Invasion History

First Non-native North American Tidal Record: 1990
First Non-native West Coast Tidal Record: 1990
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Pseudodiaptomus inopinus was described from a freshwater lake in China (Sakaguchi and Ueda 2010). It is native to lakes and estuaries of Asia from Japan, Korea and Guangdong province, China (Cordell et al. 1992; Huang 2001; Sakaguchi et al. 2012). Records from the Nansei (or Ryuku) Islands, Japan including Okinawa, are based on the morphologically similar species, P. nanseei (Sakaguchi and Ueda. 2010). It is characteristic of brackish water, and occurs from 0 to 26 PSU, and 10-31°C, but is rare above 5-8 PSU (Cordell et al. 1992; Cordell and Morrison 1996; Cordell et al. 2011; Sakaguchi et al. 2011). It is introduced in Oregon (OR) and Washington (WA) where it was first found in the Columbia River in 1990. It was subsequently found in Puget Sound (1 specimen) and estuaries from the Chehalis River, WA to Coos Bay, OR where it has established populations (Cordell and Morrison 1996; Cordell et al. 2007). In the Columbia River, it has been wholly or largely replaced by another introduced copepod, P. forbesi (Cordell et al. 2008; Bollens et al. 2012).

North American Invasion History:

Invasion History on the West Coast:

In September 1990, all the life stages of Pseudodiaptomus inopinus were found in the Columbia River estuary, Oregon/Washington, and were fairly abundant at River Mile 12 to 23, but were rare or absent near the mouth (RM 2) or upriver (RM 23) where salinity ranged from 0 to 21 PSU (Cordell et al. 1992). In 1999, another introduced copepod, P. forbesi, appeared in the Columbia River and by 2002 it had largely replaced P. inopinus (Sytsma et al. 2004). From 2005 to 2013, this species was no longer found in plankton surveys of the Columbia River estuary (Cordell et al. 2008; Bollens et al. 2012; Dexter et al. 2015).

In 1991, Cordell and Morrison (1996) surveyed zooplankton in 18 smaller estuaries of the Pacific Northwest, from Campbell River, British Columbia to Coos Bay, Oregon and found specimens of P. inopinus in seven of them. Only a single specimen was collected in the Snonomish River, a tributary of Puget Sound. Established populations were found in Chehalis, Washington (WA); Willapa, WA; Youngs, Oregon (OR); Tillamook, OR; Yaquina, OR; Umpqua, OR; Coos, OR; and Coquille and Siuslaw, OR (Cordell et al. 2010). Rivers with established populations had mean September-October temperatures of 19.3°C and salinity intrusion zones more than 1 km long, while uninvaded rivers had a mean temperature of 12.4°C and a mixing zone of less than 1 km. In the invaded rivers, it ranged from 1% (Willapa) to 55% (Chehalis) of the sampled zooplankton (Cordell and Morrison 1996). This copepod was found both in rivers with and without international shipping, suggesting either natural or human-aided dispersal along the coast, and probably an invasion of the Pacific coast some years before its detection. Pseudodiaptomus inopinus has been found in the ballast water of ships entering Puget Sound from Asian ports, and so, is likely to be introduced elsewhere on the West Coast (Cordell et al. 2009).


In adult Pseudodiaptomus inopinus, regardless of sex, the head carapace is fused with the 1st thoracic segment (Sakaguchi and Ueda 2010). In adult females, the antennules are symmetrical. Thoracic segments 4 and 5 are fused, and the prosomites on segments 2 to 4 have rows of fine spinules laterally along the posterior margins. The fused segments 4-5 have rounded corners and small spinelike process dorsally. The terminal end of segments 4-5 has a small bump, with several spinules and a row of spinules on the posterolateral corner on each side. The genital double-somite is 1.1 times longer than wide, with several spinules on each anterolateral projection, with long anterolateral setae and dorsolateral rows of spinules at one-third and two thirds anteriorly on each side. The posterior process of the genital flap is pointed and longer than that of the closely related P. nansei sp. nov. (Sakaguchi and Ueda 2010). The caudal rami are symmetrical. The medial terminal setae are greatly swollen, and equal in length to the rami. The 5th pair of swimming legs is symmetrical. The coxa (basal segment) of swimming leg 5 has spinules on posterior and anterior surfaces. The first exopodal segments have round distomedial processes. The second segments have inward pointing thum-blike distal processes. The terminal spine of the third segment lacks a notch at the base and has three terminal spines, including a short medial anterior spine with medial teeth (Fig. 7I in Sakaguchi and Ueda 2010). The length ranges from 1.38 to 1.44 mm and the egg masses are symmetrical (Shen 1979). Description based on Sakaguchi and Ueda 2010.

