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Volume 39 Issue 17 • Aug 27-Sept 2, 2009
now in our 39th season

Stealing Home

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

This week's subject is the endlessly entertaining hermit crab and my decision to write about them before summer is gone was inspired by four of them living in the Field Station aquarium who have enchanted hundreds of children (and me) over the past few weeks.  Their antics and “musical shell” games and obsessive noshing on floating detritus while simultaneously terrorizing some of the smaller fish are fodder for a pretty cool show.  Hermit crabs are not only tank entertainers in marine aquariums, but they are also very common household pets.  Every parent at one time or another has been the reluctant guardian of either a terrestrial or aquatic hermit crab.  My nephews had two land hermit crabs that lived for seven years, long after the excitement had worn off.  In fact, if properly cared for, some hermit crabs found at pet stores or at county fairs can live for decades.

Hermit crabs are in the Decapoda (literally "ten footed") order, the infraorder Anomura (which includes mole crabs and sand crabs) and the Superfamily Paguroidea (Latreille, 1802). The members of Paguroidea have oval carapaces which are usually asymmetrical.  They live either in shells (hermit crabs) or with their abdomen tucked underneath (stone, coconut, and king crabs).  These latter crabs like the king crabs which have abandoned their shell home for a free living life are called “carcinized” crabs and are thought to be more similar to true crabs.  In evolutionary biology, carcinization is a hypothesized process whereby a crustacean evolves into a crab-like form from a non-crab-like form (can be observed here on Nantucket in humans in the winter).  The term was introduced by L. A. Borradaile, who described it as "one of the many attempts of Nature to evolve a crab."  The hermit crab morphology reveals they are not closely related to true crabs and instead share more characteristics with crustaceans like lobsters.


Photos of hermit crabs in our tanks were taken by Mirabai Perfas

The online Etymology Dictionary  traces the term “hermit” from the old French word “(h)eremite” which further descended from the Late Latin word “ermita” and Greek word “eremites” which means "person of the desert," which derived originally from eremia "desert, solitude," and eremos "uninhabited.”  From the Wikipedia web site (en.wikipedia.org/wiki/Hermit_crab), we learn that there are two possible etymologies that exist for the name of "hermit crab."  One is that their habit of living in a second-hand shell is a reference to the idea of a hermit living alone in a small cave.  The second is that it is a translation of the name of one of the common species, Coenobita Clypeatus (Herbst, 1791), which is translated as the "shield-bearing monk or cloister brother."  Most hermit crabs are anything but hermits and prefer to live in groups and have developed foraging techniques that depend on group dynamics and cooperation for feeding success and for protection from predators.  Because of their gregarious nature, hermit crabs will survive as pets longer when paired with a buddy.

An early ancestor of the hermit crab may have existed 500 million years ago as theorized by geologists James W. Hagadorn and Adolf Seilacher.  They believe that the adaptation of coiling an asymmetrical body into a snail shell was a viable method to migrate a water breathing creature onto dry land to expand habitat and food sources.  They found evidence of their theory in fossilized shell “trails” preserved in ancient Cambrian intertidal sandstones recorded in a thin layer of microbial film.  These early crab-like creatures were tucked into tiny shells which only provided them with moisture, essentially bringing their aquatic home with them.

In our modern world, there are about five hundred known species of hermit crabs, most of which are aquatic and live in saltwater at depths ranging from shallow coral reefs and shorelines to deep sea bottoms.  However, in the tropics, a number of species are terrestrial, and some of these are quite large, like the Giant Hermit Crab (Petrochirus diogenes). Our “cloistered monk,” the Caribbean hermit crab (Coenobita clypeatus) is known to climb trees.  Hermit crabs have evolved over time to be uniquely suited to their lifestyle.  Their entire bodies have slowly conformed to fit into their shells.  Unlike most other crab-like species, their bodies are asymmetrical so that they can curl into the snail shells.  Hermit crabs are commonly seen in the intertidal zone, for example in tide pools.  Land crabs need dry land to survive and will die if they are in the water too long although they need some water to keep their body and lungs moist.  The ocean crab can't be taken out of the water too long or it will die.

Most species of hermit crabs have long soft abdomens which are protected from predators by the adaptation of carrying around a salvaged empty seashell into which the whole crab's body can retract.  Hermit crabs prefer to use the shells of sea snails or marine gastropod mollusks such as our northern moon snail or whelks.  The tip of the hermit crab's abdomen is adapted to clasp strongly onto the inside column of the snail shell. Their front claws have evolved to fill in the opening of a snail shell when the crab is retracted to form an operculum (Latin for “little lid” or door).

