Pagurus dalli (Benedict, 1892) Common name(s):  Whiteknee hermit
Synonyms: Eupagurus dalli
Phylum Arthropoda Subphylum Crustacea Class Malacostraca Subclass Eumalacostraca Superorder Eucarida Order Decapoda Suborder Pleocyemata Infraorder Anomura Superfamily Paguroidea Family Paguridae
	Pagurus dalli, from about 20 m depth at Mukilteo.  The "shell" on its back is composed mostly of a colony of Hydractinia milleri hydroids.
	(Photo by: Dave Cowles)

How to Distinguish from Similar Species: Pagurus stevensae has a right cheliped more triangular than elongate and a double row of spines on the eminence near the midline of the dorsal surface of the propodus of the left chela, plus it has no white band on the end of the merus of its chelipeds. Pagurus kennerlyi has a white band on the merus of the chelae but its second antennae have alternating light and dark bands and it has a tuft of setae on the dorsal side of the base of  its cornea.

Geographical Range:   Bering Sea to Oregon; widespread but not abundant.

Depth Range:  Very low intertidal to 276 m

Habitat:  Gravel, sandy, or mud bottoms; symbiotic with sponges (or hydroids)

Biology/Natural History:  This species is said to generally live symbiotically with the sponge Suberites latus or Suberites ficus, which dissolves and overgrows its shell.  This specimen was in a "house" made largely or entirely of the hydroid Hydractinia milleri.  This hydroid is said to encrust shells which have hermit crabs living in them, and eventually to overgrow the shell as the crab grows.   The hermit crab frequently rubs the flagellae of its second antennae over the hydroid colony.  In another Hydractinia-hermit crab symbiosis this behavior was found to result in the hermit crab gathering and eating some of the plankton the Hydractinia had captured.

The presence of Hydractinia on a hermit crab seems to at least partially deter predation by octopus.  Octopus usually readily capture hermit crabs and other crustaceans.  However, an octopus clearly thinks twice about attacking a hermit crab with Hydractinia on its shell.  Click here for a movie showing how octopus deal with Hydractinia-covered hermit crabs.

References:

  General References:

  Hart, 1982
  Ricketts et al., 1985

Scientific Articles:

Web sites:

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

I have rarely encountered and identified this species.  I can find no references in the literature to its living symbiotically with a hydroid (though it lives in a similar manner with sponges).  My thanks to Kirt Onthank for his sharp-eyed viewing during a dive which resulted in finding this symbiotic pair.
 

In this side view the slight coiling of the "house" is seen.  The house is made of the hydroid Hydractinia milleri.  The egglike structures are gonozooids of the hydroid.  The hermit crab gives no sign of being stung by the hydroid.  Some other species of hermit crabs, known to be symbiotic with Hydractinia, also do not seem to be vulnerable to their sting while some other hermit crab species are.

This dorsal view of the head shows the smooth carapace, the lack of tufts (setae) on the dorsal margin of the cornea, and the fully exposed eyescales with a single terminal spine and no median dorsal furrow.

The carpus of the left chela is longer than the merus.  The carpus of the right chela is about 1 1/2 times as long as wide and is not extremely flattened (the carpus is the last segment visible on the right chela here, as the propodus and dactyl are folded underneath as the animal crawls along).  The merus of both chelae has a prominent white band on the terminal end.
 
 

The 2nd and 3rd dactyls are not striped, do not have ventral spines, and are not obviously twisted in relation to the propodus.
This is a view of the merus (top right, spiny), carpus (spiny), propodus, and dactyl of the right second leg.

This ventral view of the right cheliped shows that the ventral side of the merus (to the left of the sharp turn, or "knee") does not have any prominent tubercles.
 

This X-ray view of the above live animal shows that there is a small gastropod shell at the base of the Hydractinia house.  The shell is about 5 mm long.
Digital X-ray compliments of Julie Kellogg, DDS and Tietan Dental Clinic.