Invasion History

First Non-native North American Tidal Record: 1978
First Non-native West Coast Tidal Record: 1978
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Asellus hilgendorfii, described from Japan, is native to freshwaters in eastern Siberia, Japan, and China (Henry and Magniez 1995). It has been reported from Turkey (Camur and Kigiz, 2000), but this record needs confirmation. It was identified from samples collected in the Sacramento-San Joaquin Delta, California during field sampling in 1998-1999, and was also found in survey samples taken as early as 1978 (Toft et al. 2002).

North American Invasion History:

Invasion History on the West Coast:

Asellus hilgendorfii was found in preserved invertebrate survey samples from the Sacramento-San Joaquin Delta as early as 1978. In 1998, it was found to be limited to a small portion of the central Delta, at two locations, Upper Mandeville Tip and Venice Cut (Toft et al. 2002). Over a longer span of time, it has been found over a wider range, including Sherman Lake (western Delta) and Old River (southeastern Delta) (Graening and Rogers 2013). Toft et al. (2002) found that A. hilgendorfii, unlike Caecidotea racovitzai and the amphipod Crangonyx floridanus, was not associated with floating plants, but was found mostly near emergent or riparian vegetation. They also suggested that it might be less tolerant of brackish water than C. racovitzai. In North America, Asellus hilgendorfii has not been found outside the San Francisco Bay estuary (Toft et al. 2002; Graening and Rogers 2013). Possible vectors for introduction include ballast water and the trade in ornamental aquatic plants and fishes (Toft et al. 2002).


Asellus hilgendorfii 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 pereiopods increase in length posteriorly. The coxae are fused with the body, but the other 6 segments of the pereiopods (basis, ischium, merus, carpus, propodus, and dactylus) are distinct. 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 (Pennak 1978; Brusca et al. 2007).

Distinctive morphological features of Asellus hilgendorfii are found in the gnathopods and pleopods. Two tooth-like spines occur in the palm of the propodus of the gnathopod. Asellus hilgendorfii also has a basal spur on the endopod of Pleopod 2, which is absent in Caecidotea. The endopod lacks setae and is somewhat oval. Adults of A. hilgendorfii are 7-15 mm long (Magniez and Toft 2000; Toft et al. 2003). The abdomen is somewhat divided into two ovate lateral lobes, and a central region ending in a blunt angular tip.

The genus Asellus is now considered to be a largely Old-World genus, with one native North American species, Asellus alaskensis, restricted to Alaska (Bowman and Holmquist 1975; Kensley et al. 1996). Asellus hilgendorfii is considered a species complex, widely distributed in Asia, with at least 6 recognizable geographical forms in Japan, China, and Pacific Russia (Henry and Magniez 1995).


Taxonomic Tree

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


Potentially Misidentified Species

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 racovitzai
Caecidotea racovitzai is native to the northeastern United States, and is introduced to the Sacramento-San Joaquin Delta and Puget Sound tributaries (Toft et al. 2003; 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).



Asellus hilgendorfii is a surface-water dwelling freshwater isopod. Sexes are separate, the young are brooded, and development is direct. In cultured animals, the average number of young per brood ranged from 15 to 42 per female, varying both with temperature and age of the female. The young are born at an average length of 1.03 mm and were sexually distinct at 2.5-3.5 mm. Females produced young at ~4 mm length, reached in ~35 days at 25°C and ~70 days at 15°C. The average incubation period for embryos was 9.4 days at 25°C and 18.3 days at 15°C. There was low survival and no growth or reproduction at 5°C (Miyashita and Yasuno 1984).

