Invasion History

First Non-native North American Tidal Record: 1987
First Non-native West Coast Tidal Record: 1987
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Pseudodiaptomus forbesi was described from the Yangtze River region of China (Orsi and Walter 1991). Its native range is from the 'northern South China Sea' (Huang 2001) to the Yangtze River and the 'western Taiwan Strait' (Huang 2001). It has been reported from a freshwater lake in Japan, but this could be a misidentification of P. inopinus (Kikuchi 1928, cited by Orsi and Walter 1991).

North American Invasion History:

Invasion History on the West Coast:

In October 1987, Pseudodiaptomus forbesi was first noticed in the San Joaquin River, California between Stockton and the mouth of the Mokelumne River (in freshwater), and in the Sacramento-San Joaquin Delta. In summer and fall of 1988, it was found abundantly throughout the Delta and in decreasing numbers into Suisun Bay, ranging from freshwater into salinity of 16 PSU (Orsi and Walter 1991). In 1987-1994, the peak abundance of P. forbesi occurred at 0.2-0.5 PSU, and it maintained its position in the estuary through tidally timed vertical migration (Kimmerer et al. 1998). Pseudodiaptomus forbesi has fluctuated in abundance, but remains the dominant calanoid copepod in the fresh-to-mesohaline parts of the San Francisco bay estuary (Baxter et al. 2008).

In 1999, P. forbesi was discovered in the Columbia River estuary, Oregon/Washington and by 2002 ranged from Portland to near the mouth of the river at Astoria, and had largely replaced a previous invader P. inopinus (Sytsma et al. 2004; Cordell et al. 2008b). By 2009, it had also colonized the four lowest reservoirs on the Columbia River and lower Snake Rivers, more than 240 km from the head of tide at Bonneville Dam (Cordell 2012). In continuous sampling from a pier in Vancouver, Washington, it regularly occurred as a summer-fall dominant from 2005 through 2013, and comprised 23% of mean zooplankton individuals over that time (Dexter et al. 2015).

Pseudodiaptomus forbesi is not known from other estuaries, but it was abundant in the ballast tanks of ships coming to Puget Sound from other West Coast ports, presumably from the Columbia River or the inner San Francisco estuary (Cordell et al. 2008a).


Description

In adult Pseudodiaptomus forbesi, regardless of sex, the head carapace is fused with the 1st thoracic segment (Orsi and Walter 1991). In adult females, thoracic segment 4-5 are fused, and segment 5 bears 2 small posterodorsal spines, a rounded lateral protrusion, and a cluster of ventero-lateral spinules. The antennule (1st antenna) has 21 segments, and segment 3 has a posterior row of spinules. The urosome has 4 segments. Segment 1 has prominent fine spinules dorsally, while segments 1-3 have scales on the posterior edge. The caudal rami are symmetrical, with 5 terminal plumose setae each. The lateral setae are shortest, while the 3 central setae are divided into 3 sections. The egg sacs are paired. The 5th pair of swimming legs (P5) is symmetrical. The 2nd segments of the P5s have single large dorsal spinules and 3 rows of medial spinules. The 1st segments of the endopods have 2 small dorsal spinules and 3 strong medial spinules. The distomedial corner has a large, variably shaped hyaline membrane and strongly serrate outer spines. The terminal segment of P5 has two large serrated, terminal spines. The body length ranges from 1.10 to 1.18 mm. Description based on Orsi and Walter 1991.

In adult males, the 5th thoracic segment lacks dorsal spines. The spermatophores are small and slender. The first segment of urosome 1 has two dorsal clusters of spinules. The left antennule has 21 segments, while the right antennule has 19, with posterior spinules on segment 3. Segments 13-16 are widened, and the antennule is hinged at segment 17. The 2nd urosome segment has a ventral row of spinules. The caudal rami are symmetrical, with the setae as in the female. On the right P5, the 1st basipodite segment has 2 rows of spinules and the medial margin is produced into a triangular process. The 2nd basipodite segment has a large surface spinule and a dorsolateral row of spinules. 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, and the margin is serrated between the two points. The longer point extends beyond the 1st segment of the endopod. The 2nd endopod has a serrated external spine, and the distolateral edge deeply cut by a V-shaped notch. The body length ranges from 0.97 to 1.04 mm. Description based on Orsi and Walter 1991.

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.


Taxonomy

Taxonomic Tree

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

Synonyms

Pseudodiaptomus forbesi (Burckhardt, 1913)
Schmackeria forbesi (Poppe and Richard, 1890)

Potentially Misidentified Species

Pseudodiaptomus euryhalinus
Native from San Francisco Bay to Mexico.

Pseudodiaptomus inopinus
Introduced from Coos Bay to Columbia River, Oregon.

Pseudodiaptomus marinus
Introduced from Mexico to Tomales Bay, California.