In adult males, the left antenna has 22 segments, while the right has 20 segments, with segments 6-7 incompletely fused and segments 15-17 expanded. The 19th segment has a proximal ridge, covered with fine hairs. The 2nd to 4th thoracic segments have a group of minute spinules near the anteroventral corners. The 5th thoracic segment has a spinelike process dorsally, and posterolateral spinules on each corner. The second urosomite has a patch of minute spinules anterolaterally and ventral transverse rows of spinules. The 5th swimming legs (P5) are asymmetrical. The left P5 lacks spinules at the base of the distal smaller process of the basoendopod. The second exopodal segment (based on five specimens) varies from narrow to paddle-shaped. On the right P5, the 1st basipodite segment has two rows of spinules and the medial margin is produced into a small truncated triangular process. The 2nd basipodite segment has two small surface spinules. The right P5's first exopodal segment has a proximomedial spinule with a distolateral spine extending more than mid length of the second segment with small lateral spines. The third segment is not swollen proximally. The 1st and 2nd endopod segments have large external spines. The 3rd segment is elongate, with two medial knobs and serrated edges. The left P5 has a 1st basipodite segment similar to the right leg, but without the triangular process. The 2nd segment has the medial margin produced into a very long two-pronged process, with margins serrated between the two points. The longer point extends beyond the 1st segment of the endopod. The 2nd endopod has a serrated external spine. Two morphs occur: one which the distomedial end is a thumb-like process edge, separated from the lateral side by a shallow V-shaped notch; and the other in which the whole endopod is a broad paddle-shaped lobe. The body length ranges from 1.10 to 1.15 mm. Description based on Cordell et al. 2007 and Sakaguchi and Ueda 2010.

The copepodite and naupliar stages of this copepod have not been described. Their morphology is probably similar to that of P. marinus (Uye and Onbe 1975). This copepod is characteristic of brackish and fresh waters.


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Arthropoda
Subphylum:   Crustacea
Class:   Maxillopoda
Subclass:   Copepoda
Order:   Calanoida
Family:   Pseudodiaptomidae
Genus:   Pseudodiaptomus
Species:   inopinus


Schmackeria inopina (Burckhardt, 1913)

Potentially Misidentified Species

Pseudodiaptomus forbesi
Native to Japan and China, introduced to San Francisco Bay and Columbia River

Pseudodiaptomus nansei
Closely related species, described from Nansei (or Ryuku) Islands, Japan



Planktonic calanoid copepods mate in the water column. Males use their modified antenules and 5th pair of swimming legs to grasp the female and transfer spermatophores to the female's genital segment. Female Pseudodiaptomus inopinus carry eggs in two symmetrical clusters under the abdomen (Barnes 1983; Cordell et al. 1992; Cordell et al. 2007). 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 additonal molts, with increasing numbers of swimming legs, urosomal segments, and sexual differentiation. The sixth (CVI) stage is the male or female adult (Uye and Onbe 1975; Barnes 1983).

Pseudodiaptomus inopinus, like many other copepods of its genus, is characteristic of estuaries with low-salinity waters (Walter 1989; Orsi and Walter 1991).  It is capable of completing its life cycle in freshwater, and inhabits tidal fresh waters. Late copepodites and adults of the genus Pseudodiaptomus often have strong epibenthic tendencies, particularly by day, and when carrying eggs. Adult and late copepodites of P. inopinus migrated to the surface at night in the Chehalis River (Washingotn), apparently as a means of avoiding visual predators. In the Chehalis, major predators were mysids (Neomysis mercedis) and shrimps (Crangon spp.) (Cordell et al. 2007). Migration was not seen in the Columbia River (Oregon/Washington), which is more turbid than the Chehalis (Cordell et al. 1992). Adults may cling to hard surfaces using adhesive hairs on their antennules, and filter in place (Fofonoff, personal observation of P. pelagicus, Narragansett Bay). All life stages feed on phytoplankton, although adults may also capture ciliates, rotifers, and copepod nauplii (Barnes 1983). In  experiments, P. inopinus from the Yaquina and Chehalis estuaries feed omnivroously on a mixture of phytopalnkton and ciliates (Jacobs et al. 2022).


