Potamocorbula amurensis

Invasion History

First Non-native North American Tidal Record: 1986
First Non-native West Coast Tidal Record: 1986
First Non-native East/Gulf Coast Tidal Record:

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General Invasion History:

Corbula amurensis (Asian Brackish-water Clam) is native to estuarine habitats of the Northwest Pacific from the Russian Far East to southern China (Coan 2002). Outside of its native range, it is only known from the San Francisco Bay estuary, California (Carlton et al. 1990, Cohen 2005).

North American Invasion History:

Invasion History on the West Coast:

Corbula amurensis was first collected in Grizzly Bay, in the inner brackish regions of the San Francisco estuary in 1986. By late 1987, it had spread upstream to the outer edge of the Sacramento-San Joaquin Delta (Winder et al. 2011). From 1999 through 2005, the bivalve biomass, dominated by C. amurensis, decreased by 2 orders of magnitude. This was attributed to a cold phase of the El Nino-La Nina oscillation, with increased upwelling and decreased sea-surface temperatures, favoring native cold-water predators, such as shrimp (Crangon spp.), juvenile Dungeness crab (Metacarcinus magister), and English Sole (Parophrys vetulus), resulting in increased predation, decreased bivalve biomass and grazing, and resulting blooms of phytoplankton (Cloern et al. 2007).

Ballast water is the most likely mode of transport for this bivalve to San Francisco Bay, since it has a planktonic larva, spending ~17-19 days in the water column (Nicolini and Penry 2005). However, this species could be spread outside the Bay by other modes of transport. Newly settled larvae and early juveniles secrete byssus threads which can attach to solid particles or surfaces, or to other clams, forming clumps (Nicolini and Penry 2005). Juveniles can be transported in accumulated sediment within fouling communities, and were found in surveys of surplus cargo ships moored in Suisun Bay, which were about to be towed to Texas for ship-breaking (Davidson et al. 2008).

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Description

Potamocorbula amurensis is commonly known as the Asian Brackish-water Clam, Overbite Clam, and Brackish-water Corbula. It is a small, thin-shelled bivalve, reaching 20-25 mm length. The shell is triangular to ovate in shape, with the umbos slightly projecting. The right valve is decidedly larger than the left (whence the name Overbite Clam). The beaks are slightly anterior to the midline of the shells, located at about 41% of the length from the anterior to posterior end. The anterior and posterior ends are sharply rounded. The beaks are smooth, while the rest shell surface has low, irregular ribs. The hinge plate is narrow. The right valve has a narrow tooth, which is attached to the shell wall below the hinge-line. The left valve has a long, weakly projecting chondrophore (a spoon-shaped structure, containing the cartilaginous portion of the ligament connecting the shells) that is conspicuously divided, and with a very small tooth on its posterior end. The anteroventral hinge margin is swollen into a low tooth medially. The pallial line has a small posterior sinus. The shell is white on the interior and exterior, but the exterior is covered with a brown periostracum. This is smooth in younger clams, but wrinkled and worn around the margins in older ones. (Description from: Carlton et al. 1990; Coan et al. 2000; Coan 2002; Cohen 2005; Coan and Valentich-Scott in Carlton 2007; Hallan et al. 2013)

This clam is characteristic of intertidal and shallow-water mud, sand, or clay, often in brackish and estuarine waters (Carlton et al. 1990; Cohen 2005). The planktonic larvae of CP amurensis are described by Nicolini and Penry (2000).

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Taxonomy

Ecology

General:

Potamocorbula amurensis has separate sexes and matures at about 4 mm in length (Parchaso and Thompson 2002), although adults can grow to 20-25 mm (Cohen 2005). Adults mature at a few months age and produce 220,000 eggs (Cohen 2005). Eggs and sperm are released into the water column where they are fertilized. Within 24 hours at 15°C the larvae pass through a blastula stage, reaching a trochophore stage. By 48 hours, they have grown shells and reached the straight-hinge veliger stage. After day 7, the larvae swim less actively, and by day 17, they begin to metaphose and settle at ~ 135 μm in diameter. Viable gametes were produced at 5-25 PSU, and development was fastest at 15 PSU (Nicolini and Penry 2000). Reproduction appears to occur year-round in the San Francisco Bay estuary, and regional variations in peak reproduction appear to reflect water flow and availability of food (suspended organic matter and phytoplankton food), rather than seasonal temperature (Nicolini and Penry 2000; Parchaso and Thompson 2002; Miller and Stillman 2013). At downstream sites in San Pablo Bay, reproduction was most active in dry years, probably reflecting upstream transport of food-rich ocean waters, while at upstream sites in Suisun Bay, reproduction was greatest in wet years when the flux of nutrients and suspended organic matter was greatest (Parchaso and Thompson 2002).

