Invasion History

First Non-native North American Tidal Record: 1952
First Non-native West Coast Tidal Record: 1952
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Gnorimosphaeroma rayi was collected in 1952, as G. oregonensis (Menzies 1954), and initially described as a new species in 1969, from Tomales Bay, California, and later found to be distributed from Petrov Island, off Vladivostok, Russia, along the coast of Japan to the south coast of South Korea (Hoestlandt 1973a,b; Hoestlandt 1975a,b; Nunomura and Nishimura 1976; Kwon and Kim 1987). One early specimen from Japan, identified as G. oregonensis (Richardson 1909) was later re-identified as G. rayi (USNM 43250, U.S. National Museum of Natural History 2015). Gnorimosphaeroma rayi is now recognized as native to the Northwest Pacific, and introduced to California and Hawaii (first record 1929) (Hoestlandt 1973a,b; Hoestlandt 1975a,b; Carlton 1979).

North American Invasion History:

Invasion History on the West Coast:

Gnorimosphaeroma rayi was first described from the rocky intertidal of Tomales Bay, California in 1969 (Hoestlandt 1969). It was initially collected in 1952, but was confused with G. oregonensis (Menzies 1954; Carlton 1979). Hoestlandt noted that the limited distribution of G. rayi, found only in Tomales Bay, and its presence in Japan, indicated a probable introduction to Tomales Bay with Pacific Oysters (Crassostrea gigas) (Hoestlandt 1969; Hoestlandt 1973a,b; Hoestlandt 1975a,b). This isopod has not been reported from other West Coast locations (Brusca et al. 2007). In 2009, G, rayi was collected on Bolinas Bay south of Tomales Bay (Wetzer et al. 2021)/

Invasion History in Hawaii:

Gnorimosphaeroma rayi was collected by Hoestlandt in 1972-1974 at several brackish and marine locations on Hawaii (Waiahukini Beach; Lua o Palahemo), Maui (Waianapanapa Caves), and Kauai (Waiahuakua Stream) (Hoestlandt 1973a,b; Hoestlandt 1975a,b; Carlton and Eldredge 2009; USNM 213124, United States National Museum of Natural History 2015). The locations are not near ports, but this isopod is clearly introduced on these islands. Hull fouling or Pacific Oyster (Crassostrea gigas) plantings are possible introduction vectors, and are more likely than ballast water.


Gnorimosphaeroma rayi has an oval body shape outline, and a roughly trapezoidal head with prominent dark eyes in the posterior corners and a very obtuse rostrum. Peraeonal segments 1-3 are crescent-shaped, while 4-7 are trapezoidal. The posterior corners of the peraeonal segments are rounded. The pleonites are fused in the center, but the lateral edges of all three reach the border of the pleon. [NB: In G. insulare, the lateral edge of Pleonite 3 is tucked under the edge of Pleonite 3]. The pleotelson is roughly triangular, with lateral indentations into which the endopods of the uropods are tucked.

The basal segments of Antenna 1 and 2 in G. rayi are separated by the rosturm. [NB:In G. nobeli, the basal segments of the antennae touch.] Pereiopod 1 of G. rayi has a tuft of 7-9 setae on the basis (first free joint), and 2-3 setae on the sternal crest of the ischium (2nd free joint). [NB: G. oregonensis has only 1 seta on the basis and several dense row of long setae on the ischium.] Adults of G. rayi are 8-12 mm long. Gnorimosphaeroma rayi shows great variation in its color pattern, with four basic types of color, including different arrangements of white and dark brown, with yellow lines on the borders of body segments. Description based on: Hoestlandt 1969, Hoestlandt 1973b, Hoestlandt 1975a, Hoestlandt 1975b, Nunomura and Nishimura 1976, Harrison and Ellis 1991, Brusca et al. 2007; Wetzer et al. 2022).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Arthropoda
Subphylum:   Crustacea
Class:   Malacostraca
Subclass:   Eumalacostraca
Superorder:   Peracarida
Order:   Isopoda
Suborder:   Flabellifera
Family:   Sphaeromatidae
Genus:   Gnorimosphaeroma
Species:   rayi


Potentially Misidentified Species

Gnorimosphaeroma insulare

Gnorimosphaeroma insulare (Van Name 1940) was described form a freshwater stream on San Nicolas Island, in the Channel Islands, California.  Is establishment is unknown. becuase of the scarcity of water on the islands ( Wetzer et al. 2021).

