Invasion History

First Non-native North American Tidal Record: 1963
First Non-native West Coast Tidal Record: 1963
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Oithona davisae was described from San Francisco Bay, where it was introduced and collected probably as early as 1963. Ferrari and Orsi (1984) noted that this cyclopoid copepod had affinities to small Oithona spp. of Indo-Pacific origin such as O. brevicornis, O. wellershausi, and O. aruensis. The small size of these copepods and the occurrence of many similar species, has led to under-sampling and frequent mis-identifications of this group (Uye and Sano 1995; Temnykh and Nishida 2012). Oithona davisae is known from estuarine habitats in central and southern Japan, and southern Korea (Uye and Sano 1995; Sakaguchi et al. 2011), but the full extent of its native range is uncertain.

North American Invasion History:

Invasion History on the West Coast:

Early sampling in San Francisco Bay and elsewhere (e.g., Esterly 1924) used coarse nets and tended to under-sample Oithona species. Early records of this genus were from near Golden Gate and probably referred to native O. similis and O. nana, but a survey taken by Painter (1966, cited by Ferrari and Orsi 1984), starting in 1963, found Oithona spp. occurring in Suisun and San Pablo Bays, showing patterns of distribution and seasonal abundance similar to those of later records of O. davisae. By 1981, O. davisae reached high abundances (< 20,000 m-2 in South San Francisco Bay; Hutchinson 1981, cited by Ferrari and Orsi 1984; Ambler 1985). In San Francisco Bay, this copepod was most abundant throughout San Pablo, the Central and South Bays in July thru November and scarce during times of low temperature and heavy river flows. In recent surveys it was the most abundant copepod in the Bay (Bollens et al. 2011).

Ferrari and Orsi (1984) examined samples from several other California bays, including Humboldt and Monterey Bays, and the Agua Hedionda Lagoon, and did not find any O. davisae. Fairey et al. (2002) reported the occurrence of O. davisae in samples taken in Port Huemene, Los Angeles-Long Beach Harbors, and San Diego Harbor. No information was given on abundance or seasonal patterns. Oithona davisae was frequent in ballast water of trans-Pacific and West Coast ships entering Vancouver and Puget Sound (Levings et al. 2004; Cordell et al. 2008a; Lawrence and Cordell 2010) and so is likely to be introduced to other West Coast ports. In September 2012, O. davisae was found to comprise 95% of the copepods collected in Samish Bay, Washington, just north of Puget Sound proper, and is expected to become widespread in the Sound (Cordell and Kalata 2015).

Invasion History Elsewhere in the World:

Elsewhere in the World- Oithona davisae was collected and cultured in Barcelona Harbor, Spain, in 2000, apparently its first record outside the Pacific. Specimens were identified by Prof. Shuhei Nishida, of the University of Tokyo (Saiz et al. 2003). The similar species Oithona brevicornis was reported from the Black Sea for the first time in 2005 in Ukraine (Gubanova and Altukhov 2007), but later (2010) specimens, examined in more detail, were found to be O. davisae (Temnykh and Nishida 2012). In the Bay of Varna, Bulgaria O. davisae was first identified in 2009, and became a numerically dominamt in the zooplankton by 2010 (Mihneva and Stefanova 2013). Oithona brevicornis has been widely reported in the Mediterranean (Razouls et al. 2012), but it is likely that most or all of these records refer to O. davisae. Currently, O,.davoae is reported from Spain, the Aegean Sea in Italy, the Aegean Sea and Levantine Basin in Turkey (Saiz et al. 2003; Kurt et al. 2019; Pansera et al. 2021; Terb?y?k-Kurt et al. 2022); Molecular analyses indicate that the new Oithona species in the Black Sea is distinct from O. brevicornis, but the lack of O. davisae sequences in GenBank prevent a definitive genetic identification (Shiganova et al. 2015). Despite these uncertainties, O. davisae is now reported as a dominant species in Sevastopol Bay (Altukhov et al. 2014). In 2010 and 2014, Oithona davisae was identified by morphological and genetic methods from List Tidal Basin, in the Wadden Sea, Germany, the shallow barrier-island region of the North Sea (Cornils and Wend-Heckmann 2015). In 2001, O. davisae appeared in the Ibaizabal-Nerbioi River estuary, Bilbao, Spain, in Bay of Biscay, and became one of the dominant species (Uriarte et al. 2016). The extent of the range of O. davisae is not known.
Oithona davisae was collected in the Southeast Pacific, in Aisén Fjord, Chile, near the port of Chacabuco. This copepod (81 females) was found only in a single sample during a survey, which also contained the North Pacific copepods Acartia omorii and Centropages abdominalis, suggesting that these occurrences might have resulted from a single ballast water discharge, possibly from a Japanese ship (Hirakawa 1988).


