Download Marine Invertebrates of Bermuda

Marine Invertebrates of
Bermuda
Purple Pincher Hermit Crab (Coenobita clypeatus)
By Rebecca Karis
with
James B. Wood and Melissa Parr (Eds)
Abstract Taxonomy Habitat Ecology Recent Research Commercial
Importance Bermuda Laws Personal Interest References Links
Abstract
Coenobita clypeatus is a terrestrial hermit crab found in tropical and sub-tropical areas. Living
in humid areas this terrestrial hermit crab is a nocturnal scavenger that uses a gastropod shell
to protect its soft body parts. These creatures are extremely social, foraging and traveling in
groups. While C. clypeatus is extremely social the species is known to be aggressive towards
other hermit crabs in their group to steal better fitting shells. C. clypeatus is also cannibalistic
and will eat vulnerable molting crabs. While the purple pincher hermit crab is terrestrial it
must return to the water to spawn and release it's larvae into the ocean.
In Bermuda the purple pincher hermit crab has an extremely small population. In 1994 their
populations was estimated to be only 150 individuals. The current state of the C. clypeatus
population pales in comparison to the population of the 1950s. The purple pincher hermit crab
was said to be found every where on Bermuda, now it is only found in a single bay on the
south shore. Recent research has pointed to shell availability as the determining factor in
population size. Unfortunately there are no Bermudian laws protecting the dwindling C.
clypeatus population.
Taxonomy
Phylum: Arthropoda
Class: Crustacea
Subclass: Malacostraca
Order: Decapoda
Family: Coenobitidae
Arthropods have an articulated, segmented, chininous exoskeleton (Wood 2007). Crustaceans
have 5 paired head appendages, which include antennae, mandibles and two sets of maxilla.
Crustaceans also have biramous appendages meaning they have two part appendages, each
part performing a different function. Organisms in the class Malacostraca have trunks with
fourteen segments, eight on the thoracic, six on the abdomen. Order Decopoda is defined by
three thoratic appendages and five pairs of walking legs. Coenobita animals are named so for
their communal living (Fenner 1999).
Habitat
Hermit crabs in the genus Ceonobita are found on tropical and subtropical islands in the
Pacific, Atlantic and Indian Oceans (Chance and Hobbs 1969; Ball 1972; DeWiled 1973;
Alexander 1979; Page and Willason 1982; Willason and Page 1983). Coenobita clypeatus is a
tropical land hermit crab that lives throughout the Caribbean and as far north as Florida and
Bermuda (Kurta 1982). C. clypeatus spends most of its time in the woods but is known to
travel to high sand hills near the shore (Sterrer 1986). Because C. clypeatus has gills it must
live in areas of high humidity. However their gills do not allow them to be fully submerged
under water because the hermit crabs will drown. C. clypeatus has been found to travel
across many different habitats and have climbed mountains as high as 887 meters (Walker
1994). In Bermuda these crabs once roamed all over the island but now they are only know
to inhabit Hungry Bay a small bay on the south shore of the island (Hass 1950; Walker
1994).
Ecology
Activity
Coenobita clypeatus is a slow moving nocturnal terrestrial hermit crab (Hazlett 1964; Hazlett
1966). The crabs bury themselves beneath leaves, rocks, or sand during the day to avoid
desiccation (Hazlett 1966). By being nocturnal C. clypeatus is temporally separated from its
competitors (Morrison 2006).
The hermit crabs have a complex social structure. C. clypeatus uses “social facilitation as a
foraging statagy” (Kurta 1982). This is an ordinary activity increased by the presence of
others (Wilson 1975). The hermit crabs forage in groups to find food more quickly and as an
“anti-predator device” (Powell and Nickerson 1965).
While C. clypeatus work together to find food they can become aggressive when it comes to
shell selection (Spight 1977). The attacking hermit crab will grab onto the back of another
hermit crab’s shell and rock the defending hermit crab back and forth. The attacking hermit
crab will do this until the defending crab releases its hold on the shell that sparked the conflict
(Hazlett 1966). Often the defending hermit crab will chirp, the reason for this is still unknown
(Hazlett 1966).
Molting
Hermit crabs must go through a stressful molting process (Sterrer 1986). This process allows
for growth and limb regeneration (Wood 2007). There are four stages of molting, pre-molt
(proecdysis), molt (ecdysis), post-molt (metecdysis), and intermolt (anecdysis) (Wood 2007).