The crab life cycle is an involved one with several steps.  In the late spring, you'll see hermit crabs traveling in the intertidal part of the beach in pairs with the males sometimes dragging the females around by their shells (hopefully more romantic than it sounds).  After mating, the female  keeps the eggs safe inside their shells until they are big enough to release.  The first two stages of a hermit crab's life (the nauplius and protozoea) occur within the egg.  When they are ready, she crawls partway out of her shell to brush them out using a hind leg into the ocean, where they hatch into tiny, free-swimming larvae called zoea.  Hermit crabs require this aquatic step to hatch their eggs which makes it very difficult to breed them in captivity.  Zoea grow and molt several times before becoming megalops, which are still tiny but have a crustacean-like form.  They develop their claws and antennae during these stages.  Megalops molt into juvenile hermit crabs.  Juveniles continue to grow and molt, eventually becoming adults. 

Every time the hermit crab grows, its exoskeleton doesn't, so it needs to molt and grow a new one.  It then needs to find a new shell to live in that fits it.  Once it finds a new home, unless it is checking out another one, it tends to stick close to its protection.  It is extremely difficult to pull a hermit crab out of its shell, but you can drill a hole in the shell and poke it out (obviously not high on the fun scale for them).  Hermit crabs may molt every two or three months when they are young and every 18 months as they get older.  They are able to regenerate missing appendages such as their claws during the molting phase.

Hermit crabs are omnivorous, which means they will eat anything from tiny plants and animals to decaying matter and detritus.  Hermit crabs drink by dipping their claws in water then lifting out drops of water to their gills and mouth.  They use their front claws almost like a fork and knife, holding larger food pieces with one claw and tearing them into bite sized morsels with the other.  Hermit crabs are nocturnal animals which help them survive and reduce the likelihood of them drying out in the sun.  They move around a lot more at night then during the day and some scientist have recorded them making “croaking sounds” which implies some amount of communication.  Some species have a mutualistic relationship with sea anemones that attach themselves on the shell, obtaining free transportation and scraps of food in return for protecting their hosts.  Many of the hermit crabs we find here have barnacles or slipper shells on the outside of their host shells, getting a free ride while weighing down the crab.

Several hermit crab studies have been done at the Nantucket Field Station including a population census in 1988.  Here on Nantucket we see three species on our beaches in the intertidal area: Pagurus annulipes (Stimpson), or the banded hermit crab which is the smallest one that favors periwinkle shells and has a hairy claw.  The long-clawed hermit crab, Pagurus longicarpus (Say), has long, narrow claws with a darker stripe on the “hands” of the claws.  The flat-clawed hermit crab, Pagurus pollicaris (Say), has a flat major claw with wart-like projections called tubercles.  It is the largest hermit crab species found on Nantucket.  We have a huge one in our tank living in a large whelk shell.  I like to describe these hermit crabs as walking on their knuckles.

Dr. John Ebersole is a faculty member at the University of Massachusetts Boston, which runs and funds the Nantucket Field Station, and he has mentored graduate students conducting hermit crab science here on island for more than twenty years.  Back in1995, one of his students, David Carlon, spent a few summers looking at the intricacies of our local hermit population and published several papers on the topic including “Life-History Variation Among Three Temperate Hermit Crabs: The Importance of Size in Reproductive Strategies” (Biol. Bull. 188: 329-337. June, 1995).

In a nutshell (or hermit crab shell) their research showed that the little guys (P. annulipes) reproduce soon after metamorphosis and have a high reproductive effort while the other two species took longer to become parents and had fewer offspring.  Size differences among species were related to patterns of shell usage.  Male and female P. annulipes were always found in large shells relative to body size.  In comparison, male and female P. longicarpus and P. pollicaris were found in small shells compared to body size.  They discovered that the smallest hermit crabs were reproducing at a younger age and more frequently to compensate for their high risk of mortality associated with small shells.  They also found that the shell needs (large shells which are more rare) of the bigger species were a limiting factor for the population growth.  Populations grew (or shrank) in concert with the amount of available shells.

This research supports other studies in which researchers found that if shells were provided for hermit crab populations, the populations grew (sounds like both fun research and a little weird).  Several studies show that hermit crab shell use and exchange has become ritualized and competition for shells is quite fierce.  In fact, most scientist believe and there is a fair amount of evidence to suggest that hermit crab population including those of our three common ones on Nantucket are controlled by the number of available shells.  In some parts of the world, hermit crab populations are declining because there are fewer and fewer shells for them to occupy, which sounds uncomfortably like the situation for humans around the world.

So, now we can think a bit about a very common trait that most of us have, which is the tendency to carry home a bucket full of shells from the beach.  Occasionally I have thought about whether it would deprive a hermit crab of a future home, and some of my very young students have thought of this as well, but I bet you may not have.  Next time you are on a beach and see a large whelk shell, think twice before carting it home, a nearby hermit crab may be eying it for a future renovation.

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