Oithona davisae has a shield-shaped prosome and four tapering thoracic segments. The final (5th) thoracic segment is conical and truncated, bearing very reduced P5 swimming legs (pereiopods). The urosome is slender, consisting of five segments (Ferrari and Orsi 1984). Adult females have a rostrum, which is pointed ventrally. The forehead is rounded dorsally. On the 1st urosome segment, there is a knob near the genital opening with one long and one short seta. The caudal rami have a length 3X the width, and are armed with one outward seta near the base and five seta at the tip. From the outward side inward, setae 2 and 3 are longer, with 2 being the longest. All the caudal setae are plumed and the antennules have 13 segments. The female, like other Oithonidae, often carries two symmetrical egg masses attached to the genital segment. Females range from 0.54 to 0.62 mm in size (based on 30 specimens; Ferrari and Orsi 1984). The adult male lacks a rostrum, and the forehead is somewhat squared off. The posterior-lateral edges of the cephalon are drawn into fingerlike projections. The length of the caudal rami is 2X the width, with setae like a female's. Both antennules are symmetrical and digeniculate (having two hinged joints), with 13 segments, and end in a thick subterminal projection (called an aesthete). The swimming legs (pereiopods) P1-P5 are slightly reduced in size relative to a female's. Males range from 0.47-0.53 mm in length (based on 30 specimens; Ferrari and Orsi 1984).

The copepodite and naupliar stages of this copepod have not been described. Morphology and development should be similar to that described for Oithona brevicornis by Uchima (1979).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Arthropoda
Subphylum:   Crustacea
Class:   Maxillopoda
Subclass:   Copepoda
Order:   Cyclopoida
Family:   Oithonidae
Genus:   Oithona
Species:   davisae


Potentially Misidentified Species

Oithona aruensis
NW Pacific

Oithona brevicornis
Indo-Pacific, Mediterranean (cryptogenic?), Western Atlantic (cryptogenic?)

Oithona colcarva
NW Atlantic

Oithona hebes

Oithona nana

Oithona similis
Cosmopolitan? Ranges from the poles to tropics, maybe a species complex?

Oithona wellershausi
NW Pacific



Planktonic cyclopoid copepods mate in the water column. Males use their modified antenules to grasp the female and transfer spermatophores to the female's genital segment. Female cyclopoids carry eggs in two symmetrical clusters under the abdomen (Barnes 1983). Eggs hatch into nauplii which go through six stages. The first stage, NI, has 3 pairs of appendages and is unsegmented - each molt has additional appendages and/or more differentiation of segments. The sixth stage (NVI) molts into a first copepodite stage (CI), with the basic form of the adult, and fully differentiated feeding structures, but with only two pairs of swimming legs and only one urosomal segment. The copepod goes through five additional molts, with increasing numbers of swimming legs, urosomal segments, and sexual differentiation. The sixth (CVI) stage is the male or female adult (Uchima 1979). In O. davisae (originally identified as O. brevicornis, Uye and Sano 1995), overall development time in the laboratory under food-rich conditions was about 15-18 days at 20-21 C (Uchima et al. 1979). Developmental rate is affected both by temperature and food level. At 16 C and high food levels, the nauplius phase of development took more than 8 days, but only 4-5 days at 28 C. Development times were slowed only at very low food levels, and did not decrease significantly with higher food levels (Almeda et al. 2010). Populations in areas with cold winters (Black Sea, Ukraine) overwinter as fertilized adult females, which produce eggs as temperatures rise above 8 C (Svetlichny et al. 2016). whi

Oithona davisae is characteristic of mesohaline to polyhaline estuarine waters. In the San Francisco estuary, it is uncommon at salinities below 12 PSU (Ferrari and Orsi 1984). Adult and juvenile cyclopoid copepods feed raptorially, seizing particles, and may be carnivorous or omnivorous, feeding on algae, ciliates, rotifers, and copepod nauplii (Barnes 1983). In experiments with natural assemblages, Oithona davisae fed primarily on aloricate ciliates and flagellates, while not significantly grazing diatoms in one experiment, but in another, grazed heavily on diatoms, when cilates and heterotrophic flagellates were scarce, suggesting seasonal variation in prey preference (Gifford et al. 2007).


Phytoplankton; Microzooplankton

Trophic Status:




General HabitatUnstructured BottomNone
General HabitatSalt-brackish marshNone
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatPlanktonicNone

Life History

Tolerances and Life History Parameters

Minimum Temperature (ºC)9Field, Fukuyama Harbor, Japan (Uye and Sano 1995); San Pablo Bay (Bollens et al. 2011)
Maximum Temperature (ºC)28Field, Fukuyama Harbor, Japan (Uye and Sano 1995).
Minimum Salinity (‰)0.1Field, Shira-kawa, Japan (Sakaguchi et al. 2011). O. davisae was uncommon below 12 PSU in the San Francisco estuary (Ferrari and Orsi 1985). Black Sea specimens tolerated salinities as low as 3 PSU with acclimation (Svetlichny and Hubareva 2014).
Maximum Salinity (‰)40Experimental (Svetlichny and Hubareva 2014).
Minimum Reproductive Temperature8Sevastopol Bay, Crimea (Svetlichny et al. 2016), Field, 15 C, Fukuyama Harbor, Japan (Uye and Sano 1995).
Maximum Reproductive Temperature28Field, Fukuyama Harbor, Japan (Uye and Sano 1995).
Minimum Length (mm)0.5Adult Male (Ferrari and Orsi 1985)
Maximum Length (mm)0.6Adult Female (Ferrari and Orsi 1985)
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNone Oligohaline-Euhaline

