Gnorimosphaeroma oregonensis Dana, 1854-55

Common name(s): Oregon pill bug, Pill bug

Synonyms: Exosphaeroma oregonensis, Exosphaeroma oregonense, Gnorimosphaeroma oregonense, Neosphaeroma oregonense, Sphaeroma oregonense Gnorimosphaeroma oregonensis
Phylum Arthropoda 
Subphylum Crustacea 
Class Malacostraca 
Subclass Eumalacostraca 
Superorder Peracarida 
Family Sphaeromatidae 
Gnorimosphaeroma oregonensis, about 6 mm long, found on rocks at Sares Head
(Photo by:  Dave Cowles, August 2017)

Description:  Flabelliferan isopods have uropods beside the pleotelson (photo). Their body is not more than 5 times as long as wide. Their first pereopod is usually not enlarged nor subchelate. Gnoriosphaeroma oregonensis has a small rostrum which clearly separate the bases of its first antennae (photo). It has only 3 visible pleonite plates anterior to the pleotelson (the first pleonite is hidden out of sight under the last pereonite). These 3 pleonites reach as far laterally as do the segments anterior and posterior to them. The pleopods do not have respiratory folds. The animal is very flexible and active, and can roll up into a ball, which is unusual for marine isopods but common for the terrestrial pillbug. None of the pereopods is subchelate. Gray dorsally, lighter gray ventrally. Length 1 cm or less.

How to Distinguish from Similar Species:  In Gnorimosphaeroma noblei the bases of the first antennae are not separated by a rostrum, so they touch in the front of the head. Gnorimosphaeroma insulare has only two of its three free pleonites reach the lateral edge of the animal-the other is shorter.

Geographical Range:  Alaska to southern California

Depth Range:  Intertidal to 24 m

Habitat:  Under rocks on rocky or shelly coastlines, or on floats or pilings.

Biology/Natural History:  This species lives on the open coast and in bays. Large aggregations can sometimes be found in areas with lowered salinity.  These isopods are unusual for crustaceans because they have protogynous development: When sexually mature they are first females, then change into males later in life (Brook et al., 1994). Other individuals develop directly from juveniles into males. 



Dichotomous Keys:
  Carlton, 2007
  Kozloff, 1987, 1996 (as Gnorimosphaeroma oregonense)

General References:
  Kozloff, 1993 (as Gnorimosphaeroma oregonense)
  Lamb and Hanby, 2005
  Morris et al., 1980

Scientific Articles:
Brook, Heather J., Timothy A. Rawlings, and Ronald W. Davies, 1994. Protogynous sex change in the intertidal isopod Gnorimosphaeroma oregonense (Crustacea: Isopoda). Biological Bulletin 187: pp 99-111

Rees, C. P., 1975. Competitive interactions and substratum preferences of two intertidal isopods. Marine Biology (Berl.) 30: pp21-25

Riegel, J. A., 1959a. Some aspects of osmoregulation in two species of sphaeromid isopod Crustacea. Biological Bulletin 116: pp272-284

Riegel, J.A., 1959b. A revision in the sphaeromid genus Gnoriomosphaeroma Menzies (Crustacea: Isopoda) on the basis of morphological, physical, and ecological studies on two of its "subspecies". Biological Bulletin 117: pp151-162

Standing, J. D., and D. D. Beatty, 1977. Humidity behaviour and reception in the sphaeromatid isopod Gnorimosphaeroma oregonensis (Dana). Canadian Journal of Zoology 56: pp2004-2014

Web sites:

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

This view of the posterior end shows the three free pleonites which reach to the margin of the animal, the pleotelson, and the uropods, which are beside the pleotelson. Photo by Dave Cowles, August 2017

This anterior view of the head shows the short rostrum which separates the flattened bases of the basal articles of the two first antennae. The animal must be held because otherwise it is highly active and impossible to photograph. Photo by Dave Cowles, August 2017

The animal can curl up into a loose ball, though it doesn't appear to me that it curls up as tightly nor as readily as the terrestrial pill bugs do.

Authors and Editors of Page:
Dave Cowles (2017):  Created original page
CSS coding for page developed by Jonathan Cowles

Salish Sea Invertebrates web site provided courtesy of Walla Walla University