Phytoplakton, detritus


Fishes, mysids, shrimp


Pseudodiaptomus forbesi; Eurytemora 'affinis'

Trophic Status:

Suspension Feeder



General HabitatTidal Fresh MarshNone
General HabitatUnstructured BottomNone
General HabitatSalt-brackish marshNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Salinity RangeEuhaline30-40 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone
Vertical HabitatPlanktonicNone

Life History

Tolerances and Life History Parameters

Minimum Temperature (ºC)10Field, WA-OR (Cordell et al. 2010)
Maximum Temperature (ºC)30.9Field, Japan (Oka et al. 1991; Sakaguchi et al. 2011)
Minimum Salinity (‰)0Field OR-WA (Cordell 1992); Japan (Sakaguchi et al. 2011)
Maximum Salinity (‰)26.9Field, Japan (Sakaguchi et al. 2011). But rare above 5 ppt (Cordell and Morrison 1996; Cordell et al. 2010).
Broad Temperature RangeNoneCold temperate-Warm temperate
Broad Salinity RangeNoneTidal Limnetic-Euhaline

General Impacts

The impacts of Pseudodiaptomus inopinus in Pacific Northwest rivers are unclear. In the Columbia River, Oregon/Washington, it was displaced by P. forbesi within a decade. In the Chehalis River, Washington, where it reached high abundances by 1991, it reaches peak abundance in the fall, while the native Northeast Pacific form of 'Eurytemora affinis' reaches its peak in the spring. While both species overlap, the temporal separation of the peak abundances may limit competition. Pseudodiaptomus inopinus was an important food for fish prey such as the mysid Neomysis mercedis and juvenile shrimp Crangon spp. in late summer and fall, but was rarely directly eaten by fishes. It is not clear to what extent P. inopinus has altered food webs in Pacific Northwest rivers.

Regional Impacts

P260Columbia RiverEcological ImpactCompetition
OROregonEcological ImpactCompetition

The impacts of Pseudodiaptomus inopinus in Pacific Northwest rivers are unclear. In the Columbia River, it was displaced by P. forbesi within a decade. In the Chehalis River, where it reached high abundances by 1991, it reaches its peak abundances in the fall, while the native Northeast Pacific form of 'Eurytemora affinis' reaches its peak in the spring. While both species overlap, the temporal separation of the peak abundances may limit competition. Pseudodiaptomus inopinus was an important food for fish prey such as the mysid Neomysis mercedis and juvenile shrimp Crangon spp. in late summer and fall, but was rarely directly eaten by fishes. It is not clear to what extent P. inopinus has altered foodwebs in Pacific Northwest rivers.

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NWP-2 None 0 Native Estab
NWP-3b None 0 Native Estab
NWP-4b None 0 Native Estab
NWP-4a None 0 Native Estab
NWP-3a None 0 Native Estab
NEP-IV Puget Sound to Northern California 1990 Def Estab
NEP-III Alaskan panhandle to N. of Puget Sound 1991 Def Unk
P170 Coos Bay 1991 Def Estab
P260 Columbia River 1990 Def Extinct
P280 Grays Harbor 1992 Def Estab
P180 Umpqua River 1991 Def Estab
P210 Yaquina Bay 0 Def Estab
P240 Tillamook Bay 1991 Def Estab
P270 Willapa Bay 1991 Def Estab
P290 Puget Sound 1991 Def Unk
P190 Siuslaw River 2004 Def Estab
P160 Coquille River 2004 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Bollens, Stephen M.; Breckenridge, Joanne K.; Cordell, Jeffery R.; Rollwagen-Bollens; Gretchen; Kalata, Olga (2012) Invasive copepods in the Lower Columbia River Estuary: Seasonal abundance, co-occurrence and potential competition with native copepods, Aquatic Invasions 7(1): 101-109

Bollens, Stephen M.; Cordell, Jeffrey R.; Avent, Sean: Hooff, Rian (2002) Zooplankton invasions: a brief review, plus two case studies from the northeast Pacific Ocean., Hydrobiologia 480: 87-110

