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=  Pagurus armatus  =

Pagurus armatus, the armed hermit crab or black-eyed hermit crab, is a species of hermit crab belonging to the family Paguridae. They are found in the eastern Atlantic Ocean of the United States and British Columbia, Canada.

Habitat
P. armatus lives on a variety of substrates, being particularly abundant in sea pen beds, at depths of up to 117 m

The species is commonly found in a soft, substratum habitat which includes shell fragments, sand, and mud. The bathymetric (underwater depth of ocean floors) range is 11-46 mm.

Physical Description
P. armatus is one of the largest species of hermit crab: adults may reach a carapace length of 43 mm. The mean size of P. armatus, given the length of its shell, was found to be 4.47 mm. The brain of P. armatus is broad - which is usual for crustaceans. As the hermit crab grows, it moves from the zoeal stage (which is the larval stage) to the megalopal stage (which is P. armatus in its adult form). There are many structural differences when comparing the zoeal and megalopal stages of P. armatus. The zoeal, for instance, has not developed a tail fan or mandible (the lower jaw bone) yet. The carapace (the external skeleton of the crab) in the megalopal stage has a shell that surrounding about half of its length. Therefore, a grown P. armatus favors an averagely large shell size - specifically Natica and Polinices shells.

The legs, including the claws, have bands of colour, in red, orange and white, and the claws bear short spines on the dorsal surface. The eyestalks are short, but bear large black compound eyes. P. armatus can be told apart from the similar P. ochotensis by the spines on its claws, which in P. ochotensis are replaced by granules.

Sex and Reproductivity
Hermaphroditism is common in P. armatus, along with most hermit crab species. The species also shows intersexuality, although scientists cannot determine if this is due to sequential hermaphroditism or sexual dimorphism.

For reproductivity, it was found that female P. armatus are capable of hatching 300 larvae. The larvae are able to survive with little to no sunlight and at a constant temperature of about 12 degrees celsius. The larvae of P. armatus were found to be the most resilient out of all Pagurus species, with about 80 percent surviving to the megalopal stage.

Behavior
One behavior common among most hermit crabs is its behavior against predation. P. armatus will often retreat into its shell at any sign of danger.

Feeding
P. armatus is an omnivorous scavenger that feeds on carrion (the flesh of dead animals) and plant material.

Out of most species of hermit crabs, P. armatus will form the largest groupings of scavengers. In the wild, they can signal (with their claws) and fight for food.

Interspecific and Intraspecific Competition
Interspecific competition occurs when P. armatus fights over empty Natica Clausa shells with its closely related hermit crab species. This can occur through a shell fight, although crabs with the larger body ratio (such as P. armatus), usually win.

Intraspecific competition is most common for P. armatus. Their habitat does not usually coincide with the habitats of other species of hermit crabs. Therefore, P. armatus fights among themselves for the available shells.

Type specimen
P. armatus was originally known as Bernhardus armatus from Puget Sound by James Dwight Dana. The publication of James Dana's Atlas of Crustacea had the first illustration of the species. It was not until the holotype of P. armatus was lost in the Great Chicago Fire of 1871 that it was replaced with a lectotype.

There has also been controversy surrounding the type specimen of P. armatus. Scientists recognized P. armatus as distinct from another species of hermit crab, Eupagurus ochotensis, while others grouped them together. Ultimately, five species of hermit crabs, including P. armatus was assigned to the modern bernhardus group.