Potamocorbula amurensis occur in sand, mud, and clay substrates, usually with about two-thirds of their body in the sediment, and usually in shallow subtidal environments, though they can be abundant on intertidal mudflats (Carlton et al. 1990; Cohen 2005). It has broad environmental tolerances though it seems most successful under conditions typical of large estuaries with extensive brackish-water regions. The broad latitudinal range of this clam suggests that its temperature tolerance greatly exceeds the seasonal range occurring in San Francisco Bay (Carlton et al. 1990, Cohen 2005). Experimental studies of adult/juvenile temperature tolerance, temperature-salinity interactions, and temperature-salinity effects on larval development are highly desirable. Adult clams are found at 1-30+ PSU, and tolerate a temperature range of 8-23°C in the San Francisco Bay estuary (Carlton et al. 1990; Cohen 2005). Feeding rates and metabolism were highest at a high salinity (28 PSU), compared to a control salinity (13-14 PSU) and low salinity (2 PSU), reflecting higher costs of osmoregulation at high salinity (Paganini et al. 2010).

Food:

Phytoplankton, Detritus

Consumers:

Crabs, Ducks

Competitors:

Bivalves, Zooplankton

Trophic Status:

Suspension Feeder

SusFed

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Habitats

General Habitat	Unstructured Bottom	None
General Habitat	Salt-brackish marsh	None
Salinity Range	Oligohaline	0.5-5 PSU
Salinity Range	Mesohaline	5-18 PSU
Salinity Range	Polyhaline	18-30 PSU
Tidal Range	Subtidal	None
Tidal Range	Low Intertidal	None
Vertical Habitat	Endobenthic	None

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Tolerances and Life History Parameters

Minimum Temperature (ºC)	8	Field observations, San Francisco Bay (Carlton et al. 1990)
Maximum Temperature (ºC)	23	Field observations, San Francisco Bay (Carlton et al. 1990)
Minimum Salinity (‰)	0.1	Experimental, 30 days survival (Carlton et al. 1990).
Maximum Salinity (‰)	32	Field observations, San Francisco Bay (Carlton et al. 1990)
Minimum Reproductive Temperature	6	Field observations, ripe eggs and sperm seen (Parchaso and Thompson 2002)
Maximum Reproductive Temperature	23	Field observations, ripe eggs and sperm seen (Parchaso and Thompson 2002)
Minimum Reproductive Salinity	2	Survival of 24 hr old larvae (Nicolini and Penry 2000).
Maximum Reproductive Salinity	25	Survival of 24 hr old larvae (Nicolini and Penry 2000).
Minimum Duration	17	Fertilization to settling, 15 C (Nicolini and Penry 2000).
Maximum Duration	19	Fertilization to settling, 15 C (Nicolini and Penry 2000).
Minimum Length (mm)	None	None
Maximum Length (mm)	27.5	Carlton et al. 1990
Broad Temperature Range	None	Cold temperate-Warm temperate
Broad Salinity Range	None	Tidal Limnetic-Euhaline

General Impacts

Corbula amurensis has been listed by the Invasive Species Specialist Group of the World Conservation Union (IUCN) as one of the '100 worst invasive species.' So far, San Francisco Bay is the only area that this clam has invaded, but its ecological and economic impacts may be wide-ranging.

