Paguroidea

Crabs (in general) do not have a backbone, instead rely on an hard exoskeleton for support. This exoskelton is made of a protien called chitin, containing calcium.

Hermit crabs are a group of crabs that are characterised by their use of discarded shells of marine snails for protective shelter. In hermit crabs, the exoskeleton covering the abdomen lacks calcium and is soft. As a result, hermit crabs cannot survive without a snail shell in which to live.

Aquarium Care

Hermit crabs are often included in marine aquaria as part of a clean-up crew. Despite popular opinion, they are not necessarily effective herbivores in an aquarium setting, and so are unlikely to be helpful in control of nuisance algae. They are opportunistic omnivores, and can be very effective at utilising leftover food, or disposing of deceased tank specimens. Unfortunately, they can also predate on living snail populations (both as food, and as potential sources of new shells), as well as be destructive to populations of beneficial live rock and sand bed infauna. For this reason, many aquarists choose not to include hermit crabs in their herbivore team.

Large specimens have also been considered capable of killing fish, although in many cases they may not necessarily have been the cause of the fish death, but have just been very quick to enter the scene of the crime in search of a quick meal.

There are some very attractive species that are worthy aquarium specimens in their own right, and their behaviour can be very entertaining.

Hermit crabs in the aquarium should be presented with a number of suitably sized shells as potential homes as they grow. Availability of spare shells may help to prevent excessive predation on live snail populations. Some hermit crab species have a preference for a particular species or shaped snail shell, whilst other are less choosy, and will select shells of any type, and may continually change shells in their quest for the 'perfect' home.

Hermit crabs can also be somewhat clumsy, and be the cause of disloding rockwork and corals, due to their bulky shells and 'bulldozer' tendencies. However careful mounting and securing of corals and aquascaping can be effective in preventing unwanted rockslides.