Q. I have a 55-gallon tank with several damselfishes, a pair of percula clownfish, a blue tang and a yellow tang. Recently I went to a local pet store and decided I would buy a large hermit crab to help clean up the bits of food my fish do not eat. But another customer at the store talked me out it saying that they will eat my fish! I find this hard to believe (a lady who works at the store told me they are harmless) and I really find the hermit crabs interesting. So, I would like to buy one. That is why I am writing to AFI. I thought you might be able to give me the correct answer on this subject.
A. Well, both the customer you spoke to and the shop attendant are correct. Most hermit crabs are opportunistic and seem to have insatiable appetites. They will feed on anything they can catch and tear up with their claws. This includes fish food that reaches the substrate, dead fish and live fish if they can capture them. Most of these crabs that prey on live fish feed on smaller fish when they are torpid (“asleep”) at night. So, although you can keep hermit crabs, you need to be selective about which species you purchase.
Let’s start with the undesirable species first. One species that should definitely be avoided is the red hairy or white spotted hermit crab (Dardanus megistos). This species is voracious, and attains a length of about 4 inches. Although it can be housed with larger fish, I would avoid placing it in a tank with any fish less than four or five times its size.
The species of Dardanus hermits that carry anemones on their shells (namely D. penduculatus) are also predaceous and potentially dangerous to smaller fishes, as is the leviathan — sometimes referred to as the large hermit crab (Aniculus maximus). The latter species attains a maximum length of 4 inches and is sometimes imported from Hawaii. The giant hermit crab (Petrochirus diogenes) is a huge Caribbean species (it reaches 12 inches according to one source) that must be avoided, while the starry-eye hermit (Dardanus venosus) and white speckled hermit (Paguristes puncticeps) are medium-size crabs that are occasionally available from Florida and are a real threat to their fish tankmates.
There are number of smaller crabs that should be okay with your fish as long as there is a significant size disparity between the crabs and your fish (the fish should be at least two times their size or larger). These crabs are also a greater threat to sick or injured fish.
Some of the more desirable forms include the orangeclaw hermit crab (Calacinus tibicen), the red reef hermit (Paguristes cadenati), the red-stripe hermit (Phimochirus holthuisi) and the polka-dotted hermit (Phimochirus opercualtus). All these species are found in the tropical western Atlantic and reach approximately 1 inch in length. Remember, even these species could catch and feed on fish that are of similar size or only slightly larger.
A very attractive, medium-size hermit that is relatively innocuous is the striped or Halloween hermit (Trizopagurus strigatus). This crab has red and orange stripes on its appendages. Although it is a potential threat to fish of similar size or smaller, I have never had it cause problems.
There are several smaller hermit crabs that are currently available that are used to help control algae growth. These species tend to remain small and pose very little threat to fish. The most attractive member of this group is the blue-legged hermit crab (Clibanarius tricolor). This species grows to less than an inch in length. The blue-eyed spotted hermit crab (Clibanarius digueti) is an even smaller species that will scavenge on uneaten food, eat algae and is too small to be much of a threat to your fish.
Not only are hermit crabs potentially dangerous to fishes, they will also eat tube worms and ailing corals. They may also irritate (and even kill) small clams.