Economic Impacts

From a human economic point of view, the impacts of the Corbula invasion are mixed. Filtering by the clams has resulted in increased water clarity in the Delta, probably with some aesthetic and recreational benefits. Some recreationally and aesthetically important species, such as sturgeon and diving ducks may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect. Declines in many recreationally important fish stocks, as well as native endangered species, have been attributed, in part to the food-web changes, although these effects are difficult to separate from the many other human impacts on the inner Bay-Delta system. The Corbula invasion is seen as contributing to a general phenomenon of 'Pelagic Organism Decline', in which the energy and nutrients from primary production (phytoplankton) are being shifted from the pelagic foodweb (phytoplankton > small zooplankton > small fishes > gamefishes) to a benthic foodweb (bivalves and other filter feeders > crabs and shrimp > bottom feeding fishes), which is less desirable economically (Sommer et al. 2007; MacNally 2010; Glibert et al. 2011; Winder and Jassby 2011).

Ecological Impacts

The invasion of C. amurensis has had dramatic effects on the San Francisco Bay estuary. A flood in 1986 may have assisted its invasion by eliminating much of the dry-season benthic community from the upstream portions of the estuary. Within a year of its first detection, huge biomasses of this clam had developed, largely replacing the previous dry-season benthic community, which had included the introduced bivalve Mya arenaria (Softshell Clam), the introduced filter-feeding amphipods Monocorophium acherusicum and Ampelisca abdita, and the introduced polychaete Streblospio benedicti. Corbula’s dominance of the benthos has continued through successive periods of drought and flood, owing to its great tolerance of salinity fluctuations (Nichols et al. 1990). In many locations in Suisun and San Pablo Bay, it now comprises 95% of the filter-feeding biomass (Nichols et al. 1990; Cohen 2005).

The development of a huge biomass of filter-feeding bivalves has dramatically altered the food-web of San Francisco Bay, by suppressing phytoplankton blooms in the upstream reaches of the estuary and diverting biomass from the plankton to the benthos, resulting in sharp decreases in chlorophyll, zooplankton biomass, and in turn, the survival of fish larvae, which depend on zooplankton (Alpine and Cloern 1992; Feyrer et al. 2003). The effects on the zooplankton include direct predation as copepod larvae (nauplii) and microzooplankton are filtered out of the water (Kimmerer et al. 1994; Greene et al. 2011) as well as food deprivation. Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis (Sommer 2007; MacNally 2010; Glibert et al. 2011). In South San Francisco Bay, this impact was reduced from 1999 to 2005, apparently by a cold phase of the El Nino-La Nina oscillation, resulting in reduced water temperatures, increased predation by cold water predators, greatly decreased biomass of C. amurensis and other bivalves, resulting in phytoplankton blooms (Cloern et al. 2007). However, it is not clear whether these effects have extended to the upstream parts of the estuary, including the Delta.

The invasion of C. amurensis has created a major new food source for predators, including mollusk-eating fishes, such as sturgeons, diving ducks, and others. However, as an efficient filter-feeder, C. amurensis also concentrates toxins, such as selenium, pesticides, etc., from the water column (Cohen 2005). While the invasion has resulted in increased aggregations of diving ducks, e.g. Lesser Scaup (Athya affinis), the toxin load from feeding on the clams may be contributing to decreasing success in breeding on the bird’s nesting grounds (Richman and Lovvorn 2004).

