“Delightful” is perhaps the best way to describe an aquarium housing a colony of hermit crabs. These inquisitive, active creatures are constantly in motion — squabbling over bits of food, settling territorial disputes, and, in general, exploring their environment. Hermit crabs lack a hard exoskeleton on the rear of their bodies, but they make up for this shortcoming by sheltering in the empty shells of other creatures, such as snails. Hermit crabs carry these appropriated homes around, exchanging them for larger “models” as the crab increases in size. Investigating empty shells and trying to dislodge tenants from occupied ones takes up a good deal of their time.

The appetite of the typical hermit crab knows no bounds, and they make excellent scavengers. These crabs can coexist well with all but the largest of fish, protected as they are by their hard shells, and they are one of the few species of crab that can be kept in colonies. Be sure to include plenty of extra empty shells of various sizes, so that the animals can relocate as they grow.

Many species are commercially available, for example the long-clawed hermit crab (Pagurus longicarpus). They are also easily collected along shores and in tidal pools, where doing so is lawful. A particularly interesting member of the family is the anemone hermit crab (Pagurus prideauxi). This species often places a stinging sea anemone on its shell as a deterrent to predators, carefully relocating its protector when the time comes to change shells. The anemone, it seems, benefits from the arrangement as well by gaining access to bits of leftovers from the crab's meals.

The arrow crab (Sterorhynchus seticornis) is a particularly amiable fellow. Except for particularly large individuals, it rarely disturbs other aquarium inhabitants. Thinly built, with long, spindly legs that give it its alternative name of spider crab, this species makes an unusual and interesting addition to the marine aquarium that lacks aggressive predators. Amazingly, the arrow crab will impale extra pieces of food upon its pointy carapace, to be consumed at its leisure or in times of need. Like many of its relatives, the arrow crab is usually intolerant of company, and individuals confined together will usually fight to the death.

The boxer crab (Lybia tessellata) has a fascinating habit of carrying small sea anemones in its claws as a deterrent against predators. When threatened, this tiny (1.2 inches) crab will rear up on its hind legs and wave its “weapons” at the interloper. Due to their small size, boxing crabs will mix well with many species of fish, but be aware that, like all crabs, they will consume animals smaller than themselves.

The exoskeleton of all crabs and other crustaceans is very soft for several hours after the animal sheds. Animals in this state are extremely vulnerable and will be sought out and consumed by fish and invertebrate neighbors. Be sure that there are adequate numbers of secure retreats and substrates suitable for burrowing.