Invasion History

First Non-native North American Tidal Record: 1997
First Non-native West Coast Tidal Record: 1997
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

The freshwater isopod Caecidotea racovitzai is native to eastern North America from southeastern Canada to Florida and Georgia (Williams 1970). It was first collected in Washington in 1955, and now occurs in the Snohomish River estuary, flowing into Puget Sound (first record 1997), and the Columbia River estuary (first record 1999, Sytsma et al. 2004). It was collected in the Sacramento-San Joaquin Delta in 1999 (Toft et al. 2002). This isopod has also been collected in the Provo River, Utah (in 1973, U.S. National Museum of Natural History 2006).

North American Invasion History:

Invasion History on the West Coast:

Caecidotea racovitzai was first collected on the West Coast in 1955, in Echo Lake, Seattle, Washington (WA) (Williams 1970), possibly introduced with stocked fish or aquatic plants. In 1997, it was found in the Snohomish River estuary, near Everett, WA (Toft et al. 2002). In 1999, it was collected at several sites in the tidal fresh Columbia River estuary, from Portland, Oregon (OR) to Longview, WA (Sytsma et al. 2004). In later sampling (2006), it was found in the middle and upper Columbia, from Hood River upstream to the Hanford Reach and the lower Snake River (Draheim et al. 2007; USGS Nonindigenous Aquatic Species Program 2015). In 1998-1999, C. racovitzai was collected at many sites in the Sacramento-San Joaquin Delta, California. It has been collected from Browns Island, upstream to Mandeville Island (Toft et al. 2002; Graening and Rogers 2013). In 2007, it was found in dredge tailing ponds along the Merced River in Merced County, far above tidal influence (Graening and Rogers 2013). Likely vectors for transport into West Coast estuaries include ballast water, aquatic plants, and fish-stocking (Toft et al. 2002; Graening and Rogers 2013).


Caecidotea racovitzai is an isopod of the widespread freshwater genus Asellidae. The body is dorsoventrally flattened, with a length about 4X its width. The 'head' is actually a cephalothorax, representing a fusion of the head and the first thoracic segment. In asellids, Antenna 1 is short, with a segmented peduncle. Antenna 2 has a 5-segmented flagellum and is 2-3X longer than Antenna 1. Both antennae have many-segmented flagella. The eyes are dorsal. There are 6 separated thoracic segments. The first pair of thoracic appendages are a prominent pair of gnathopods. There are 7 pairs of walking legs (pereiopods). The gnathopods and pereiopods are each about equal or greater than the body width, and extend out laterally; the perieopods increase in length posteriorly. The coxae are fused with the body, but the other 6 segments of the perriopods (basis, ischium, merus, carpus, propodus, and dactylus) are distinct. The abdomen is divided into two ovate lateral lobe and a central region ending in a blunt angular tip. There are 5 pairs of pleopods hidden beneath the abdomen. The pleopods are sexually dimorphic and important for species identification. The 6th pair (uropods) are unmodified and biramous, projecting well behind the body. Description based on: Pennak 1978 and Brusca et al. 2007.

Caecidotea racovitzai has distinctive morphological features on the gnathopods and pleopods. The gnathopod bears a large triangular process in the middle of the palm, and smaller spines, where it meets the dactyl, near its tip. The endopod is narrow and blade-like, about 3X as wide as long, ending in 3 beak-like processes. The uropod is about the same length as the telson and is biramous. Adults are 4 to 15 mm in size. Description based on: Williams 1970 and Toft et al. 2002.

The species is divided into two subspecies, Caecidotea racovitzai, occurring from Virginia to Quebec and west to Indiana and Ontario, and C. r. australius, found from North Carolina to Florida (Williams 1970).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Arthropoda
Subphylum:   Crustacea
Class:   Malacostraca
Subclass:   Eumalacostraca
Superorder:   Peracarida
Order:   Isopoda
Suborder:   Asellota
Family:   Asellidae
Genus:   Caecidotea
Species:   racovitzai


Asellus racovitzai (Williams, 1970)

Potentially Misidentified Species

Asellus hilgendorfii
Asellus hilgendorfii is native to northeastern Asia, and has been introduced to the Sacramento-San Joaquin Delta (Toft et al. 2002).

Caecidotea communis
Caecidotea communis is native to the northeastern United States, but has been introduced to ponds in Berkeley, California, and near Tomales Bay (Graening and Rogers 2013).

Caecidotea occidentalis
Caecidotea occidentalis is native to the western US from southern British Columbia to Santa Cruz, California (Williams 1970; Graening and Rogers 2013).

Caecidotea tomalensis
Caecidotea tomalensis is native to northern California in shallow, slow-water habitats, including springs and lagoons on the edge of Tomales and San Francisco Bay (Graening and Rogers 2013).



Caecidotea racovitzai is a widespread surface-water dwelling freshwater isopod. Sexes are separate, the young are brooded, and development is direct. Females begin reproducing at 4 mm (Toft et al. 2002). The mean size of reproducing females in Lake Erie in March and June was 5.8-6.1 mm. Females measuring around 6 mm had an average of 59 eggs while 7 mm females had a mean of 89 eggs (Kerr 1978). Young individuals were dominant in June and July, suggesting an annual life cycle in Lake Erie (Kerr 1978).

