Ligia (Ligia) pallasii Brandt, 1833

Common name:  Rock louse, Common rock louse
Synonyms:  None
Phylum Arthropoda 
 Subphylum Crustacea 
  Class Malacostraca 
   Subclass Eumalacostraca 
    Superorder Peracarida 
     Order Isopoda 
      Suborder Oniscoidea
       Family Ligiidae
Ligia (Ligia) pallasii found on Swirl Rocks, Washington. 
Photo taken by Dave Cowles, at Swirl Rocks. July 2002
The bodies of these isopods are usually robust and approximately 35 mm in length, however I found that around Swirl Rocks these isopods greatly vary in size.  The eyes are separated by a distance equal to twice the greatest diameter of an eye.  They have short uropods, and a basal segment as long as broad. Their uropods are terminal and close to the midline (photo).

How to Distinguish from Similar Species:
They may be confused with their relative, L. (M.) occidentalis in central California because of geographic overlap.  However, where overlap occurs the two species are generally ecologically segregated in that the L.(L.) pallasii prefers sea cliffs and the L. (M.) occidentalis prefers rocky beaches.  Essentially terrestrial, they prefer to live near the high tide line.

Habitat and Range:
These isopods are often found in caves and crevices on rocky cliffs. They live in the high intertidal zone from the Aleutian Islands (Alaska) to Santa Cruz (California).

Biology/Natural History:
A researcher, Wilson (1970), found that the osmoregulatory responses might differ according to their behavior and ecology.   The slower moving Ligia pallasii that live permanently in cool, moist habitats are characterized by fluctuating hyposaline conditions, whereas their faster moving relative, L. (M.) occidentalis alternates between dry and wet conditions so it can replace water previously lost by evaporation.  This species of isopods are aerobic and breathe using pleopods, which need to be moist in order to function properly.  To keep this moistness, the isopod must be immersed or dip the tail in water so that the uropods can serve as capillary siphons. Ligia pallasii feed on dead plants and animals, and the algal film from upper intertidal rocks.  They are primarily fed upon by birds and crabs. The life span of these isopods is about 1.5-2 years with the breeding period in the spring and early summer months.   Mothers brood their young, which do not have a planktonic larval stage.  Given this life cycle, the exchange of genes among populations up and down the coast may be limited.  Molecular genetic studies (Renate, 2013) show that this species is most diverse genetically in the southern (California) part of its range and that much of the northern range may have only a few closely related haplotypes.

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Dichotomous Keys:
Carlton et al., 2007
Kozloff, 1987

General References:
Lamb and Hanby, 2005
Morris, Abbott, and Haderlie, 1992

Scientific Articles:

Carefoot, T.H., B.E. Taylor, and K. Brett, 1998.  A day in the life of an isopod: time and energy allocations in the semiterrestrial Ligia pallasii.  Israel Journal of Zoology 44: pp 463-471

Eberl, R., 2010.  Sea-land transitions in isopods:  pattern of symbiont distribution in two species of intertidal isopods Ligia pallasii and Ligia occidentalis in the Eastern Pacific.  Symbiosis 1:1 pp. 107-116

Eberl, R., 2012.  Distribution, habitat and food preferences of sympatric high intertidal isopod species, Ligia occidentalis and Ligia pallasii (Ligiidae: Oniscidea). Journal of Natural History 46: pp 29-30

Eberl, Renate, 2013.  Phylogeography of the high intertidal isopod Ligia pallasii Brandt, 1833 (Isopoda: Oniscidea) from the Aleutian Islands to Monterey Bay.  Journal of Crustacean Biology 33:2 pp. 253-264

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

Another view of L. pallasii tail with uropods.  Photo by Dave Cowles, June 21, 2005 on Swirl Rocks.

A closeup of the uropods, showing their terminal position (characteristic of Oniscoidea) and the fact that the basal segment is as long as it is broad.

The underside of the pleon (abdomen) shows the arrangement of the pleopods and uropods in this species.

A closeup view of the head.  Note the multiple articles on the flagellum of the second antenna.  Photo by Dave Cowles, June 21, 2005 on Swirl Rocks

An individual crawling on a rock near Fucus.  Photo by Dave Cowles at Swirl Rocks, June 21, 2005

A scattering of yellowish-white spots on the dorsum is common.  Photo by Dave Cowles, July 2005

A wider individual from Swirl Rocks.  Note how it is the coxal plates that make the animal appear so wide.  Photo by Dave Cowles, July 2009 

Skittering across rock
In a typical intertidal view, this Ligia pallasii skitters across a Rialto Beach rock at low tide. Photo by Dave Cowles, July 2019

Authors and Editors of Page:
Created Original Page: Heidee Leno (2002)
Edited by:  Hans Helmstetler  11-2002