Pagurus armatus (Dana, 1851)

Common name(s): Armed hermit, blackeyed hermit, black-eyed hermit

Synonyms: Bernhardus armatus, Eupagurus armatus, Eupagurus ochotensis, Pagurus ochotensis Pagurus armatus
Phylum Arthropoda 
Subphylum Crustacea 
Class Malacostraca 
Subclass Eumalacostraca 
Superorder Eucarida 
Order Decapoda 
Suborder Pleocyemata 
Superfamily Paguroidea 
Pagurus armatus found at about 20 m depth in Admiralty Bay, Whidbey Island.
(Photo by:  Dave Cowles, August 2011)

Description:  This hermit crab has a carapace which is smooth and only the shield is calcified (photo).  The abdomen is asymmetrically coiled.  The carpus of the right cheliped is longer than wide (photo)(photo).  The cheliped is shorter than the walking legs (photo).  The dactyls of legs 2 and 3 are longer than the merus, twisted in relation to the propodus, and have 2 reddish-brown stripes (photo).  The upper margin of the dactyl is serrated and the lower margin has close-set corneous spines (photo).  The merus of the walking legs is compressed laterally, with spines and toothed ridges.  The carpus and propodus of the walking legs have serrated dorsal margins and small spines on the surface.  The chelipeds are spiny dorsally and marginally (photo) but are nearly smooth ventrally (photo).  The dorsal side of the left (smaller) cheliped is flat (photo).  The eystalks are short and the eyes are large and black (photo).  The legs and chelipeds have orange transverse bands.  The ischium of the chelipeds has brown bands, the merus is pink, brown, and white with brown margins and teeth plus an orange distal band.  The palm of the propodus is pale yellow.  It has dark brown medial spines and red marginal spines.  The finger of the propodus is white or gray-blue with a white tip and white teeth (photo).  The walking legs have a yellow and white ischium, a merus with white, tan, orange, and mahogany with scattered brown spots; a yellow carpus with dark spines plus a mahogany stripe posteriorly and a white patch near the end, a propodus with white, yellow, orange, and brown and a mahogany stripe, and a dactyl which is white, orange, and violet-blue with dark spines and two narrow brownish-red stripes.  The legs have a green or gold iridescent sheen to them.  The eyestalks have a red band at the base and white and brown bands (photo).  The antennal flagellum is pale yellow (photo).  Carapace shield length to 19.6 mm and total carapace length up to 4.5 cm.

How to Distinguish from Similar Species:  The large black eyes plus orange bands on the legs are distinctive. P. aleuticus and P. ochotensis have spines (or grains) on the ventral side of the chelipeds, plus P. ochotensis has greenish-yellow eyes.  Most other hermit crabs do not have the twisted dactyls on legs 2 and 3.

Geographical Range: Northern Alaska to southern California

Depth Range:  Intertidal to 146 m; usually subtidal

Habitat:  Usually on sand, mud, shell, or gravel bottoms.

Biology/Natural History:  This is one of the largest and most common hermit crabs found in subtidal sandy areas.  As seen in many other crab species, males carry females around prior to the female molting so that they can mate. This species can grow large and often lives in moon snail shells,Euspira lewisii. Small individuals may have shells covered with hydroids and have partly yellow eyes.  It is often found among sea pens (Ptilosarcus gurneyi) in Puget Sound.  Females may carry up to three broods of eggs per year.



 

References:

Dichotomous Keys:
  Carlton, 2007
  Coffin, 1952
  Kozloff, 1987, 1996

General References:
  Hart, 1982
  Jensen, 1995
  Lamb and Hanby, 2005
  Niesen, 1997
  O'Clair and O'Clair, 1998

Scientific Articles:

Web sites:


General Notes and Observations:  Locations, abundances, unusual behaviors:


Another view

Another view of the animal, partly out of the shell.  The carapace shield is clearly visible.  Notice also how the dactyl of the walking legs twists slightly in relation to the propodus.

Underside

This view of the underside shows that few spines are found on those surfaces.  Note that the animal is in a moon snail shell, which is the characteristic choice of this species.
 
Views of the right cheliped:
Righ dorsal view Ventral view Right chela spines closeup Right chela closeup
Dorsal view of entire cheliped.  Segments that can be seen (from base to tip):  Merus, Carpus, Propodus, Dactyl Ventral view of entire cheliped.  Segments that can be seen (from base to tip):  Basis, Ischium, Merus, Carpus, Propodus, Dactyl A closeup of the spines on the dorsal side of the chela (propodus) A closeup of the teeth on the right chela.  Note the setae hanging down, which are probably sensory.

Views of the left cheliped:
Dorsal view of entire cheliped Ventral view of entire cheliped A closeup of the spines on the dorsal side of the chela Left chela
Dorsal view of entire cheliped.  Segments that can be seen (from base to tip):  Merus, Carpus, Propodus Ventral view of entire cheliped.  Segments that can be seen (from base to tip):  Basis, Ischium, Merus, Carpus, Propodus, Dactyl A closeup of the spines on the dorsal side of the chela (propodus) A closeup of the teeth on the left chela.  Note the fine setae lining the edges.

Propodus and dactyl of walking leg

The end of the propodus and base of the dactyl of  a walking leg (leg #2).  Notice how the dorsal edge has many teeth but the ventral edge has few.

Eyes

This closeup dorsal view of the head clearly shows the black eyes.

Out of shell

One last view



Authors and Editors of Page:
Dave Cowles (2010):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)

Rosario Invertebrates web site provided courtesy of Walla Walla University