Pseudodiaptomus pelagicus
East Coast of North America

Ecology

General:

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 forbesi carry eggs in two symmetrical clusters under the abdomen (Uye and Onbe 1975; Barnes 1983; Orsi and Walter 1991). 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 (Uye and Onbe 1975; Barnes 1983).

Pseudodiaptomus forbesi, 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, and has colonized four reservoirs of the Columbia River (Cordell et al. 2008). Late copepodites and adults of the genus Pseudodiaptomus have strong epibenthic tendencies, particularly by day and when carrying eggs. 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, adult P. forbesi reduced the abundance of tintinnids and aloricate cilates, but had no net effect on phytoplankton biomass (York et al. 2014). In the Columbia River, P. forbesi showed a preference for diatoms and ciliates, but an avoidance of chlorophytes and cyanobacteria (Bowen et al. 2015). In brackish portions of the San Francisco estuary, abundance is correlated with high spring freshwater flow, due largely to physical transport of the copepods from freshwater. Modeling inidicates that high spring freshwater flows havor copepod populations, but fall flows amy decrease populations (Kimmerer et al. 2018; Hamilton et al. 2021).

Food:

Phytoplankton, detritus, tintinnids, ciliates

Consumers:

Fishes, Delta Smelt

Competitors:

Pseudodiaptomus inopinus

Trophic Status:

Suspension Feeder

SusFed

Habitats

General HabitatTidal Fresh MarshNone
General HabitatUnstructured BottomNone
General HabitatMarinas & DocksNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Tidal RangeSubtidalNone
Vertical HabitatEpibenthicNone
Vertical HabitatPlanktonicNone


Tolerances and Life History Parameters

Minimum Temperature (ºC)3Field, Columbia River estuary (Bollens et al. 2012)
Maximum Temperature (ºC)27Maximum in San Joaquin river, near Stockton (Orsi and Mecum 1986)
Minimum Salinity (‰)0Field, San Francisco Bay (Orsi and Walter 1991)
Maximum Salinity (‰)16Field, San Francisco Bay (Orsi and Walter 1991). In experiments, nauplii of P. forbesi tolerated salinities at least as high as 14 PSU, and adults, at least 8 PSU. but it has become rare at salinities above 2 PSU, probably because of predation by A. sinensis and by Corbula amurensis.
Minimum Length (mm)1Minimum adult length, male (Orsi and Walter 1991)
Maximum Length (mm)1.2Maximum adult length, female (Orsi and Walter 1991)
Broad Temperature RangeNoneWarm temperate
Broad Salinity RangeNoneNontidal Limnetic-Mesohaline

General Impacts

Ecological Impacts

Competition: After its invasion in 1987, Pseudodiaptomus forbesi has remained the dominant calanoid copepod in low-salinity regions of the San Francisco estuary, while Eurytemora carolleeae (formerly E. affinis) has declined (Orsi and Walter 1991; Baxter et al. 2005), suggestive of competition between the two species. In the Columbia River, Pseudodiaptomus forbesi, by 2002, had replaced a previous invader, P. inopinus (Systsma 2004; Cordell 2008a; Dexter et al. 2015). Competition with the native form of 'Eurytemora affinis' is possible, since the two species have a substantial temporal overlap (Bollens et al. 2012).

Food/Prey: Pseudodiaptomus forbesi was selected against by larval Striped Bass (Morone saxatilis), apparently because of superior escape abilities (Meng and Orsi 1991). However, Pseudodiaptomus spp., mostly P. forbesi, have become the predominant food of small (under 50 mm) Striped Bass (Bryant and Arnold 2007). Pseudodiaptomus forbesi is also now the principal food organism of the endangered Delta Smelt (Hypomesus transpacificus) and the threatened Longfin Smelt (Spirinchus thaleichthys) in the Sacramento-San Joaquin Delta (Hobbs et al. 2006). It is considered to be inferior to the previous dominant copepod, Eurytemora affinis, as a food item for the smelts (Moyle et al. 1992; Nobriga 2002).