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Regional Impacts

NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Herbivory
By 1988, Corbula amurensis had become a dominant filter-feeder in the San Francisco Bay benthic community (Carlton et al. 1990). Its huge biomass resulted in the disappearance of the summer phytoplankton maximum, which normally occurs in years of low river flow (Alpine and Cloern 1992; Jassby et al. 2002)
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Trophic Cascade
The Corbula amurensis invasion and suppression of phytoplankton biomass has had effects throughout the estuary's food-web, resulting in diminished food supplies for other benthic filter-feeders (Nichols et al. 1990), for filter-feeding zooplankton (Kimmerer et al. 1994), and for predators on benthos and zooplankton, such as fishes (Feyrer et al. 2003). Declines in zooplankton biomass resulting from reduced phytoplankton food and direct predation by clams on copepod nauplii have apparently contributed to sharp declines in mysids and fishes (Feyrer et al. 2003). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis and its predatory impact on zooplankton.The development of the large Corbula biomass has also affected the overall flow of nutrients in the ecosystem, including C02, which is released in the process of shell formation (Chauvaud et al. 2003) and dissolved Si (silicon), which is taken up by diatoms, whose biomass has been greatly decreased by grazing (Kimmerer 2005). The invasion has resulted in a significant increase in carbon release by the estuary (Chauvaud et al. 2003) and a sharp decrease in silica uptake (Kimmerer 2005). The decreased diatom biomass in the estuary has also resulted in increased light penetration and a shift in production to other phytoplankton, such as flagellates and cyanobacteria, and to macrophytes (larger floating and submerged plants) such as Egeria densa (Brazilian Waterweed) and Eichornia crassipes (Water Hyacinth) (Jassby 2008). The clam invasion, combined with climate change is beleived to have resulted in a shift in peak phytoplankton and zooplankton biomass to earlier times of year, resulting in a potential mismatch for larvae of Delta Smelt and other planktivorous species (Merz et al. 2018).
P090	San Francisco Bay		Ecological Impact	Herbivory
By 1988, Corbula amurensis had become a dominant filter-feeder in the San Francisco Bay benthic community (Carlton et al. 1990). Its huge biomass resulted in the disappearance of the summer phytoplankton maximum, which normally occurs in years of low river flow (Alpine and Cloern 1992; Jassby et al. 2002).
P090	San Francisco Bay		Ecological Impact	Competition
The invasion of Corbula amurensis was accompanied by declines in the previously dominant, largely introduced, dry-season benthos, including Mya arenaria, Gemma gemma, Ampelisca abdita, Monocorophium acherusicum and Streblospio benedicti (Nichols et al. 1990; Poulton et al. 2004).
P090	San Francisco Bay		Ecological Impact	Predation
The effects on the zooplankton include direct predation as copepod larvae (nauplii) are filtered out of the water (Kimmerer et al. 1994) as well as food deprivation. Predation on copepod nauplii and copepodites, together with decreases in phytoplankton abundance, have led to the decline of the formerly domonant copepod Eurytemora carolleeae (=E. affinis (Kimmerer and Lougee 2015). Grazing rates of C. amurensis on cilate microzooplankton also were significant, exceeding estimated growth rates, and potentially disrupting a link in the microbial food-web (Greene et al. 2011). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis (Feyrer et al. 2003).
P090	San Francisco Bay		Ecological Impact	Trophic Cascade
The Corbula amurensis invasion and suppression of phytoplankton biomass has had effects throughout the estuary's food-web, resulting in diminished food supplies for other benthic filter-feeders (Nichols et al. 1990), for filter-feeding zooplankton (Kimmerer et al. 1994), and for predators on benthos and zooplankton, such as fishes (Feyrer et al. 2003). Declines in zooplankton biomass resulting from reduced phytoplankton food and direct predation by clams on copepod nauplii have apparently contributed to sharp declines in mysids and fishes (Feyrer et al. 2003). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis and its predatory impact on zooplankton.The development of the large Corbula biomass has also affected the overall flow of nutrients in the ecosystem, including C02, which is released in the process of shell formation (Chauvaud et al. 2003) and dissolved Si (silicon), which is taken up by diatoms, whose biomass has been greatly decreased by grazing (Kimmerer 2005). The invasion has resulted in a significant increase in carbon release by the estuary (Chauvaud et al. 2003) and a sharp decrease in silica uptake (Kimmerer 2005). The decreased diatom biomass in the estuary has also resulted in increased light penetration and a shift in production to other phytoplankton, such as flagellates and cyanobacteria, and to macrophytes (larger floating and submerged plants) such as Egeria densa (Brazilian Waterweed) and Eichornia crassipes (Water Hyacinth) (Jassby 2008). Since the invasion of C. amurensis, the peak of phytoplankton productivity has shifted to earlier in the year, shifting the peak of zooplankton abundance, resulting in a possible mismatch between the availability of prey and the larval period of the Delta Smelt (Merz et al. 2016).
