Ligia (Megaligia) occidentalis Dana, 1853

Common name(s): Rock louse

Phylum Arthropoda
 Subphylum Crustacea
  Class Malacostraca
   Subclass Eumalacostraca
    Superorder Peracarida
    Order Isopoda
      Suborder Oniscoidea
       Family Ligiidae
Ligia occidentalis from Dana Point, CA
(Photo by: Dave Cowles, April 1997)
Description:  This isopod is a member of suborder Oniscoidea (uropodsterminal not ventral; body usually large and robust, mostly terrestrial, pleon generally with 5 free pleonites plus the pleotelson, antenna 1 with only 2-3 articles).  Flagellum of antenna 2 is much larger, with more than 10 articles when mature.  Up to 2.5 cm long.  Eyes separated by one full eye diameter. Uropods are nearly half as long as the body, with basal segments several times as long as broad (photo).

How to Distinguish from Similar Species: Does not get as large as Ligia pallasii, plus the very long uropods (trail out of the picture above) are nearly half as long as the body, while in L. pallassi they are much shorter.

Geographical Range: Sonoma County, CA to Central America.

Depth Range: Intertidal

Habitat:  Rock crevices or under stranded Macrocystis algae in the high intertidal during high tides; ranges through intertidal at evening, especially at low tide.

Biology/Natural History: Hides most of the day in crevices or under stones just above the high tide line.  At night and at low tides in cool weather, forages throughout the intertidal zone.  Is a scavenger, plus feeds on microscopic algae.  This species is nearly terrestrial--it must keep its gills moist (by dipping the back of its abdomen into the water) but will drown if forced to stay underwater.  This species is very tolerant to water loss.  They are paler at night than during the day due to clustering and dispersal of chromatophores.  Animals on dark backgrounds remain darker than do those on light backgrounds.  Ovigerous females have been observed at Monterey Bay in March, May, and June.

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Dichotomous Keys:
Kozloff 1987, 1996
Smith and Carlton, 1975

General References:
Kozloff, 1993
Morris et al., 1980
Niesen, 1994

Scientific Articles:
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

Hurtado, L.A., M. Mateos, and C.A. Santamaria, 2010.  Phylogeography of supralittoral rocky intertidal Ligia isopods in the Pacific region from Central California to Central Mexico.  PLOS One 5:7 e11633

Markow, T.A. and E. Pfeiler, 2012.  Mitochondrial DNA evidence for deep genetic divergences in allopatric populations of the rocky intertidal isopod Ligia occidentalis from the eastern Pacific.  Molecular Phylogenetics and Evolution 56: pp 468-473

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

Here is another, small individual from Little Corona del Mar, CA.

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The basal segment of the uropods of this species is several times longer than its diameter, as can be seen in the dorsal (left) and ventral(right) views above.

Authors and Editors of Page:
Dave Cowles (2005):  Created original page