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.
History: This species
lives on the open coast and in bays. Large aggregations can sometimes
found in areas with lowered salinity. These isopods are
crustaceans because they have protogynous
development: When sexually mature they are first females, then change
males later in life (Brook et al., 1994). Other individuals develop
from juveniles into males.
Kozloff, 1987, 1996 (as Gnorimosphaeroma oregonense)
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
General Notes and Observations: Locations, abundances, unusual behaviors:
Authors and Editors
Salish Sea Invertebrates web site provided courtesy of Walla