House hunting with friends: the life of Dardanus Hermit Crabs

Hermit crabs plod about the sea-floor using someone else’s shell for a home. They always use empty shells and never kill the original occupant.

When the crab becomes to large for its shell, it looks for another. On finding a likely looking one, it tries it on for size. If it isn’t pleased with the fit, or the shell is too heavy, the crab returns to its old castoff and continues it search. It may try the old and new several times before it makes its decision.

Where there is a large population of hermit crabs and a shortage of shells, the crab will accept a sub-standard home: maybe a cracked or uncomfortable shell. But in good conditions it will be very particular about the new shell it chooses. For particularly good shells, two hermit crabs will fight for possession. Some will even attack an inhabitant to steal it’s particularly desirable abode. These fights don’t tend to damage the combatants, probably due to complex ritualised fighting

Sea anemone “friends”

Some of the Dardanas species of Hermit crab stick sea anemones on their shells. Sometimes the anemone assists with the process, loosening their connection to the substrate to ease transfer. The aneomone camoflauges the crab and its stinging cells may protect the crab from predators such as octopus.

The anemone also benefits from the arrangement. It becomes effortlessly mobile and shares the crab’s meals. The crab will also defend the anemone from predators trying to eat them. It’s even possible that the hermit crab feeds the anemone directly.

However, if the crab is starving and can’t find anything to eat, it will eat the anemones, either from its own back or from the backs of other crabs.

Night dives in temperate and tropical water

Dardanas species are nocturnal with dark-adapted eyes. Look out for them on night-dives. They live in the Indian, Pacific and Atlantic oceans and the Mediterranean Sea.

Phylum: Arthropoda > Subphylum Crustacea Class: Malacostraca > Order: Decapoda (10 legs) > Infraorder: Anomura (reduced/hidden last legs) > Genus: Dardanus

Sources and References

Karplus, I. The partnerships between hermit crabs and sea anemones: an overview. Symbiosis 93, 1-15 (2024).

Chryssanthi, Antoniadou et al. (2013). Symbiosis of Sea Anemones and Hermit Crabs in Temperate Seas.

Invertebrate Zoology, Fox, Ruppert and Barnes, ISBN 0030259827

M. Imafuku , T. Yamamoto & M. Ohta. Predation on symbiont sea anemones by their host hermit crab Dardanus pedunculatus. Journal Marine and Freshwater Behaviour and Physiology, Volume 33, 2000 – Issue 4. 10 Jan 2000.