P090	San Francisco Bay		Ecological Impact	Food/Prey
Since its invasion, Corbula amurensis had become a major prey item for sturgeon and diving ducks, such as the Lesser Scaup (Athya affinis). Numbers of scaup aggregating in San Pablo and Suisun Bay increased following the invasion (Richman and Lovvorn 2004). During a period of colder water, in 1999-2004, the abundance of C. amurensis and other bivalves decreased, apparently as a result of an influx of cool-water predators, including shrimp (Crangon sp.), Dungeness Crabs (Metacarcinus magister) and English Sole (Parophrys vetulus) (Cloern et al. 2007). Corbula amurensis has become the dominant prey item of the White Sturgeon (Acipenser transmontanus), but the poorer food quality of the invading clams, and a reduction in benthic diversity, have led to a dietary shift including an increased consumpiton of fish (Zeug et al. 2014).
P090	San Francisco Bay		Ecological Impact	Toxic
Corbula amurensis efficiently concentrates toxins, such as selenium, pesticides, etc., from the water column (Cohen 2005; Lee et al. 2006). While the invasion has resulted in increased aggregations of diving ducks, e.g. Lesser Scaup (Athya affinis), the toxin load from feeding on the clams may be contributing to decreasing success in breeding on the bird's nesting grounds (Richman and Lovvorn 2004).
P090	San Francisco Bay		Economic Impact	Fisheries
Some recreationally and aesthetically important species, such as sturgeon (Cohen 2005) and diving ducks (Richman and Lovvorn 2004) may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect. Declines in many recreationally important fish stocks, such as Striped Bass (Morone saxatilis), as well as native endangered species, such as Delta Smelt (Hypomesus transpacificus) have been attributed, in part to the food-web changes (Feyrer et al. 2003), although these effects are difficult to separate from the many other human impacts on the inner Bay-Delta system.
P090	San Francisco Bay		Economic Impact	Aesthetic
Filtering by the clams has resulted in increased water clarity in the Delta, probably with some aesthetic and recreational benefits. Some recreationally and aesthetically important species, such as sturgeon and diving ducks may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect.
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Competition
The invasion of Corbula amurensis was accompanied by declines in the previously dominant, largely introduced, dry-season benthos, including Mya arenaria, Gemma gemma, Ampelisca abdita, Monocorophium acherusicum and Streblospio benedicti (Nichols et al. 1990; Poulton et al. 2004).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Predation
The effects on the zooplankton include direct predation as copepod larvae (nauplii) are filtered out of the water (Kimmerer et al. 1994) as well as food deprivation. Grazing rates of C. amurensis on cilate microzooplankton also were significant, exceeding estimated growth rates, and potentially disrupting a link in the microbial food-web (Greene et al. 2011). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to the decreased food availability resulting from the huge filtering biomass of C. amurensis (Feyrer et al. 2003).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Food/Prey
Since its invasion, Corbula amurensis had become a major prey item for sturgeon and diving ducks, such as the Lesser Scaup (Athya affinis). Numbers of scaup aggregating in San Pablo and Suisun Bay increased following the invasion (Richman and Lovvorn 2004). During a period of colder water, in 1999-2004, the abundance of C. amurensis and other bivalves decreased, apparently as a result of an influx of cool-water predators, including shrimp (Crangon sp.), Dungeness Crabs (Metacarcinus magister) and English Sole (Parophrys vetulus) (Cloern et al. 2007).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Toxic
Corbula amurensis efficiently concentrates toxins, such as selenium, pesticides, etc., from the water column (Cohen 2005; Lee et al. 2006). While the invasion has resulted in increased aggregations of diving ducks, e.g. Lesser Scaup (Athya affinis), the toxin load from feeding on the clams may be contributing to decreasing success in breeding on the bird's nesting grounds (Richman and Lovvorn 2004).
