Hermit Crabs and Their Allies of French Frigate Shoals

Because so many people understand in general what a “crab” is, that common name has been applied to a wide variety of species, only some of which are related.  For example, the common and well-known “horseshoe crab” of the east coast of the United States is not really a crab, or even a crustacean.  It is, instead, an aquatic relative of the arachnids, the same group that contains spiders, ticks, and scorpions.             

Hermit crabs are not true crabs either.  But they are indeed crustaceans, and they are decapod crustaceans, like the true crabs.  But in the true crabs (Brachyura, for short tail), the abdomen is reduced in size and is tucked beneath the body.  Thus the tail, and the eggs that it will carry (in females), are safely protected beneath the main body of the crab.  In contrast, in the hermit crabs (members of a group called the Anomura, for “anomalous tail”), the abdomen is long, relatively soft, and usually twisted, all of which are adaptations for living inside another object, which is usually the shell of a gastropod (snail).  Not content with the protection afforded by their chitinous external skeleton, the hermit crabs use the shells of snails as their home.  Sometimes these homes are inviting to other species, and some hermit crabs harbor a host of other species, including worms and other crustaceans, as guests in “their” shells.  Indeed, many hermit crabs encourage other species to live in or on their shell, and they then benefit from the newcomers strengths.  The most obvious examples of this type of “symbiosis” are the many hermit crabs that carry living anemones on their shells.  For many hermit/anemone relationships, it seems to be an exclusive arrangement – that is, one species of hermit crab always carries the same species of anemone on its shell.  For other species, this is not the case.  How and when such arrangements are made is the subject of much interest to the marine biologists who study hermit crabs.

Living inside a snail shell offers the obvious advantage of a secure home that can be taken with you when you leave.  It must be a successful strategy, as hermit crabs are a very diverse group.  The many species of hermit crabs include some that are fully terrestrial, some that can climb trees, some that live in the intertidal zone, and still others living at great depths in the ocean.  But life in a snail shell also has its disadvantages.  When a hermit crab reaches a size that is too large for its current shell, it must locate another, larger, one, and the transfer is a dangerous time for the hermit, as it exposes itself to predators for the brief time it takes to switch from the old to the new shell.  There is also a supply and demand issue – such that hermits would have a hard time living where there is not a ready source of available snail shells for them to inhabit.  Life in a snail shell also means that the body of hermit crabs is usually asymmetrically twisted, like the snail shell itself.  For the hermit, this often means the loss of legs on one side of the body, including the appendages that carry eggs, such that most hermit crabs carry eggs only on one side of their body.  They also will often have an enlarged claw that serves as the “operculum” to fill and guard the opening when the hermit retracts into its shell.

At French Frigate Shoals, we are finding hermit crabs just about everywhere we look.  They are the most common crustaceans coming up from the deep sea in our deep-set baited traps, and we have found them in the shallowest areas we have explored.   It will be many years before we can figure out even how many species there are living out here.  But our expedition has already encountered several unknown species, some with what appear to be new species of anemones on their shells, and there are undoubtedly many more out there ready to be discovered.

By Joel Martin