Common hermit crabs are found in the near-shore North Atlantic waters of northwestern Europe, from the White Sea to the British Isles, including the North, Baltic, and Barents Seas. They are found as far south as Portugal, including in the Mediterranean Sea, as well as along the coasts of the Azorean Islands. ("ARKive", 2012; Anger, 1989; Reese, 1969; Reiss, et al., 2005; Sokolov, 2006; Tϋrkay, 2013)
These crabs live along coasts in most types of seabeds, including rocky and shell bottoms, in sea grass beds, and sandy or silty sediments, but excluding muddy bottoms. Larvae live mainly in pools and may be found under objects such as rocks and seaweed. Common hermit crabs are most often found at depths up to 80 m, although they may be found as deep as 200 m. Smaller individuals live in shallower waters than larger individuals. (Burton and Burton, 2002; Lancaster, 1990; Tϋrkay, 2013)
Common hermit crabs are relatively large (maximum body length 8 cm), with bodies that are divided into two segments: cephalothorax and abdomen. The cephalothorax is encased by a carapace consisting of three thick cuticle layers: epicuticle, exocuticle, and endocuticle (maximum carapace length is 4.5 cm). The abdomen is soft and coiled to the right and body color is typically reddish or brown. These crabs inhabit the abandoned shells of animals, such as edible periwinkles (Littorina littorea), flat periwinkles (Littorina obtusata), dog whelks (Nucella lapillus) and other whelks (Buccinum sp.), using them as protection for their soft bodies. Their last two pairs of walking legs are greatly reduced, and are used to hold the shell in place. Compared to most hermit crabs, Common hermit crabs prefer a lighter shell. The size of the shell is important because it affects the fitness of the hermit crab-a shell that is too large does not offer the best protection and a shell that is too small restricts its growth. Individuals may attack each other in attempts to claim ownership of a shell. These crabs have 5 pairs of walking legs; the first pair are enlarged claws (known as chelae); those of males are larger than those of females. Chelae are used for gathering food and for protection. Of the two claws, one is larger and is covered by an operculum; this claw is used in fighting and defense. Chelae and walking leg surfaces are rough. Common hermit crabs have compound eyes and four short antennae. ("ARKive", 2012; "World Association of Zoos and Aquariums", 2012; Babu and Anger, 1987; Briffa, et al., 2008a; Briffa, et al., 2008b; Doake, et al., 2010; Elwood and Briffa, 2001; Reiss, et al., 2005; Tϋrkay, 2013)
Eggs of this species are black in color. Females carry eggs until they hatch (2 months, on average). Common hermit crabs pass through four zoeal stages and a glaucothoe/megalopa stage before reaching adulthood. Zoeal stages take 39-47 days to complete, with the megalopa stage requiring an additional 13 days on average; total development time averages 60 days. Zoeal larvae are thin, long, and yellowish-red in color. More chromatophores are added in each stage of zoeal development: megalops have areas of red and pale-yellow coloration and adults are reddish-brown. Megalops do not eat and must seek out a suitable shell in which to complete metamorphosis. Shell type and size effectively determine survival odds, as well as body size, time to sexual maturity, and longevity. Sexual differentiation occurs during the zoeal stages; female development is complete upon reaching adulthood, while males continue to develop morphological characteristics after adulthood. Sexual maturity is typically reached within a year. ("ARKive", 2012; Anger, 1989; Bookhout, 1964; Carvacho, 1988; Darwirs, 1979; Lancaster, 1990; Macdonald, 1957)
Unlike some species of crabs, copulation does not necessarily follow a female's molting. Before copulation, a male carries or pulls a female around by her shell opening for hours, even days in some cases, using his minor cheliped. Just prior to copulation, he holds her by one of her right walking legs, pulling her toward him and tapping her appendages with his major cheliped. This is followed by a period where both male and female tap each other with their chelipeds (the female also palpates the male's mouthparts) for 15-20 minutes. When a female is ready to mate, she gives either a tactile or chemical signal, causing a male to turn her around to face him. Both individuals leave their shells and copulate for 4-6 minutes. They remain together, outside of their shells, for up to 10 minutes after copulation. (Hazlett, 1970; Lancaster, 1988)
Unlike other Pagurus hermit crabs, Common hermit crabs are able to mate outside of the period after a female molts; there have been records of breeding occurring during all phases of a female molt cycle. There are many environmental factors that affect whether or not breeding occurs, such as photoperiod, water temperature (itself and in relation to air temperature), salinity, and availability of resources. Common hermit crabs may mate continuously throughout the year, although populations living in shallower water tend to have reproductive peaks in January and Feburary. After one encounter, a female produces 200-300 eggs, up to 30% of which may by lost or destroyed. ("ARKive", 2012; "World Association of Zoos and Aquariums", 2012; Burton and Burton, 2002; Hazlett, 1970; Lancaster, 1988)
After fertilization, a female carries eggs until they hatch (43 days, on average). During this time, they exhibit greater care in shell choice, switching shells more often than when they are not brooding. (Lancaster, 1988; Neil and Elwood, 1985)