Gnorimosphaeroma nobeli
Gnorimosphaeroma nobeli is native to central California.

Gnorimosphaeroma oregonensis
Gnorimosphaeroma oregonensis Dana 1853 (=Gnorimosphaeroma oregeonense, in Hoestlandt 1973 and Brusca et al. 2007) is known from marine and brackish waters from central California to Alaska (Schultz 1969; Brusca et al. 2007; Graening and Rogers 2013).

Gnorimosphaeroma sp.

A distinct species of Gnorimosphaeroma was collected from San Francisco Bay in 2002 and 2005, and identified as a distinct species by molecular means.  Gnorimosphaeroma  sp. was distinct from G. rayi, but closely related. to it.  The native region of the new species is unknown, but is assumed to be the northwest Pacific, based on gentic relationships.



Gnorimosphaeroma rayi is a marine isopod with separate sexes and internal fertilization. The young are brooded (Schultz 1969; Hoestlandt 1975a; Hoestlandt 1975b, Brusca et al. 2007). Populations of G. rayi from Tomales Bay had a one-year lifespan, breeding in the spring, with only a few adults surviving through the winter to reproduce, contrasting with a 2-3 year lifespan and longer breeding season in G. oregonensis (Hoestlandt 1969).

It inhabits marine and brackish rocky intertidal and shallow subtidal areas. In Tomales Bay, California, it was found among stones and pebbles in the polyhaline rocky intertidal zone (Hoestlandt 1969; Hoestlandt 1975a; Hoestlandt 1975b). In Osaka Bay, Japan, it was also found in the rocky intertidal, including colonies of the barnacle Amphibalanus amphitrite (Nunomura and Nishimura 1976). In the Yodo River estuary, Japan, it occurred at salinities of 1-30 PSU and a temperature range of 8-27°C. On the island of Hawaii, it was found in a brackish pond, at a salinity of 22 PSU (Carlton and Eldredge 2009). We have not found information on the feeding of this isopod. However, most sphaeratomids are herbivorous (Brusca et al. 2007).


Detritus, benthic microalgae, invertebrate eggs

Trophic Status:




General HabitatOyster ReefNone
General HabitatRockyNone
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Tidal RangeMid IntertidalNone
Vertical HabitatEpibenthicNone

Life History

Tolerances and Life History Parameters

Minimum Temperature (ºC)8Field, Yodo River estuary, Osaka, Japan (Yamanishi et al. 1998)
Maximum Temperature (ºC)27Field, Yodo River estuary, Osaka, Japan (Yamanishi et al. 1998)
Minimum Salinity (‰)1Field, Yodo River estuary, Osaka, Japan (Yamanishi et al. 1998)
Maximum Salinity (‰)35Typical marine salinity
Minimum Length (mm)8Hoestlandt 1969; Hoestlandt 1975a; Hoestlandt 1975b;
Maximum Length (mm)12Hoestlandt 1969; Hoestlandt 1975a; Hoestlandt 1975b;
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNonePolyhaline-Euhaline

General Impacts

No impacts have been reported for introduced populations of Gnorimosphaeroma rayi.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-V Northern California to Mid Channel Islands 1952 Def Estab
NWP-4a None 0 Native Estab
SP-XXI None 1972 Def Estab
NWP-3b None 0 Native Estab
NWP-3a None 0 Native Estab
NWP-4b None 0 Native Estab
P110 Tomales Bay 1952 Def Estab
P095 _CDA_P095 (Tomales-Drakes Bay) 2009 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Brusca, Richard C.; Coeljo, Vania R. Taiti, Stefano (2007) The Light and Smith Manual: Intertidal invertebrates from Central California to Oregon (4th edition), University of Calfiornia Press, Berkeley CA. Pp. 503-542