NEP-V	Northern California to Mid Channel Islands		Ecological Impact	Habitat Change
Intense grazing of phytoplankton by Corbula amurenisis has affected the sediment by adding large quantities of pseudofeces, increasing the amount of suspended particles (Carlton et al. 1990). Grazing by C. amurenisis has decreased phytoplankton biomass, potentially increasing water clarity, and favoring submersed vegetation (Jassby 2008). Pseudofeces, bound by mucus, produced by Corbula, as well as the mucus produced by other native and introduced deposit feeding and tube-building benthos, contributes to a surface layer of flocculent fluff, which may trap much more phtyoplankton than is actually consumed by the animals (Jones et al. 2009).
NEP-V	Northern California to Mid Channel Islands		Economic Impact	Fisheries
Some recreationally and aesthetically important species, such as sturgeon (Cohen 2005) and diving ducks (Richman and Lovvorn 2004) may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect. Declines in many recreationally important fish stocks, such as Striped Bass (Morone saxatilis), as well as native endangered species, such as Delta Smelt (Hypomesus transpacificus) have been attributed, in part to the food-web changes (Feyrer et al. 2003), although these effects are difficult to separate from the many other human impacts on the inner Bay-Delta system.
NEP-V	Northern California to Mid Channel Islands		Economic Impact	Aesthetic
Filtering by the clams has resulted in increased water clarity in the Delta, probably with some aesthetic and recreational benefits. Some recreationally and aesthetically important species, such as sturgeon and diving ducks may be benefiting from increased food supplies (Richman and Lovvorn 2004) but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect (Cohen 2005).
P090	San Francisco Bay		Ecological Impact	Habitat Change
Intense grazing of phytoplankton by Corbula amurenisis has affected the sediment by adding large quantities of pseudofeces, increasing the amount of suspended particles (Carlton et al. 1990). Grazing by C. amurenisis has decreased phytoplankton biomass, potentially increasing water clarity, and favoring submersed vegetation (Jassby 2008). Pseudofeces, bound by mucus, produced by Corbula, as well as the mucus produced by other native and introduced deposit feeding and tube-building benthos, contributes to a surface layer of flocculent fluff, which may trap much more phtyoplankton than that actually consumed by the animals (Jones et al. 2009).
CA	California		Ecological Impact	Competition
The invasion of Corbula amurensis was accompanied by declines in the previously dominant, largely introduced, dry-season benthos, including Mya arenaria, Gemma gemma, Ampelisca abdita, Monocorophium acherusicum and Streblospio benedicti (Nichols et al. 1990; Poulton et al. 2004)., The invasion of Corbula amurensis was accompanied by declines in the previously dominant, largely introduced, dry-season benthos, including Mya arenaria, Gemma gemma, Ampelisca abdita, Monocorophium acherusicum and Streblospio benedicti (Nichols et al. 1990; Poulton et al. 2004).
CA	California		Ecological Impact	Food/Prey
Since its invasion, Corbula amurensis had become a major prey item for sturgeon and diving ducks, such as the Lesser Scaup (Athya affinis). Numbers of scaup aggregating in San Pablo and Suisun Bay increased following the invasion (Richman and Lovvorn 2004). During a period of colder water, in 1999-2004, the abundance of C. amurensis and other bivalves decreased, apparently as a result of an influx of cool-water predators, including shrimp (Crangon sp.), Dungeness Crabs (Metacarcinus magister) and English Sole (Parophrys vetulus) (Cloern et al. 2007)., Since its invasion, Corbula amurensis had become a major prey item for sturgeon and diving ducks, such as the Lesser Scaup (Athya affinis). Numbers of scaup aggregating in San Pablo and Suisun Bay increased following the invasion (Richman and Lovvorn 2004). During a period of colder water, in 1999-2004, the abundance of C. amurensis and other bivalves decreased, apparently as a result of an influx of cool-water predators, including shrimp (Crangon sp.), Dungeness Crabs (Metacarcinus magister) and English Sole (Parophrys vetulus) (Cloern et al. 2007). Corbula amurensis has become the dominant prey item of the White Sturgeon (Acipenser transmontanus), but the poorer food quality of the invading clams, and a reduction in benthic diversity, have led to a dietary shift including an increased consumpiton of fish (Zeug et al. 2014).
CA	California		Ecological Impact	Habitat Change
