Invasion History

First Non-native North American Tidal Record: 1904
First Non-native West Coast Tidal Record: 1904
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Iais californica was first described by Harriet Richardson in 1904, from specimens collected in Sausalito, San Francisco Bay, California, taken from the pleopods of the isopod Sphaeroma quoianum (as S. pentadon). 'Iais pubescens' from New Zealand, identified by Chilton in 1892, was eventually recognized as synonymous with I. californica, while 'S. pentadon' was synonymized with Australia-New Zealand native S. quoianum (Menzies and Barnard 1051; Hurley 1956). Bot species are now recognized as introductions to the Northeast Pacific (Rotramel 1972). Iais californica is found over most of the introduced range of S. quoianum, from Coos Bay, Oregon to San Diego Bay, California (Rotramel 1972; Iverson 1974; Carlton 1979; Cohen and Carlton 1995; Davidson 2006).

North American Invasion History:

Invasion History on the West Coast:

Iais californica was described from Sausalito, in San Francisco Bay in 1904 (Menzies and Barnard 1951; Carlton 1979), associated with the isopod Sphaeroma quoianum. It is known from the South and Central Bays, and San Pablo Bay (Richardson 1904; Menzies and Barnard 1951: Robinson et al. 2011). It was also found with S. quoianum in fouling on an obsolete cargo vessel moored in Suisun Bay (Llansó et al. 2011), and appears to share some of its host's tolerance for brackish water. The spread of I. californica has depended on the spread of its host, but reports of the commensal have matched, or lagged behind those of the host. This could reflect either slower dispersal of I. californica, or its lesser obviousness.

Iais californica is found in most of the West Coast estuaries where S. quoianum is found, although Iais californica has not been reported from Los Angeles Harbor, where S. quoianum was first reported in 1927 (Johnson and Snook 1927, cited by Carlton 1979), or from northern Yaquina Bay, Oregon (Davidson 2006), or the southern edge of S. quoianum's introduced range at Bahia San Quintin, Baja California (Menzies 1962a). Of course, I. californica could just be overlooked at these sites. Both isopods were probably transported in the fouling communities on hulls of cargo ships coming from New Zealand (Carlton 1979; Cohen and Carlton 1995).


Iais californica is a small isopod, living commensally on the pleopods of the isopod Sphaeroma quoianum, a wood-borer in marine and estuarine habitats. Iais californica has an elongated egg-shaped body, which is broadest in pereaonal segment 7 and tapered towards the head. Coxal plates are visible in dorsal view. The pleotelson is shield-shaped, with long biramous uropods protruding beyond it. The head is roughly oval, with medium-sized eyes. Antenna 1 is 1/6 the size of the body length and composed of 6 segments, while Antenna 2 is 6/10 of the body length, with a flagellum of 24 segments. In males, Pleopods 1 and 2 are modified for copulation. Pleopods 1 are fused to form a narrow appendage (the sympod), not widening near the tip and ending in a pair of rounded processes, each bearing 5 setae (Menzies and Barnard 1951, California). Pleopods 2 are not fused, but the inner ramus is modified into a needle-like stylus. The pleotelson is wider than it is long and the uropods are about half as long as the pleotelson. One male was 2.7 mm long (Menzies and Barnard 1951), and Schultz (1969) gives an average length of 1.9 mm. Description based on: Hurley 1956, Menzies and Barnard 1951, Schultz 1969, and Wilson and Wagele 1994.

Iais pubescens (s. l.) was described by Dana from Tierra del Fuego. Chilton reported this species from New Zealand in 1893, as I. pubescens var. longistylis, associated with S. quoianum, but it was later found to be identical with I. californica (Menzies and Barnard 1951; Hurley 1956).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Arthropoda
Subphylum:   Crustacea
Class:   Malacostraca
Subclass:   Eumalacostraca
Superorder:   Peracarida
Order:   Isopoda
Suborder:   Asellota
Family:   Janiridae
Genus:   Iais
Species:   californica


Iais pubescens var. longistylis (Chilton, 1912)
Ianiropsis californica (Richardson, 1904)

Potentially Misidentified Species



Iais californica lives as a commensal on the pleopods of the boring/burrowing isopod Sphaeroma quoianum spp. It has separate sexes and direct development (Schultz 1969). Iais californica presumably feeds on the detritus and phytoplankton gathered by Sphaeroma quoianum’s suspension feeding (Rotramel 1975). The salinity and temperature tolerances of I. californica have not been studied experimentally, but it appears to match most or all of the environmental range of S. quoianum. It was found on the obsolete cargo ship 'Florikan', moored in Suisun Bay, with S. quoianum, in a fouling community typical of brackish water (Llansó et al. 2011).


detritus and phytoplankton


Commensal on Sphaeroma quoyanum

Trophic Status:




General HabitatUnstructured BottomNone
General HabitatCoarse Woody DebrisNone
General HabitatMarinas & DocksNone
General HabitatSalt-brackish marshNone
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Tidal RangeLow IntertidalNone
Vertical HabitatEndobenthicNone
Vertical HabitatEpibenthicNone