Asellus hilgendorfii occurs over a wide latitudinal range in Asia, from Siberia to southern China, and all the islands of Japan, including Okinawa (Birstein 1964; Magniez and Toft 2000). We do not have salinity tolerance data for A. hilgendorfii, but the conspecific European species A. aquaticus, which occurs in the mesohaline region of the Baltic Sea, tolerates salinities up to 15 PSU and can reproduce at up to 5 PSU (Gresens 1928, cited by Remane and Schleiper 1971). Asellus hilgendorfii is known from fresh and low salinity regions of the Sacramento-San Joaquin Delta, but avoids more brackish waters (Toft et al. 2002; Graening and Rogers 2013).

Normal habitats of A. hilgendorfii include lakes, ponds, rivers, and ditches, and it is considered an indicator of polluted waters (Miyashita and Yasuno 1984). Toft et al. (2003) found that A. hilgendorfii was not associated with floating plants, but was found mostly near emergent or riparian vegetation. Asellus hilgendorfii, like other Asellus species, is a deposit-feeder on decaying vegetation, fungi, and bacteria. In laboratory culture, it was fed on decaying chestnut leaves (Miyashita and Yasuno 1984). This isopod was found in the guts of four species of freshwater fishes: two native (Sacramento Pike minnow- Ptychocheilus grandis; Tule Perch- Hysterocarpus traskii) 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 HabitatTidal Fresh MarshNone
General HabitatGrass BedNone
General HabitatNontidal FreshwaterNone
General HabitatFresh (nontidal) MarshNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)4Field, in a lake with winter ice-cover, Japan (Miyashita and Yasuno 1984)
Maximum Temperature (ºC)30Field, in the River Tama, Japan (Miyashita and Yasuno 1984)
Minimum Salinity (‰)0Restricted to freshwater habitats in Sacramento-San Joaquin Delta (Toft et al. 2002)
Minimum Length (mm)7Adults (Toft et al. 2002)
Maximum Length (mm)15Adults (Toft et al. 2002)
Broad Temperature RangeNoneCold temperate- Warm temperate
Broad Salinity RangeNoneNontidal Limnetic-Tidal Limnetic

General Impacts

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

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
P090 San Francisco Bay 1978 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
35691 Toft et al. 2002 1998 1998-06-01 Upper Mandeville Tip Def 38.0613 -121.5385
35692 Toft et al. 2002, 1998 1999-06-01 Venice Cut Def 38.0554 -121.5243
35693 Toft et al. 2002 1998 1998-06-01 Mildred Island Def 37.9999 -121.5191
35695 Graening and Rogers 2013 2006 1006-01-01 Franks Tract Def 38.0488 -121.6377
35696 Graening and Rogers 2013 2006 2006-01-01 Old River Def 37.9999 -121.5707


Birstein, Ya. A. (1964) <missing title>, Zoological Institute of the Academy of Science of the USSR, Leningrad (St. Petersburg), Russia. Pp. 148

Bowman, Thomas E.; Holmquist, Charlotte (1975) Asellus alaskensis, n. sp., the first Alaskan Asellus, with remarks on its Asian affinities (Crustacea: Isopoda: Asellidae), Proceedings of the Biological Society of Washington 88: 59-72

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

Camur, Belgin, Kigiz, Timur (2000) Freshwater isopod species of Turkish Thrace and their distribution, Turkish Journal of Zoology 24: 17-22

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

Henry, Jean-Paul; Magniez, Guy (1995) [New data on the epigeal Asellidae of the Far East (Crustacea, Isopoda, Asellota)], Contributions to Zoology 65(2): 101-122

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

Magniez, Guy J.; Toft, Jason (2000) [Presence of an epigean asellid, originally form the Far East, in California], Bulletin Mensuel de la Société Linnéenne de Lyon 69: 127-132

Miyashita, Mamoru; Yasuno, Masayuki (1985) Growth and reproduction of Asellus hilgendorfii (Crustacea,Iisopoda) under laboratory conditions, Japanese Journal of Limnology 45(3): 213-219

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

Remane, Adolf; Schleiper, Carl C. (1971) Biology of Brackish Water, In: (Eds.) . , New York. Pp. 1-210

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