Intense grazing of phytoplankton by Corbula amurenisis has affected the sediment by adding large quantities of pseudofeces, increasing the amount of suspended particles (Carlton et al. 1990). Grazing by C. amurenisis has decreased phytoplankton biomass, potentially increasing water clarity, and favoring submersed vegetation (Jassby 2008). Pseudofeces, bound by mucus, produced by Corbula, as well as the mucus produced by other native and introduced deposit feeding and tube-building benthos, contributes to a surface layer of flocculent fluff, which may trap much more phtyoplankton than is actually consumed by the animals (Jones et al. 2009)., Intense grazing of phytoplankton by Corbula amurenisis has affected the sediment by adding large quantities of pseudofeces, increasing the amount of suspended particles (Carlton et al. 1990). Grazing by C. amurenisis has decreased phytoplankton biomass, potentially increasing water clarity, and favoring submersed vegetation (Jassby 2008). Pseudofeces, bound by mucus, produced by Corbula, as well as the mucus produced by other native and introduced deposit feeding and tube-building benthos, contributes to a surface layer of flocculent fluff, which may trap much more phtyoplankton than that actually consumed by the animals (Jones et al. 2009).
CA	California		Ecological Impact	Herbivory
By 1988, Corbula amurensis had become a dominant filter-feeder in the San Francisco Bay benthic community (Carlton et al. 1990). Its huge biomass resulted in the disappearance of the summer phytoplankton maximum, which normally occurs in years of low river flow (Alpine and Cloern 1992; Jassby et al. 2002), By 1988, Corbula amurensis had become a dominant filter-feeder in the San Francisco Bay benthic community (Carlton et al. 1990). Its huge biomass resulted in the disappearance of the summer phytoplankton maximum, which normally occurs in years of low river flow (Alpine and Cloern 1992; Jassby et al. 2002).
CA	California		Ecological Impact	Predation
The effects on the zooplankton include direct predation as copepod larvae (nauplii) are filtered out of the water (Kimmerer et al. 1994) as well as food deprivation. Grazing rates of C. amurensis on cilate microzooplankton also were significant, exceeding estimated growth rates, and potentially disrupting a link in the microbial food-web (Greene et al. 2011). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to the decreased food availability resulting from the huge filtering biomass of C. amurensis (Feyrer et al. 2003)., The effects on the zooplankton include direct predation as copepod larvae (nauplii) are filtered out of the water (Kimmerer et al. 1994) as well as food deprivation. Predation on copepod nauplii and copepodites, together with decreases in phytoplankton abundance, have led to the decline of the formerly domonant copepod Eurytemora carolleeae (=E. affinis (Kimmerer and Lougee 2015). Grazing rates of C. amurensis on cilate microzooplankton also were significant, exceeding estimated growth rates, and potentially disrupting a link in the microbial food-web (Greene et al. 2011). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis (Feyrer et al. 2003).
CA	California		Ecological Impact	Trophic Cascade
The Corbula amurensis invasion and suppression of phytoplankton biomass has had effects throughout the estuary's food-web, resulting in diminished food supplies for other benthic filter-feeders (Nichols et al. 1990), for filter-feeding zooplankton (Kimmerer et al. 1994), and for predators on benthos and zooplankton, such as fishes (Feyrer et al. 2003). Declines in zooplankton biomass resulting from reduced phytoplankton food and direct predation by clams on copepod nauplii have apparently contributed to sharp declines in mysids and fishes (Feyrer et al. 2003). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis and its predatory impact on zooplankton.The development of the large Corbula biomass has also affected the overall flow of nutrients in the ecosystem, including C02, which is released in the process of shell formation (Chauvaud et al. 2003) and dissolved Si (silicon), which is taken up by diatoms, whose biomass has been greatly decreased by grazing (Kimmerer 2005). The invasion has resulted in a significant increase in carbon release by the estuary (Chauvaud et al. 2003) and a sharp decrease in silica uptake (Kimmerer 2005). The decreased diatom biomass in the estuary has also resulted in increased light penetration and a shift in production to other phytoplankton, such as flagellates and cyanobacteria, and to macrophytes (larger floating and submerged plants) such as Egeria densa (Brazilian Waterweed) and Eichornia crassipes (Water Hyacinth) (Jassby 2008). The clam invasion, combined with climate change is beleived to have resulted in a shift in peak phytoplankton and zooplankton biomass to earlier times of year, resulting in a potential mismatch for larvae of Delta Smelt and other planktivorous species (Merz et al. 2018)., The Corbula amurensis invasion