabs are common in the marine aquarium trade. These omnivorous or herbivorous species are useful in the household aquarium as scavengers because they eat algae and other debris.

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 do well in 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 (for tropical species), with 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. In certain regions of the Caribbean, such as Jamaica and the Florida Keys, hermit crabs are commonly used in a sport known as crab racing, where they are placed in the center of a ring and whichever crab reaches the outside of the ring first wins.

Hermit Crabs used to be thought of as a "throwaway pet" that would only live a few months, but with proper care, including mimicking the environment they originated from, and proper feeding, they can thrive for many years. For example, Coenobita clypeatus is commonly listed as having a 20 year lifespan.

In general, hermit crabs are social animals and should be kept in groups. They need higher moisture and heat than an air conditioned home, and they must have an adequate depth of substrate to allow them to dig while moulting. and must be kept accordingly.

There is some controversy surrounding the use of painted shells. The detractors claim that the paint is toxic and uncomfortable, that the use of painted shells encourages buyers to select a crab based on its shell (which will be vacated eventually) and not on the health or personality of the crab, that crabs often get stuck in the shells, and that the methods for moving the crabs into painted shells are inhumane. Some anecdotal evidence suggests that crabs will prefer natural shells if given the choice (and if size and shape is not a factor).

A hermit crab will sometimes inexplicably die in the first few weeks after purchase, sometimes called Post Purchase Syndrome. This is thought to be linked to the stress of changing surroundings. Usually the process of being shipped, moved into the pet store, and then brought home is fairly quick, causing a great deal of change in a short period of time. Thus it is important to touch a new crab as little as possible for the first few weeks, possibly even isolating it from other crabs.

A crab preparing to moult will sometimes go limp and apparently lifeless. Thus it is important to distinguish between live and dead crabs by the strong fishy odor given off by dead crabs.