Fish Facts

.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.