Carlton, James T. (1979) History, biogeography, and ecology of the introduced marine and estuarine invertebrates of the Pacific Coast of North America., Ph.D. dissertation, University of California, Davis. Pp. 1-904

Carlton, James T.; Eldredge, Lucius (2009) Marine bioinvasions of Hawaii: The introduced and cryptogenic marine and estuarine animals and plants of the Hawaiian archipelago., Bishop Museum Bulletin in Cultural and Environmental Studies 4: 1-202

Graening, G. O.; Rogers, D. Christopher (2013) Checklist of inland aquatic Isopoda (Crustacea: Malacostraca) of California, California Fish and Game 99(4): 176-192

Harrison, K.; Ellis, J. P. (1991) The genera of the Sphaeromatidae (Crustacea: Isopoda): a key and distribution list, Invertebrate Taxonomy 5: 915-952

Hoestlandt, Henri (1973a) Présence de Gnorimosphaeroma rayi Hoestlandt (Isopode flabellifère) sur les côtes du Japon, de Sibérie orientale et d’Hawaii, ainsi qu’ indication sommaires de son polychromatism génétique., Comptes Rendus de l’ Academie des Sciences du Paris, Serie D 276: 2820-2823

Hoestlandt, Henri (1973b) [Systematic and genetic studies of three Pacifc North American species of the genus Gnorimosphaeroma Menzies (Isopoda, Flabellifera) I. General considertations and systematics], Archives de Zoologie Experimental et Generale 114: 349-395

Hoestlandt, Henri (1975a) Occurrences of the isopoda Flabellifera Gnorimosphaeroma rayi Hoestlandt on the coasts of Japan, eastern Siberia and Hawaii, with a brief note on its genetic polychromatism, Publications of the Seto Marine Biological Laboratory 22(1/4): 31-46

Hoestlandt, Herni (1975b) [Systematic and genetic studies of three Pacifc North American species of the genus Gnorimosphaeroma Menzies (Isopoda, Flabellifera) II. Genetics of polychromatism and populations], Archives de Zoologie Experimental et Generale 116: 521-548

Hoestlandt, Hernti (1969) ]On a new sphaeratomid isopod on the American Pacific coast, Gnorimosphaeroma rayi], Comptes Rendus de l’ Academie des Sciences du Paris, Serie D 268: 325-327

Kwon, Do Heon; Kim, Hoon, Soon (1987) A new species of the genus Gnorimosphaeroma (Crusttacea, Sphaeratomidae) from the Nakong River, with a kwy to Korean species of the genus, Korean Journal of Systematic Zoology Molluscan Research 3(1): 51-56

Menzies, Robert James (1954) A review of the systematics and ecology of the genus "ExospIaeroma," with the description of a new genus, a new species, and a new subspecies (Crustacea, Isopoda, Sphaeromidae), American Museum Novitates 1683: 1-24

Nunomura, Noburu; Nishimura, Saburo (1976) Marine Isopoda from the rocky shore of Osaka Bay, middle Japan (2), Bulletin of the Osaka Museum of Natual History 30: 19-26

Schultz, G.A. (1969) The Marine Isopod Crustaceans, Wm. C. Brown Company, Dubuque, Iowa. Pp. <missing location>

U.S. National Museum of Natural History 2002-2021 Invertebrate Zoology Collections Database.

Wetzer, Regina; Wall, Adam; Bruce, Niel L. (2021) Redescription of Gnorimosphaeroma oregonense (Dana, 1853) (Crustacea, Isopoda, Sphaeromatidae), designation of neotype, and 16S-rDNA molecular phylogeny of the north-eastern Pacific species, ZooKeys 1037: 23-56
doi: 10.3897/zookeys.1037.63017

Yamanishi, Ryohei; Yokoyama, Hisashi; Ariyama, Hiroyukii (1991) Distributional and seasonal changes of intertidal attaching organisms in relation to water quality along the brackish reaches of the Yodo River., Occasional Papers from the Osaka Museum of Natural History 2(7): 83-96