Common hermit crabs are known to live for up to four years in the wild. Shell selection, as well as molting, affects longevity. Molting, in particular, has short-term benefits and long-term costs; short term benefits include limb regeneration, while long-term costs include potential negative shifts in dominance hierarchy, reproductive success, feeding, communication, and locomotion. (Burton and Burton, 2002; Lancaster, 1988)
Common hermit crabs are nocturnal. They aggregate into loose communities, living around each other but not necessarily interacting. In these populations, breeding occurs at almost any time during the year and there is usually intense competition over resources, often resulting in physical damage to individuals. Each community has a dominant male; this individual wins the most fights and may assert its dominance by taking resources from others. (Briffa and Bibost, 2009; Briffa and Dallaway, 2007; Hazlett, 1968; Hazlett, 1970; Jackson and Elwood, 1989; Lancaster, 1988; Lancaster, 1990)
Before any physical encounter, Common hermit crabs engage in visual displays. During aggressive encounters, they either raise their bodies high off the ground, displaying their chelipeds and walking legs, or lower their bodies in submission. If neither individual submits, the crabs may try to dislodge each other from their shells, an interaction known as shell fighting. One hermit crab will crawl onto its opponent's shell, hitting its opponents entire body, including its shell, and using its chelipeds to rap its opponent's shell (over 500 raps have been observed in a single encounter). Around half of such encounters result in the aggressor exchanging shells with its opponent. Larger crabs, and those with larger chelipeds, are usually more successful than small- and medium-sized crabs in these encounters. (Briffa and Bibost, 2009; Briffa and Dallaway, 2007; Hazlett, 1968; Hazlett, 1970; Jackson and Elwood, 1989; Lancaster, 1988; Lancaster, 1990)
Another behavior that has been extensively studied is shell selection. Common hermit crabs are more likely to exchange and fight over shells when a new shell confers an advantage, such as more space. They are also able to remember shells that they have rejected before. If a crab is naked or in a shell that does not offer enough protection or space, then it will make the decision to move into a shell very quickly, regardless of advantages it may confer. If a new shell is of a much higher quality than the currently inhabited shell, the decision to move will also be made very quickly. (Briffa and Bibost, 2009; Briffa and Dallaway, 2007; Hazlett, 1968; Hazlett, 1970; Jackson and Elwood, 1989; Lancaster, 1988; Lancaster, 1990)
Studies have shown that individuals may move within a range of 16-312 meters per month. (Bell, 2009)
Males and females communicate with each other through tactile and chemical signaling, and visual cues are used during aggressive encounters. (Briffa and Bibost, 2009; Briffa and Dallaway, 2007; Hazlett, 1968; Hazlett, 1970; Lancaster, 1988)
Common hermit crabs are scavengers. Their diet includes a wide variety of food sources, including microscopic bivalves, scraps of dead animals, and plants; they are known to be cannibalistic when resources are low. This diet is good for survival in environments that are unpredictable and can seasonally change. (Lancaster, 1990)
Predators of common hermit crabs include fishes, starfish, octopi, other crabs, and some birds. Seagulls are known to pick up common hermit crabs in their beaks and drop them on rocks to break their shells. These hermit crabs may also exhibit cannibalism. (Haug, et al., 2002; Lancaster, 1988)
Common hermit crabs are valuable members of their ecosystems as scavengers and detritovores. As detritovores, these hermit crabs help decompose dead materials and contribute to nutrient cycles. Common hermit crabs may host a variety of epibionts on their shells, including protozoans, hydrozoans, entroprocts, barnacles, and polychaete worms. These hermit crabs prefer shells with epibonts over clean shells; some epibionts, such as anenomes, can protect the hermit crab from predators. They may also host parasites (most often isopods or barnacles), which bore into their shells, residing most often in the abdomen or brachial cavities, sometimes even causing castration in males. Infestation levels are as low as 1.5%. Eggs of this species have antibiotic properties. ("World Association of Zoos and Aquariums", 2012; Bell, 2009; Fernandez-Leborans and Gabilondo, 2006; Haug, et al., 2002; Lancaster, 1988)
Common hermit crabs play an important role in their environments as decomposers. They are also kept by some people in aquaria and may be used as fishing bait, once removed from their shells. (Lancaster, 1988)
There are no known adverse effects of common hermit crabs on humans.
Kathleen Wang (author), University of Michigan-Ann Arbor, Jeremy Wright (editor), University of Michigan-Ann Arbor.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
helps break down and decompose dead plants and/or animals
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
an animal that mainly eats decomposed plants and/or animals
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
parental care is carried out by females
union of egg and spermatozoan
An animal that eats mainly plants or parts of plants.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
seaweed. Algae that are large and photosynthetic.
A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
an animal that mainly eats all kinds of things, including plants and animals
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
the business of buying and selling animals for people to keep in their homes as pets.
an animal that mainly eats plankton
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
uses sight to communicate
breeding takes place throughout the year
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