The Anemone hermit crab, Dardanus pedunculatus has a mutualistic symbiotic relationship with the anemone Calliactis polypus. The anemones are collected by the crab and placed on its shell. The Anemone Hermit crab benefits from the protection and camouflage that it obtains from the anemones and the anemone benefits from scraps that drift off the crabs prey and from being moved around with the crab.
As with all hermit crabs they spend their lives in empty gastropod shells. Some people refer to these crabs as Decorator Crabs although technically they are decorating the shells and not themselves.
Anemone hermit crabs are easily recognisable by the anemones that they carry on their shells. The body of the Anemone Hermit Crab is a pale pinkish colour, interspersed with white and orange blotches. The eye stalks are red and white and the eyes are a light green colour. The body and legs are covered in light brown coloured bristles. Similar to the spiny lobsters they have two sets of antennae, one short pair in the middle with sensory organs on it and the other longer and extendable outwards.
Usually only the legs and head are seen protruding from the shell. The first set of legs has claws, the left one being larger than the right. Aside from being used for predation and self defence, the claws are also used to seal themselves into their shell when threatened. The second and third legs are used for walking. The rear of the body is adapted to fold into the shell. The Anemone hermit crab grows up to 5 cm in width.
IN THE WILD
Anemone hermit crabs are usually only seen at night.During the day they tend to hide in crevices and under rocks. Being quite shy they tend to scuttle off when lit up by dive lights at night. They have developed the skill of coaxing the anemones off rocks and onto their shells. When they change shells they remove the anemones off the old shell by gripping them with their claws and slowly massaging the base of the anemone until it lets go. They then place it on their new shell. When the anemones are disturbed they give off sticky threads which are distasteful to predators thus providing protection for the crab.
The Anemone hermit crabs are found on coral and rock reefs across the west Indian Ocean in depths of up to 40 meters.
They are carnivorous and although they are mainly scavengers, they will break open and eat any mollusc that they can open.
The Males are able to chemically detect when a female is fertile and approach them, often holding onto the female until she is ready to mate with them. Once the eggs are fertilised they are kept on the underside of the female inside their shell. When the eggs hatch the larvae are planktonic until they are large enough to find small shells.
Anemone Hermit crabs use two species of anemone on their shells. The larger anemone is Calliactus polypus and the smaller flatter anemone is Verrillactis paguri. When both species of anemone are on the shell then the crabs usually place the Calliactus polypus on the outer part of the shell. There are amphipods of an unknown species that inhabit these anemones and live on the shells. These can be seen in the images below.
They are not reef safe and being carnivorous can do a considerable amount of damage in a tank. If kept they need a supply of larger shells in the tank to switch to. They are easy to feed and will devour large amounts of food. These are crustaceans that should probably not be kept as pets as they are unlikely to survive for very long unless cared for properly.
ANEMONE HERMIT CRAB CLASSIFICATION
Species: Dardanus pedunculatus
- The Reef Guide fishes, corals, nudibranchs & other invertebrates: East and South Coasts of Southern Africa by Dennis King & Valda Fraser