Proecdysis is the process of laying down a new exoskeleton and storing water for the arduous
molt. Ecdysis is the shedding of the exoskeleton, at this point the crab is extremely vunerable
without the protection of a hard exoskeleton. Metecdysis is the process of hardening the new
exoskeleton; often the crab will eat its old exoskeleton to provide calcium for the new
exoskeleton. Anecdysis is the periods in-between molts where the crab gains energy to
prepare for the next molt (Wood 2007).
The molting process can take from three weeks to three months depending on the size of the
hermit crab (Walker 2005).
Feeding
Coenobita clypeatus is a generalist omnivore scavenger (Morrison 2002). These hermit crabs
will eat almost anything from carrion, dead fish, and fallen fruit to rotting wood and sea turtle
eggs (Vannini 1976). This feeding strategy is very similar to many other animals, so C.
clypeatus is nocturnal to avoid competitors (Morrison 2006). The crab uses its chelipeds to
bring food to its mouth parts to be ingested (Kurta 1982).
Reproduction and Life Cycle
Female hermit crabs have gonophores on their fourth pair of legs (Sterrer 1986). Males
fertilize the females by passing spermocytes over her gonophores, to do this they must both
emerge from their shells slightly (Walker 2005). The female then lays eggs and holds them
inside her shell using pleopods (Sterrer 1986). The female broods the eggs for approximately
a month before returning to the ocean to release the eggs (Walker 2005). The eggs burst and
release zoea, planktonic larvae of Coenobita clypeatus. The zoea remain in the planktos for
about two months constantly going through metamorphosis to become a megalopa. The
megalopa find its first shell and lives both on land and in the water for a month before
completing its first molt. After the molt the hermit crab emerges as a juvenile and is
completely terrestrial (Walker 2005).
Threats
Coenobita clypeatus uses its shell to protect its soft body parts from predators and other
threats. C. clypeatus is cannibalistic and will eat other hermit crabs especially over shell
disputes. C. clypeatus is also known to dig up hermit crabs during molting periods and eat
them, often stealing the shell of the molted crab in the process (personal observation).
Humans have been attributed to removing shells from the C. clypeatus habitat and can stunt
the growth of individuals as well as reduce population size (Nieves-Rivera and Williams
2003).
Recent Research
Much of the current research on Coenobita clypeatus has concentrated on shell selection and
competition between land hermit crabs and ants.
Sally E. Walker did a study in Bermuda in 1994 on shell modifications by Coenobita
clypeatus. As a paleontologist Walker (1994) was looking at how terrestrial hermit crabs often
use fossilized Cittarium pica shells and the problem it caused when trying to establish time
lines. C. clypeatus carried shells all over Bermuda making it appear that C. pica lived on land
confusing the fossil record. The re-use of shells also can confuse time lines as extremely old
shells that have been fossilized for thousands of years, are released by hurricanes and used by
present day C. clypeatus. Walker (1994) made a standard way to evaluate C. pica shells to
determine if they had been used by C. clypeatus. Walker (1994) also estimated the population
of C. clypeatus on Bermuda to be 150 individuals, very different than previous observations
made by Haas (1950) when terrestrial hermit crabs were found in large numbers roaming the
entire island.
A more recent study tested the patterns of gastropod shell use of C. clypeatus on islands in
the Bahamas. Morrison and Spiller (2006) also estimated population densities on these small
Bahamian islands. Densities on the islands varied, due to shell availability. Shell use of C.
clypeatus appeared dependant on size of the hermit crab. Larger hermit crabs used C. pica
shells while smaller crabs used a variety of shells such as periwinkle or nerite shells.
Both studies found a strong correlation between cheliped length and body size. Both studies
also suggested that population sizes of C. clypeatus are restricted by shell availability.
Morrison (2002 and 2006) studied the competition between C. clypeatus and ant species.
Both species are scavengers and they compete over food sources. The hermit crabs tend to
find food quickly but large groups of ants drive the hermit crabs away. The ants can become
extremely aggressive and have been known to kill hermit crabs. This competition seems to
explain C. clypeatus nocturnal foraging as an attempt at temporal separation from its
competitor.