Tolerances and Life History Parameters

Maximum Length (mm)2.7None
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNoneMesohaline-Euhaline

General Impacts

No impacts have been reported for Iais californica, although its host organism, Sphaeroma quoianum is regarded as an important ecosystem engineer in tidal marshes, and as a pest, boring and burrowing into wooden docks, earthen dikes, and plastic floats (Carlton 1979; Cohen and Carlton 1995).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-V Northern California to Mid Channel Islands 1904 Def Estab
NEP-VI Pt. Conception to Southern Baja California 1957 Def Estab
NEP-IV Puget Sound to Northern California 1931 Def Estab
AUS-VIII None 0 Native Estab
SEP-I None 1975 Def Unk
NZ-IV None 0 Native Estab
P170 Coos Bay 1995 Def Estab
P090 San Francisco Bay 1904 Def Estab
P130 Humboldt Bay 1931 Def Estab
P112 _CDA_P112 (Bodega Bay) 1972 Def Estab
P020 San Diego Bay 1973 Def Estab
P040 Newport Bay 1957 Def Estab
P080 Monterey Bay 1998 Def Estab
P095 _CDA_P095 (Tomales-Drakes Bay) 1972 Def Estab
P110 Tomales Bay 1948 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Boyd, Milton J.; Mulligan, Tim J; Shaughnessy, Frank J. (2002) <missing title>, California Department of Fish and Game, Sacramento. Pp. 1-118

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.; Iverson, Ernest W. (1981) Biogeography and natural history of Sphaeroma walkeri Stebbing (Crustacea: Isopoda) and its introduction to San Diego Bay, California, Journal of Natural History 15: 31-48

Cohen, Andrew N.; Carlton, James T. (1995) Nonindigenous aquatic species in a United States estuary: a case study of the biological invasions of the San Francisco Bay and Delta, U.S. Fish and Wildlife Service and National Sea Grant College Program (Connecticut Sea Grant), Washington DC, Silver Spring MD.. Pp. <missing location>

Davidson, Timothy M. (2006) <missing title>, University of Oregon, MS Thesis, Eugene. Pp. <missing location>

Davidson, Timothy M.; Hewitt, Chad L.; Campbell, Marnie (2008) Distribution, density, and habitat use among native and introduced populations of the Australasian burrowing isopod Sphaeroma quoianum, Biological Invasions 10: 399-410

Davidson, Timothy M.; de Rivera, Catherine E. (2010) Accelerated erosion of saltmarshes infested by the non-native burrowing crustacean Sphaeroma quoianum, Marine Ecology Progress Series 419: 129–136

Hurley, D. E. (1956) The New Zealand species of Iais (Crustacea Isopoda), Transactions of the Royal Society of New Zealand 83(4): 715-519

Iverson, Ernest W. (1974) Range extensions for some California marine isopod crustaceans, Bulletin of the Southern California Academy of Sciences 73: 164-169

Llansó, Roberto J.; Sillett, Kristine; Scott, Lisa (2011) <missing title>, Versar, Inc., Columbia MD. Pp. <missing location>

Menzies, R. J.; Barnard, J. L. (1951) The isopod genus Iais, Bulletin of the Southern California Academy of Sciences 50(3): 136-151

Menzies, Robert J. (1962a) The marine isopod fauna of Bahia de San Quintin,Baja California, Mexico, Pacific Naturalist 3(11): 337-348

Poore, G. C. B.; Storey, M. (1999) Soft Sediment Crustacea of Port Phillip Bay, In: Hewitt, Campbell, Thresher & Martin(Eds.) Marine Biological Invasions of Port Phillip Bay, Victoria. , Hobart, Tasmania. Pp. 150-170

Richardson, Harriet (1905) A monograph on the isopods of North America, United States National Museum Bulletin 54: 1-727

Robinson, April; Cohen, Andrew N.; Lindsey, Brie; Grenier, Letitia (2011) Distribution of macroinvertebrates across a tidal gradient, Marin County, California, San Francisco Estuary and Watershed Science 9(3): published online

Rotramel, George (1972) Iais californica and Sphaeroma quoyanum, two symbiotic isopods introduced to California (Isopoda, Janiridae and Sphaeromatidae), Crustaceana Suppl. Ill: 193-197

Rotramel, George L. (1975) Observations on the commensal relations of Iais californica (Richardson, 1904) and Sphaeroma quoyanum H. Milne Edwards, 1840 (Isopoda), Crustaceana 28(3): 247-256

Talley, T. S.; Crooks, J. A.; Levin, L. A. (2001) Habitat utilization and alteration by the invasive burrowing isopod, Sphaeroma quoyanum, in California salt marshes, Marine Biology 138: 561-573

Wasson, Kerstin; Zabin, C. J.; Bedinger, L.; Diaz, M. C.; Pearse J. S. (2001) Biological invasions of estuaries without international shipping: the importance of intraregional transport, Biological Conservation 102: 143-153

Wilson, George D. F.; Wagele, Johann-Wolfgang (1994) Review of the family Ianiridae (Crustacea: Isopoda: Asellota), Invertebrate Taxonomy 8: 683-747