Hermit crabs are closely related to the robber crab, with the two genera ( Birgus and Coenabita) making up the whole of the family Coenobitidae.
Most hermit crab species have a long, spirally curved abdomen, which is soft, unlike the hard, calcified abdomens of related crustaceans. The asymmetrical shape of their body structure is an adaptation to their lifestyle. Hermits protect their vulnerable soft abdomen from predators by living in a salvaged empty seashell, into which they can retract their whole body. Most frequently hermit crabs use the shells of sea snails but on Christmas Island the species that venture farthest from the shoreline also opportunistically use shells of the introduced Giant African land snail.
The rear two pairs of small, hook-like limbs or legs are used by the hermit crab to anchor its body into the shell. The front two pairs of legs extend from the shell opening and are used for walking and climbing. The tip of the hermit crab’s abdomen is also adapted to clasp strongly onto the internal spiral of the snail shell.
As the hermit crab grows, it must find a larger shell and in some dense populations, fighting for shells is a frequent event. Hermit crabs frequently try on different abandoned snail shells to find one that fits
the delicate abdomen closely. A close fitting shell is less cumbersome to move about in because the crab is more easily able to hold on to the shell, and for terrestrial hermits, minimises evaporation.
Infra-order Anomura, Family Coenobitidae
The genus Coenobita contains the sixteen species of terrestrial hermit crabs. The majority of the species are found in the Indo-Pacific region, with only one species in West Africa, one species occurring along the western Atlantic Ocean, and one species occurring on the Pacific coast of the Americas. Three species of terrestrial hermit crabs have been recorded on Christmas Island.
Red hermit crab | Coenobita perlatus H. Milne Edwards, 1837
The red hermit crab's distinctive coloring varies from a striking overall red to white / pink with red about joints on the legs and a dark variation of the overall red. They have flattened triangular eyestalks and grow up to 16mm shield length.
Restricted to the intertidal zone to 5 metres above sea level. Found on most of the beaches around the island. More
prevalent on coral rubble beaches than sand beaches.
Also referred to as Strawberry hermit crab.
Purple hermit crab | Coenobita brevimanus Dana,1852
Colour varies from a distinctive purplish-brown colour, bluish-gray, dusky or reddish-violet. The eye stalks are cylindrical. The largest of the hermits found on the island with a shield length of up to 30 mm.
Sea level to 50 metres above sea level. On Christmas Island they are found mainly on the beaches and on shore terraces. Penetrates inland and is occasionally found on the plateau. May be found under shore terrace vegetation and debris, sometimes in small to large groups. Although fairly common, they are shy and not often seen.
Tawny hermit crab | Coenobita rugosus H. Milne Edwards, 1837
Tawny hermit crabs are quite variable in color; cream, purplish, bluish grey to brownish, or a mixture of these colors. Often has a dark spot on the larger left claw. Shield length up to around 16 mm. Flattened triangular eye stalks.
Intertidal zone to 5 metres above sea level. Has been found on all the beaches of the island. Can penetrate inland where fresh water is available. Has also been found on the shore terrace above Greta Beach and West White Beach. Fairly common. Most often found foraging about the strand line on beaches at night.
Read more about Christmas Island's terrestrial hermit crabs in the book
"Crabs of Christmas Island" by Max Orchard
© 2013 Max Orchard.
Terrestrial hermit crabs