Brodskii, K. A. (1967) <missing title>, !srael Program for Scientific Translations, Jerusalem, Israel. Pp. <missing location>

Cordell, J. R., Morgan, C. A., Simenstad, C. A. (1992) Occurrence of the Asian calanoid copepod Pseudodiaptomus inopinus in the zooplankton of the Columbia River Estuary, Journal of Crustacean Biology 12(2): 260-269

Cordell, Jeffery R., Morrison, Sean M. (1996) The invasive Asian copepod Pseudodiaptomus inopinus in Oregon, Washington, and British Columbia estuaries, Estuaries 19(3): 629-638

Cordell, Jeffery R.; Rasmussen, Mikelle; Bollens, Stephen M. (2007) Biology of the introduced copepod Pseudodiaptomus inopinus in a northeast Pacific estuary., Marine Ecology Progress Series 333: 213-227

Cordell, Jeffery R.; Tear, Lucinda M.; Bollens, Stephen M. (2010) Modelling physico-chemical factors affecting occurrences of a non-indigenous planktonic copepod in northeast Pacific estuaries, Biological Invasions 12: 1427-1445

Cordell, Jeffrey (1998) Asian copepods in Pacific Northwest estuaries., Puget Sound Notes 41: 1-7

Cordell, Jeffrey R. and 5 authors (2009) Factors influencing densities of non-indigenous species in the ballast water of ships arriving at ports in Puget Sound, Washington, United States, Aquatic Conservation: Marine and Freshwater Ecosystems 19(3): 322-343

Cordell, Jeffrey; Bollens, Stephen M.; Draheim, Robyn; Sytsma, Mark (2008) Asian copepods on the move: Recent invasions in the Columbia-Snake River system, USA, ICES Journal of Marine Science 65: 753-758

Danziger, Ariella; Frederich, Markus (2022) Challenges in eDNA detection of the invasive European green crab, Carcinus maenas, Biological Invasions <missing volume>: https://link.springe

Dexter, Eric; Bollens, Stephen M.; Rollwagen-Bollens, Gretchen; Emerson, Josh; Zimmerman, Julie (2015) Persistent vs. ephemeral invasions: 8.5 years of zooplankton community dynamics in the Columbia River, Limnology and Oceanography 60: 527-539

Eyun, Seong-il; Lee, Youn-Ho; Suh, Hae-Lip; Kim, Sung; Soh, Ho Young (2007) Genetic identification and molecular phylogeny of Pseudodiaptomus species (Calanoida, Pseudodiaptomidae) in Korean waters, Zoological Science 24(1): 265-271

Huang, Zongguo (Ed.), Junda Lin (Translator) (2001) Marine Species and Their Distributions in China's Seas, Krieger, Malabar, FL. Pp. <missing location>

Jacobs, Jade; Rollwagen-Bollens, Gretchen; Bollens, Stephen M. (2022) Feeding dynamics of the invasive calanoid copepod Pseudodiaptomus inopinus in two northeast Pacific estuaries, Aquatic Ecology 31: 49-64

Oka, Shin-ichiro; Saisho, Toshio; Hirota, Reiichiro (1991) Pseudodiaptomus (Crustacea: Copepoda) in the brackish waters of mangrove regions in the Nansei Islands, southwestern Japan, Bulletin of the Biogeographical Society of Japan 46(8): 83-88

Sakaguchi, Sakiko Orui; Hiroshi Ueda (2010) A new species of Pseudodiaptomus (Copepoda: Calanoida) from Japan, with notes on the closely related P. inopinus Burckhardt, 1913 from Kyushu Island, Zootaxa 2623: 52-68

Sakaguchii, Sakiko Orui; Ueda, Hiroshi; Ohtsuka, Susumu; Soh, Young Ho; Yoon, Ho Yoon (2011) Zoogeography of planktonic brackish-water calanoid copepods in western Japan with comparison with neighboring Korean fauna, Plankton & Benthos Research 6: 18-25

Shen, C-J.; Lee, F.-S. (1963) [The estuarine copepods of Chiekong and Zaikong Rivers, Kwangtung Province, China] (Chinese), Studia Marina Sinica 3: 571-596

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>

Uye, Shin-ichi; Onbe, Takashi (1975) The developmental stages of Pseudodiaptomus marinus reared in the laboratory, Bulletin of the Plankton Society of Japan 21(2): 1-12