Regional Impacts

NEP-VNorthern California to Mid Channel IslandsEcological ImpactFood/Prey
Pseudodiaptomus forbesi was selected against by larval Striped Bass (Morone saxatilis), apparently because of superior escape abilities (Meng and Orsi 1991). However, Pseudodiaptomus spp., mostly P. forbesi, have become the predominant food of small (under 50 mm) Striped Bass (Bryant and Arnold 2007). Pseudodiaptomus forbesi is also now the principal food organism of the endangered Delta Smelt (Hypomesus transpacificus and the threatened Longfin Smelt (Spirinchus thaleichthys) in the Sacramento-San Joaquin Delta (Hobbs et al. 2006). It is considered to be inferior to the previous dominant copepod, Eurytemora carolleeae as a food item for the smelts (Moyle et al. 1992; Nobriga 2002). The shift in dominance from Eurytemora to Pseudodiaptomus was associated with a decrease in average length for Delta Smelt (Glibert 2010). Overall, changes in the fatty-acid and overall nutritional quality in the foodweb are complex, but the shift from more herbivorous native cladocerans to more omnivorous copepods may have increased transfer of contaminants up the food-chain, and increased dominance of cyanobacteria (Kratina and Winder 2015). The effects of introduced copepods are additionally complex, because of the varying size of the life-stages, and the interaction of different species of fish predators (Sullivan et al. 2016).
P090San Francisco BayEcological ImpactFood/Prey
Pseudodiaptomus forbesi was selected against by larval Striped Bass (Morone saxatilis), apparently because of superior escape abilities (Meng and Orsi 1991). However, Pseudodiaptomus spp., mostly P. forbesi, have become the predominant food of small (under 50 mm) Striped Bass (Bryant and Arnold 2007). Pseudodiaptomus forbesi is also now the principal food organism of the endangered Delta Smelt (Hypomesus transpacificus and the threatened Longfin Smelt (Spirinchus thaleichthys) in the Sacramento-San Joaquin Delta (Hobbs et al. 2006; Slater and Baxter 2014; Kimmerer et al. 2018; Hamilton et al. 2020). It is considered to be inferior to the previous dominant copepod, Eurytemora affinis as a food item for the smelts (Moyle 1992; Nobriga 2002). Overall, changes in the fatty-acid and overall nutritional quality in the foodweb are complex, but the shift from more herbivorous native cladocerans to more omnivorous copepods may have increased transfer of contaminants up the food-chain, and increased dominance of cyanobacteria (Kratina and Winder 2015). The effects of introduced copepods are additionally complex, because of the varying size of the life-stages, and the interaction of different species of fish predators (Sullivan et al. 2016).
P090San Francisco BayEcological ImpactCompetition
After its invasion in 1987, Pseudodiaptomus forbesi has remained the dominant calnaoid copepod in low-salinity regions of the San Francisco estuary, while Eurytemora carolleeae has declined (Orsi and Walter 1991; Baxter et al. 2005). One of many possible factors is that Pseudodiaptomus forbesi was better able to graze selectively than E. caroleeae, thus avoiding the toxic cyanobacterium Microcystis sp., which has been blooming in low-salinity waters of the estuary (Ger et al. 2010) .
NEP-VNorthern California to Mid Channel IslandsEcological ImpactCompetition
After its invasion in 1987, Pseudodiaptomus forbesi has remained the dominant calanoid copepod in low-salinity regions of the San Francisco estuary, while Eurytemora carolleeae (formerly E. affinis) has declined (Orsi and Walter 1991; Baxter et al. 2005). One of many possible factors is that Pseudodiaptomus forbesi was better able to graze selectively than E. caroleeae, thus avoiding the toxic cyanobacterium Microcystis sp., which has been blooming in low-salinity waters of the estuary (Ger et al. 2010).
NEP-IVPuget Sound to Northern CaliforniaEcological ImpactCompetition
In the Columbia River, Pseudodiaptomus forbesi, by 2002, has largely replaced a previous invader, P. inopinus (Sytsma et al. 2004; Cordell 2008). Psudodiaptomus forbesi has a substantial period of temporal overlap and potential competition with the native northeast Pacific form of 'Eurytemora affinis' in the Columbia estuary (Bollens et al. 2012).
P260Columbia RiverEcological ImpactCompetition
In the Columbia River, by 2003, P. forbesi had replaced a previous invader, P. inopinus(Sytsma et al. 2004). Psudodiaptomus forbesi has a substantial period of temporal overlap and potential competition with the native northeast Pacific form of 'Eurytemora affinis' in the Columbia estuary (Bollens et al. 2012).
P260Columbia RiverEcological ImpactFood/Prey
In experiments, 3 species of native fishes (Chinook Salmon- Oncorhynchus tschawytscha, Three-spined Stickleback- Gasterosteus aculeatus, and Northern Pikeminnow- Ptychocheilus oregonensis) consumed P. forbesi together with native copepods, but salmon and pikemnnows showed a preference for naitve cladocerans over copepods (Adams et al. 2015).
NEP-IVPuget Sound to Northern CaliforniaEcological ImpactFood/Prey
In experiments, 3 species of native fishes (Chinook Salmon- Oncorhynchus tschawytscha, Three-spined Stickleback- Gasterosteus aculeatus, and Northern Pikeminnow- Ptychocheilus oregonensis) consumed P. forbesi together with native copepods, but salmon and pikemnnows showed a preference for native cladocerans over copepods (Adams et al. 2015).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
NWP-3a None 0 Native Estab
NWP-2 None 0 Native Estab
NEP-V Northern California to Mid Channel Islands 1987 Def Estab
NEP-IV Puget Sound to Northern California 1999 Def Estab
P260 Columbia River 1999 Def Estab
P090 San Francisco Bay 1987 Def Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude
26921 Orsi and Walter 1991 1989 1989-01-01 Delta - Port of Stockton Def 37.9522 -121.3277

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