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

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.

References:

Dichotomous Keys:

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., 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

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