Caecidotea racovitzai occurs over a wide latitudinal range in North America, from the Great Lakes to Georgia (Williams 1970). Caecidotea sp. from Hudson River brackish marshes (probably C. racovitzai) tolerated salinities of 5 and 10 PSU, but had high mortality at 15-20 PSU (Connolly et al. 2014). The occurrence of C. racovitzai in central and western Sacramento-San Joaquin Delta (Toft et al. 2002) is consistent with this level of salinity tolerance.

Caecidotea racovitzai inhabits large and small lakes, ponds, rivers, and swamps, and is considered an indicator of polluted waters (Williams 1970; Kerr 1978). In the Delta, C. racovitzai was associated with floating plants, the native Marsh Pennywort (Hydrocotyle umbellata), and less frequently with the introduced Water Hyacinth (Eichonria crassipes) (Toft et al. 2003). Caecidotea racovitzai, like other Asellus species, is a deposit-feeder on decaying vegetation, algae, fungi, and bacteria (Kerr 1978). In laboratory culture, Caecidotea sp. was reared on decaying leaves of the Common Reed (Phragmites australis) (Connolly et al. 2014). This isopod was found in the guts of three species of freshwater fishes: one native (Prickly Sculpin- Cottus asper) and two introduced (Bluegill- Lepomis macrochirus; Largemouth Bass- Micropterus salmoides) in the Sacramento-San Joaquin Delta (Toft et al. 2002).


Vascular plants, fallen leaves, algae, detritus



Trophic Status:

Deposit Feeder



General HabitatFresh (nontidal) MarshNone
General HabitatGrass BedNone
General HabitatCoarse Woody DebrisNone
General HabitatSwampNone
General HabitatNontidal FreshwaterNone
General HabitatTidal Fresh MarshNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Tidal RangeSubtidalNone

Tolerances and Life History Parameters

Minimum Salinity (‰)0This is a freshwater species.
Maximum Salinity (‰)10Experimental, Caecidotea sp., probably C. racovitzai, Hudson River (Connolly et al. 2014)
Minimum Length (mm)4Adults (Toft et al. 2002)
Maximum Length (mm)15Adults (Toft et al. 2002)
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNoneNontidal fresh-Oligohaline

General Impacts

Caecidotea racovitzai is locally common in freshwater portions of the Sacramento-San Joaquin Delta, and is a frequent food item for fish (Toft et al. 2002). However, impacts on fish populations have not been reported.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
M130 Chesapeake Bay 0 Native Estab
N040 Blue Hill Bay 0 Native Estab
N150 Merrimack River 0 Native Estab
GL-I Lakes Huron, Superior and Michigan 0 Native Estab
M040 Long Island Sound 0 Native Estab
S180 St. Johns River 0 Native Estab
P090 San Francisco Bay 1998 Def Estab
P260 Columbia River 1999 Def Estab
P290 Puget Sound 1997 Def Estab
GRSALTLK Great Salt Lake 1973 Def Estab
GL-II Lake Erie 0 Native 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

Connolly, Craig T.; Sobczak, William V.; Findlay, Stuart E. G. (2014) Salinity effects on Phragmites decomposition dynamics among the Hudson River’s freshwater tidal wetlands, Wetlands 34: 575-582

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

Kensley, Brian; Schotte , Marilyn; Schilling, Steve 1996 World list of marine, freshwater and terrestrial isopod crustaceans. <missing URL>

Kerr, Jeff R. (1978) Some aspects of life history and ecology of the isopod Asellus r. racovitzai in western and central Lake Erie, Ohio Journal of Science 78(6): 298-300

Pennak, Robert W. (1978) <missing title>, John Wiley & Sons, New York. Pp. <missing location>

Sytsma, Mark D.; Cordell, Jeffrey R.; Chapman, John W.; Draheim, Robyn, C. (2004) <missing title>, Center for Lakes and Reservoirs, Portland State University, Portland OR. Pp. <missing location>

Sytsma, Mark; Cordell, Jeff; Chapman, John; Miller, Rich; Draheim, Robyn; (2004) Lower Columbia River aquatic nonindigenous species survey 2001-2004: Final technical report, Portland State University, Portland. Pp. <missing location>

Toft, Jason D.; Cordell, Jeffrey R.; Fields, Wayne C. (2002) New records of crustaceans (Amphipoda, Isopoda) in the Sacramento/San Joaquin Delta, California, and application of criteria for introduced species, Journal of Crustacean Biology 22(1): 190-200

Toft, Jason D.; Simenstad, Charles A.; Cordell, Jeffrey R.; Grimaldo, Lenny F. (2003) The effects of introduced water hyacinth on habitat structure, invertebrate assemblages, and fish diets., Estuaries 26(3): 746-758

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

USGS Nonindigenous Aquatic Species Program 2003-2024 Nonindigenous Aquatic Species Database.

Williams, W. D. (1970) A revision of the North American epigean species of Asellus (Crustacea: Isopoda), Smithsonian Contributions to Zoology 49: Washington, D. C.

Wilson, Sarah; Partridge, Valerie (2007) <missing title>, Washington State Department of Ecology, Olympia. Pp. 244