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Coenobita variabilis (Family Coenobitidae)

Terrestrial Hermit Crab

Distribution:

The Terrestrial Hermit Crab has a tropical Australian distribution. It ranges from Exmouth Gulf, WA across the NT, to northern Qld (WA, NT, QLD).

Features:

The Terrestrial Hermit Crab is a small terrestrial (land-living) hermit crab, that is commonly found on sandy shores behind mangroves, hiding under debris and rocks in northern Australia. They grow to a carapace length of 40 mm. Unlike many of the other crabs; they do not have a hard exoskeleton for protection. Terrestrial Hermit Crabs protect themselves by choosing to live within a spiral mollusc shell.

They have soft body parts at the hind end of their body, including the curved, long, abdomen that contains the liver and the gonads. The outer bend of the abdomen has small appendages that allow the crab to hold on to the inside of the shell. The 4th and 5th pairs of legs are reduced in size for this purpose. The Terrestrial Hermit Crab has fewer spines on its carapace and legs than other hermit crabs. Their eyestalks are compressed from side to side. The anennule stalks are very long but the feelers (flagella) are short and compressed from side to side (Jones & Morgan).

Males and females only differ in the placement of their gonopores.

The Terrestrial Hermit Crab is coloured cream to pale brown, with darker markings.

Ecology/Way of Life:

Terrestrial Hermit Crabs occur between the tides and higher on the shore, often hundreds of metres above high tide level. They are found near mangroves, but also occur on sand and rocky beaches. They are very active nocturnal (night-preferring) scavengers, attracted to decaying material, such as dead fish on the beach, and even to household garbage and pet food bowls. They can live well over a decade and some may live as long as 20 years.

While walking the animal drags its spiral mollusc shell house along. As the crab grows and does not fit in its shell any longer, it looks for a bigger one. The original occupant, if it is still present, is plucked out. Then, quickly and nervously, the hermit crab moves over into its new home. They live near trees or other growths for protection from predators. Natural predators are birds, other crabs, fish, octopus, and some mammals (including man).

Female Terrestrial Hermit Crabs return to the ocean to deposit their eggs that hatch in the ocean. The small crab larvae are part of the plankton. After three moults, all of their legs are gained and they crawl up onto the beach. The gills have changed from being adapted to being submerged in seawater and now they can gain oxygen from the air. The crabs will now drown if they submersed for long periods. After having crawled up onto the beach they must find their first shell.

Small crabs moult more often than larger crabs and this puts them at risk during this time. The most dangerous time in a crab's life is just before, through, and just after the moulting process. Terrestrial Hermit Crabs regularly swap their shells when they find another that more suitable. Hermit Crabs grow around 3-5mm per year and shed their skin each summer.

They have large gill chambers and as long as they keep their gill filaments moist, the chamber acts like a lung. This allows the hermit crab to wander far from water. Crabs don't return to the ocean again until the female lays her eggs. Hermit crabs collect water from tide pools the keep their gills moist. Terrestrial Hermit Crabs also require fresh drinking water each day. At night, they venture down the shore to search for food, check out larger shells and to meet other Hermit Crabs.

In case of danger the Terrestrial Hermit Crab withdraws into the shell as deep as possible. When hiding in the shell it uses its largest chelae to guard the entrance. Terrestrial Hermit Crabs will never fully leave the shell, except when they are switching their shells, unless they moult, are sick, or are dying or dead.

Other Comments:

Coenobita variabilis, McCulloch. Coen comes from the Greek word koinobion or koinos that means common. The current English word coenobite refers to a monk who lives in a community. Variabilis comes from the Latin word variare or variatum, meaning to vary.

Further Reading:

Bennett, I. (1987). W. J. Dakin's classic study: Australian Seashores: a guide to the temperate shores for the beach-lover, the naturalist, the shore-fisherman and the student. Angus & Robertson.

Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. p.46, New Holland Press, Sydney.

Harvey, A. (1992) Abbreviated Larval Development in the Australian Terrestrial Hermit Crab, Coenobita variabilis, McCulloch (Anomura, Coenobitidae), Journal of Crustacean Biology, 12(2), 196-209.

Jones, D. & Morgan, G. (1994). A Field Guide to Crustaceans of Australian waters. p.123, Reed.

Morgan, G. J. (1987) Hermit Crabs (Decapoda: Anomura: Coenobitidae, Diogenidae, Paguridae) of Darwin and Port Essington, Beagle, 4(1), 165-86.

Acknowledgments:

Text, map and photograph by Keith Davey.

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