Most hermit crabs have no protective hardened shell for their abdominal soft parts. This feature separates them from true crab status. In order to survive they utilize the shells of dead univalve shells and at various times must change into larger shells as they grow.
Although Hermit Crabs in general are thought of as scavengers, they are in life, efficient hunters and voracious predators on other kinds of mobile invertebrates and even cannibalize their own species, and other Hermit Crabs.
(Photo Neville Coleman)
FEATURES
They are the ultimate "House Hunters" and will investigate any empty shell they find, often changing "houses" several times in the same day if there are lots of empty chells on the bottom.
The Hairy Red Hermit Crab Dardanus lagapodes showing the naked abdomen common to most hermit crabs.
(Photo Neville Coleman)
The giant land hermit crab known as the Coconut Crab (Birgus latro ) only uses shells in its juvenile stage, developing a hardened shell for its soft parts as it grows.
Coconut, or Robber Crabs Birgus latro are the giants of the Hermit Crab world. The ability to evove a hardened carapace has made them independent of mollusc shells and as such allowed them to grow to very large size. Like most hermit crabs they are efficient predators and opportunistic scavengers feeding on animal, or plant matter, depending on what is available.
(Photo Neville Coleman)
Most hermit crabs have short, strong legs, long eye stalks with small eyes for diurnal species and larger eyes on nocturnal species.
The Diogenids and the Coenobitids have larger left hand claws which they use to plug the aperture of the mollusc shell home. The Pagurids have a larger right hand claw.
LIFESTYLE
Found crawling around in the sea grass meadows in the lagoon at Lord Howe Island, this large specimen of the Red - Banded Hermit Crab Dardanus lagapodes hides away in the reef during the day and only comes out to hunt at night.
(Photo Neville Coleman)
Hermit crabs can be found on coral reefs, rocky reefs, rubble, sand, mud,
around mangroves and even in fresh water sinks and rivers open to the ocean.
They are very predatory and will also scavenge and some can use the ciliary processes of their hairy antennae and flagellums to collect planktonic food.
Rarely observed, this specimen of the Fimbriate Hermit Crab Pylopaguropsis fimbriata lives from 8 to 100 metres and has been recorded in Japan and Papua New Guinea. The lines on its legs and the long claw make it relatively easy to identify.
(Photo Neville Coleman)
One of the most attractive and easily recognised species the Strigated Hermit Crab Ciliopagurus strigatus generally lives in cone or stromb shells and has a very compressed white back. It occurs across the Indo Pacific.
(Photo Neville Coleman)
Easily identified, the Blue - spotted Hermit Crab Dardanus guttatus inhabits areas of coral reefs, rubble and rocky reefs in shallow water. The white - spotted dark red legs and the large blue spots on the first leg segments make it very characteristic. This one has chosen a large Spider Shell for its home, which, although very thick and serving well as excellent protection, is a dificult "home " to move over the bottom with, due to its projections.
(Photo Neville Coleman)
The Line - legged Hermit Crab Calcinus lineapropodus is a smaller species often inhabiting shells that appear way too large and heavy for it to carry. However, the hermit crab is not looking for a light shell to make its life easier, its looking for a thick shell to protect itself. It ranges from Japan to Papua New Guinea, Australia and down into the South Pacific.
(Photo Neville Coleman)
With white legs tipped with orange, the Minute Hermit Crab Calcinus minutus is very easy to recognise. It is distributed from Japan to Papua New Guinea and Australia and out into the South Pacific.
(Photo Neville Coleman)
While some species of Hermit Crabs are very drab, others can be quite colourful with very distinctive patterns.
The common land Hermit crab Coenobita perlata is present on many offshore islands throughout the Indo - Pacific. Although on some Coral Sea sand cays it may be seen out during the day, in general the species hides away from the heat of the tropical sun and only ventures forth at night. It is prevelant at the Cocos (Keeling) Islands and Christmas Island in the Indian Ocean and throughout the Great Barrier Reef.
(Photo Neville Coleman)
There are several species of land hermit crabs in the tropics and in some areas they can occur in large numbers, crawling around at night amongst decaying leaf litter and scavenging the tidelines for cast up carrion. Although the land hermit crabs spend their adult lives on land they never seem to venture too far from the sea shore and after mating the females go back to the sea to release hatching larvae.
This Red land Hermit Crab Coenibita perlata has reacted to potential danger by retreating into its shell and using its large claw and very thick - shelled legs as a very efficient armour - plated "door" which would thawt any predator, even its own kind.
(Photo Neville Coleman)
REPRODUCTION
In some species the males may be larger than the females, in others they appear to be the same size. Sexes are separate and after fertilization the female lays her eggs on small tendril - like pleopods along the side of the abdomen. They stay in this position until they hatch and the larvae are released.
IDENTIFICATION
Visual identification can be made on many species, once the specific characteristics have been pointed out using leg colour and pattern, claw shapes and eye stalk colours as compared to in an identified colour reproduction.
However, until we have close up images of all the species and their variations, there is still aways to go before it can be stated that thay are all identifiable in the field.
Growing to over 100 mm the White - Spotted Hermit Crab Dardanus megistos is the largest species of hermit crab in the Indo - Pacific and is generally seen more at night than during the day.
(Photo Neville Coleman)
The Groove - clawed Hermit Crab Aniculus retipes occurs from Africa across the Indo - Pacific to Papua New Guinea and out into the South Pacific to at least Samoa. Generally nocturnal it is found on reefs off Lord Howe Island and the Great Barrier Reef.
(Photo Neville Coleman)