A Hermit Crab.....

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Watch our short intro about Hermit Crabs.

​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 colour 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 (although a social sharing of shells is known to happen. Hermit crabs will form an orderly line and swap shells).
It is not unusual to find bristle worms inside the Hermit Crab shell. They eat leftovers from the hermit crabs. In return they remove any parasites from the abdomen.
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 defence (The large claw acting as a shield or door)..
Common hermit crabs have compound eyes and four short antennae.
 
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)
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)
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)
 
https://animaldiversity.org/accounts/Pagurus_bernhardus/

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