Commercial Importance
Hermit crabs are popular pets in homes around the world. All hermit crab pets are taken from
the wild because they can not be bred in captivity.
Bermuda Laws
There are currently no laws in Bermuda pertaining to Ceonobita clypeatus.
Personal Interest
I have several different hermit crab species as pets and they make wonderful companions.
They are very easy to care for and are their behavior is very interesting to watch. Each crab
has their own personality and I find the social interactions of Coenobita clypeatus extremely
interesting. Their communication and relations are fascinating as they work together to find
food yet compete with each other for shells. I would love to find a way to breed C. clypeatus
in captivity in order to protect the wild populations from being overexploited.
The current state of the C. clypeatus population in Bermuda is alarming. I would like to find a
way to bring the plight of C. clypeatus to the attention of the Bermudian Government with
the goal of having laws put in place to protect these intriguing animals. I also think it would be
beneficial to run a program to increase the population and possibly introduce C. clypeatus to
Nonsuch island, the “Living Museum” of Bermuda.
References
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crustaceans in the Aldabran ecosystem. Philisophical Transactions of the Royal Society of
London Series B 286:241
Ball, E. E. 1972. Observations on the biology of the hermit crab, Coenobita compressus H.
Milne Edwards (Decapoda; Anomura) on the west coast of the Americans. Revista de
Biologica Tropical 20:265
Chance, F. A. and Hobbs, H. H., Jr. 1969. The fresh water and terrestrial crabs o fthe West
Indies with special reference to Dominica. Smithsonian Institute, U.S. National Museum of
Natural History Bulletin, 292:1
DeWilde, P. A. W. J. 1973. On the ecology of Coenobita clypeatus in Curacao with
reference to reproduction, water economy and osmoregulation in terrestrial hermit crabs.
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clypeatus (Herbst). Ecology 47:316
Hazlett, B. A. 1981. The behavioral ecology of hermit crabs. Annual Review of Ecology and
Systematics 12:1
Kurta, A. 1982. Social facilitation of foraging behavior by the hermit crab, Coenibita
compressus, in Costa Rica. Biotropica 14:132
Morrison, L. W. 2002. Interspecific competition and coexistence between ants and land
hermit crabs on small Bahamian islands. ACTA Oecologica International Journal of Ecology
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Morrison, L. W. 2006. Mechanisms of coexistence and competition between ants and land
hermit crabs in Bahamian archipelago. ACTA Oecologica International Journal of Ecology
29:1
Morrison, L. W. and Spiller, D. A. 2006. Land hermit crab (Coenobita clypeatus) densities
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Nieves-Rivera, A. M. and Williams, E. H., Jr. 2003. Annual migrations and spawning of
Coenobita clypeatus (Herbst) on Mona Island (Puerto Rico) and notes on insland
crustaceans. Crustaceana 76:547
Osorno, J. L., Fernandez-Casillas, L., and Rodriquez-Juarez, C. 1998. Are hermit crabs
looking for light and large shells? Evidence form natural and field induced shell exchanges.
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Page, H. M. and Willason, S. W. 1982. Distribution patterns of terrestrial hermit crabs at
Enewetak Atoll, Marshal Islands. Pacific Science 36:107
Powell, G. C. and Nickerson, R. C. 1965. Aggregations among juvenile king crabs
(Paralithodes camischatica Tilesius) Kodiak, Alaskan Animal Behavior 13:374
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Indian topshell, Cittarium pica (Linnaeus). Bulletin of Marine Science of the Gulf and
Caribbean 14:424
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152:1
Sterrer, W. ed. 1986. Marine Fauna and Flora of Bermuda: A Systematic Guide to
Identification of Marine Organisms. New York: John Wiley and Sons.
Vannini, M. 1975. Researches on the coast of Somalia, The shore and the dune of Sar Uanle,
4, Orientation and anemotaxis in the land hermit crab, Coenobita rugosus Milne Edwards.
Monitore Zoologico Italiano, N. S. 6(4):57
Vannini, M. 1976. Researches on the coast of Somalia. The shore and the dune of Sar Uanle.
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crab, Coenobita clypeatus, on Bermuda. Palaios 9:403
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Links
The Crabbage Patch
Florida Marine Research
Smithsonian Think Tank: Land Hermit Crabs