General Impacts

The historical increase in abundance of Oithona davisae, in San Francisco Bay proper, combined with the decrease in the abundance of larger calanoid copepods may be a consequence both of eutrophication and the modification of Bay food webs by the invasion of Corbula amurensis. Eutrophic conditions may favor flagellates and ciliates, as part of the microbial food web over diatoms (Uye and Sano 1995; Gifford et al. 2007; Bollens et al. 2011). In turn, the dominance of small oithonids, such as O. davisae and Limnotihona tetraspina may worsen feeding conditions for larval fishes (Bollens et al. 2011; Winder 2011). However, we have no specific information on the impact of Oithona davisae on larval fish feeding.

Regional Impacts

MED-IXNoneEcological ImpactCompetition
Oithona davisae, from its appearance in 2000 to 2015 increased to greatly outnumber the native O. nana. It has greater fecundity, greate low salinity tolerance (3 vs. 10), and greater escape speed (Isinibilir et al. 2016).
MED-VIINoneEcological ImpactCompetition
Oithona daviae appears to have replaced the native, more marine, O. nana, in brackish, more eutrophic parts of the lagoon (Pansera et al. 2021)

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NEP-V Northern California to Mid Channel Islands 1963 Def Estab
NWP-3b None 0 Native Estab
NWP-4a None 0 Native Estab
SEP-B None 1983 Def Unk
MED-II None 2000 Def Estab
NEP-VI Pt. Conception to Southern Baja California 2000 Def Estab
P090 San Francisco Bay 1963 Def Estab
P062 _CDA_P062 (Calleguas) 2000 Def Estab
P050 San Pedro Bay 2001 Def Estab
P020 San Diego Bay 2000 Def Estab
P093 _CDA_P093 (San Pablo Bay) 1963 Def Estab
MED-IX None 2009 Def Estab
NWP-3a None 0 Native Estab
P293 _CDA_P293 (Strait of Georgia) 2012 Def Estab
NEP-III Alaskan panhandle to N. of Puget Sound 2012 Def Estab
NEA-II None 2010 Def Estab
NEA-V None 2001 Def Estab
MED-VIII None 2014 Def Estab
MED-VI None 2015 Def Estab
MED-VII None 2014 Def Estab
P040 Newport Bay 2015 Def Estab
MED-VI None 2018 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
26557 Fairey et al. 2002 2002 2002-03-04 San Diego Bay Plankton 04 Def 32.6885 -117.1495
26559 Fairey et al. 2002 2001 2001-12-19 San Diego Bay Plankton 04 Def 32.6885 -117.1495
27135 Fairey et al. 2002 2002 2002-03-04 San Diego Bay Plankton 02 Def 32.7204 -117.2180
27136 Fairey et al. 2002) 2001 2001-12-19 San Diego Bay Plankton 02 Def 32.7204 -117.2180
27159 Fairey et al. 2002 2002 2002-03-04 San Diego Bay Plankton 05 Def 32.6706 -117.1285
27166 Fairey et al. 2002 2001 2001-12-19 San Diego Bay Plankton 05 Def 32.6706 -117.1285
27719 Ferrari and Orsi 1984 1963 1963-01-01 Suisun Bay Def 38.0713 -122.0581
27720 Cohen and Carlton, 1995 1979 1979-12-01 Suisun Bay Def 38.0713 -122.0581
29744 Fairey et al. 2002 2002 2002-03-04 San Diego Bay Plankton 01 Def 32.6932 -117.2306
29747 Fairey et al. 2002 2001 2001-12-19 San Diego Bay Plankton 01 Def 32.6932 -117.2306
29749 Fairey et al. 2002 2000 2000-09-14 Port Hueneme Plankton 01 Def 34.1510 -119.2067
30371 Ferrari and Orsi 1984; Cohen and Carlton, 1995 1980 1980-01-01 South San Francisco Bay Def 37.5457 -122.1645
31397 Fairey et al. 2002 2002 2002-03-04 San Diego Bay Plankton 03 Def 32.7223 -117.1849
31404 Fairey et al. 2002) 2001 2001-12-19 San Diego Bay Plankton 03 Def 32.7223 -117.1849
32131 Ambler 1980; Cohen and Carlton, 1995 1980 1980-01-01 Central San Francisco Bay Def 37.8595 -122.3884
32478 Fairey et al. 2002 2002 2002-03-04 LA Harbor Plankton 01 Def 33.7323 -118.2294
32933 Fairey et al. 2002) 2001 2001-12-19 San Diego Bay Plankton 06 Def 32.6437 -117.1236
33360 Ambler 1985; Cohen and Carlton 1995 1980 1980-01-01 San Pablo Bay Def 38.0600 -122.3900
33367 Ferrari and Orsi 1984 1963 1963-01-01 San Pablo Bay Def 38.0600 -122.3900
33528 Ferrari and Orsi 1984 2001 2001-10-11 LA Harbor Plankton 03 Def 33.7694 -118.2259


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