and suppression of phytoplankton biomass has had effects throughout the estuary's food-web, resulting in diminished food supplies for other benthic filter-feeders (Nichols et al. 1990), for filter-feeding zooplankton (Kimmerer et al. 1994), and for predators on benthos and zooplankton, such as fishes (Feyrer et al. 2003). Declines in zooplankton biomass resulting from reduced phytoplankton food and direct predation by clams on copepod nauplii have apparently contributed to sharp declines in mysids and fishes (Feyrer et al. 2003). Decreased recruitment of many species of fishes, including the economically important introduced Striped Bass (Morone saxatilis) and the endangered Delta Smelt (Hypomesus transpacificus) has been attributed, in part, to decreased food availability resulting from the huge filtering biomass of C. amurensis and its predatory impact on zooplankton.The development of the large Corbula biomass has also affected the overall flow of nutrients in the ecosystem, including C02, which is released in the process of shell formation (Chauvaud et al. 2003) and dissolved Si (silicon), which is taken up by diatoms, whose biomass has been greatly decreased by grazing (Kimmerer 2005). The invasion has resulted in a significant increase in carbon release by the estuary (Chauvaud et al. 2003) and a sharp decrease in silica uptake (Kimmerer 2005). The decreased diatom biomass in the estuary has also resulted in increased light penetration and a shift in production to other phytoplankton, such as flagellates and cyanobacteria, and to macrophytes (larger floating and submerged plants) such as Egeria densa (Brazilian Waterweed) and Eichornia crassipes (Water Hyacinth) (Jassby 2008). Since the invasion of C. amurensis, the peak of phytoplankton productivity has shifted to earlier in the year, shifting the peak of zooplankton abundance, resulting in a possible mismatch between the availability of prey and the larval period of the Delta Smelt (Merz et al. 2016).
CA	California		Ecological Impact	Toxic
Corbula amurensis efficiently concentrates toxins, such as selenium, pesticides, etc., from the water column (Cohen 2005; Lee et al. 2006). While the invasion has resulted in increased aggregations of diving ducks, e.g. Lesser Scaup (Athya affinis), the toxin load from feeding on the clams may be contributing to decreasing success in breeding on the bird's nesting grounds (Richman and Lovvorn 2004)., Corbula amurensis efficiently concentrates toxins, such as selenium, pesticides, etc., from the water column (Cohen 2005; Lee et al. 2006). While the invasion has resulted in increased aggregations of diving ducks, e.g. Lesser Scaup (Athya affinis), the toxin load from feeding on the clams may be contributing to decreasing success in breeding on the bird's nesting grounds (Richman and Lovvorn 2004).
CA	California		Economic Impact	Aesthetic
Filtering by the clams has resulted in increased water clarity in the Delta, probably with some aesthetic and recreational benefits. Some recreationally and aesthetically important species, such as sturgeon and diving ducks may be benefiting from increased food supplies (Richman and Lovvorn 2004) but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect (Cohen 2005)., Filtering by the clams has resulted in increased water clarity in the Delta, probably with some aesthetic and recreational benefits. Some recreationally and aesthetically important species, such as sturgeon and diving ducks may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect.
CA	California		Economic Impact	Fisheries
Some recreationally and aesthetically important species, such as sturgeon (Cohen 2005) and diving ducks (Richman and Lovvorn 2004) may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect. Declines in many recreationally important fish stocks, such as Striped Bass (Morone saxatilis), as well as native endangered species, such as Delta Smelt (Hypomesus transpacificus) have been attributed, in part to the food-web changes (Feyrer et al. 2003), although these effects are difficult to separate from the many other human impacts on the inner Bay-Delta system., Some recreationally and aesthetically important species, such as sturgeon (Cohen 2005) and diving ducks (Richman and Lovvorn 2004) may be benefiting from increased food supplies, but negative effects from concentrated toxins on the populations and human consumers will be more difficult to detect. Declines in many recreationally important fish stocks, such as Striped Bass (Morone saxatilis), as well as native endangered species, such as Delta Smelt (Hypomesus transpacificus) have been attributed, in part to the food-web changes (Feyrer et al. 2003), although these effects are difficult to separate from the many other human impacts on the inner Bay-Delta system.

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