1

Corbula amurensis Conceptual Model 

Janet K. Thompson (jthompso@usgs.gov) and Francis Parchaso 

U.S. Geological Survey 

Peer Reviewed 

October 2010 
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PREFACE 

This Conceptual Model is part of a suite of conceptual models that collectively 

articulate the current scientific understanding of important aspects of the SacramentoSan Joaquin River Delta ecosystem. The conceptual models are designed to aid in the 

identification and evaluation of ecosystem restoration actions in the Delta. These 

models are designed to structure scientific information such that it can be used to 

inform sound public policy. 

The Delta Conceptual Models include both ecosystem element models (including 

process, habitat, and stressor models) and species life history models. The models 

were prepared by teams of experts using common guidance documents developed to 

promote consistency in the format and terminology of the models 

http://www.delta.dfg.ca.gov/erpdeltaplan/science_process.asp . 

The Delta Conceptual Models are qualitative models that describe current 

understanding of how the system works. They are designed and intended to be used 

by experts to identify and evaluate potential restoration actions. They are not 

quantitative, numeric computer models that can be “run” to determine the effects of 

actions. Rather they are designed to facilitate informed discussions regarding 

expected outcomes resulting from restoration actions and the scientific basis for those 

expectations. The structure of many of the Delta Conceptual Models can serve as the 

basis for future development of quantitative models. 

Each of the Delta Conceptual Models has been, or is currently being subject to a 

rigorous scientific peer review process. The peer review status of each model is 

indicated on the title page of the model. 

The Delta Conceptual models will be updated and refined over time as new 

information is developed, and/or as the models are used and the need for further 

refinements or clarifications are identified. 
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CONTENTS 

I. Introduction ......................................................................................................................... 4

A. A note on Corbula amurensis taxonomy and nomenclature ........................................ 5

B. Status and invasion of Corbula amurensis in San Francisco Estuary.......................... 5

II. Biology .......................................................................................................................... 6

III. Distribution .................................................................................................................... 7

IV. Ecology ......................................................................................................................... 8

A. Ecological Influences on Corbula’s Life Cycle .............................................................. 8

1. Gametogenesis ........................................................................................................ 8

2. Spawning and Fertilization ....................................................................................... 9

3. Pelagic Larvae .......................................................................................................... 9

4. Recruitment ............................................................................................................ 10

5. Adult Corbula .......................................................................................................... 12

B. Corbula’s Control of Phytoplankton Biomass ............................................................. 14

C. Negative Influence of the Trophic Transfer of Contaminants ..................................... 16

D. Constraints on Restoration ......................................................................................... 17

V. Future research .......................................................................................................... 18

VI. References ................................................................................................................. 20

VII. Tables ......................................................................................................................... 25

VIII. Figures ........................................................................................................................ 28
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I. Introduction 

C. amurensis is a pest in the San Francisco Estuary (hereafter SFE) due to its ability 

to heavily graze pelagic food resources and its accumulation of toxic substances that 

are then moved into the food web through its predators. We begin by describing what 

we know of its life history and establish its life cycle within the framework of the 

ecological setting of the SFE. This model should be used within the context of a 

thought experiment, as there is not enough known about the species to design a 

strategy to control it in the estuary or Delta. The ultimate purpose of this conceptual 

model of Corbula amurensis is to explore its weaknesses. 

The geographic focus of the DRERIP models is the Delta. However we examine the 

life history patterns of Corbula amurensis as far down bay as San Pablo Bay for two 

reasons. First, because some of the largest ecosystem effects attributed to C. 

amurensis are its consumption of pelagic carbon resources, it is necessary to include 

areas adjacent to the Delta because of the effects of tidal dispersion. Second, many of 

the life cycle patterns that are visible in San Pablo Bay are transferrable to upstream 

locations during drought periods. 

Finally, we expect this model to evolve as a result of the many ongoing studies on C. 

amurensis. Some of the data shown in this discussion are the direct result of current 

CALFED studies being jointly done by California Department of Water Resources 

(EMP Program D. Messer and K. Gerhts) and the USGS (Analysis of Archived 

Samples to Assess Patterns of Historic Invasive Bivalve Biomass). Our coinvestigators on that study have been gracious enough to allow us to show some of 

that data here, before it has been published. 

We start the model discussion with a brief description of the taxonomic status of this 

bivalve because the utility of the literature available to us is based on assumptions of 

the correct identification. We also very briefly describe its introduction into the 

estuary as a reminder of the facts about its invasion. 
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A. A note on Corbula amurensis taxonomy and nomenclature 

Corbula amurensis (Schrenck, 1861) was first identified as Potamocorbula amurensis

by Carlton et al. (1990) with the aid of A. Matsukuma (National Science Museum, 

Tokyo) who was starting a revision of the genus at that time. Coan (2002) in his 

analysis of Eastern Pacific Corbulidae revised the identification to Corbula amurensis

but he warned that it may be changed again once the Asian Corbula group is revised. 

The confusion in the taxonomy of this group makes it difficult for us to establish a 

definitive world-wide geographic distribution and for us to confidently reference 

literature from areas outside SFE. Sato and Azuma (2002) have examined the C. 

amurensis from the SFE and believe it to be more similar to a newly invasive 

Potamocorbula species observed in the Ariake Sea than to the native Potamocorbula 

amurensis seen in northeast Japan. The species described by Sato and Azuma (2002) 

as Potamocorbula sp. was not seen in Japan before 1990 and probably arrived from 

China or Korea with shipments of Corbicula. If it is the same species as found in SFE, 

it would be the second invasion location for the species. 

B. Status and invasion of Corbula amurensis in San Francisco Estuary 

Carlton et al. (1990) hypothesize that C. amurensis (hereafter Corbula) was 

introduced into the SFE in ballast water, most likely originating from an Asian port. 

The first specimen was discovered in late 1986, when juveniles were collected in the 

northern estuary. Within a year the species had spread throughout the northern 

estuary, and within 18 months had become the dominant bivalve in the northern 

estuary (Nichols et al., 1990). It spread to the southern estuary within two years 

where it remains a common and frequently dominant bivalve (Thompson, et al. 2008) 

during years when bivalves occur in South Bay (Cloern et al. 2007). Corbula is on 

the alert list for ISSG (Invasive Species Specialist Group) where it is listed as among 

the 100 World’s Worst Invaders, is listed as a Pest by NIMBUS (National Introduced 

Marine Pest Information System) (Hewitt et al., 2002), and has classified as one of 

“six exotic high impact species” included in an early detection surveillance system in 

New Zealand (Global Invasive Species Database, 2005). It’s presence on these lists is 

due in part to its ecosystem effects in the SFE. 
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II.Biology 

Corbula amurensis is a dioecious (sexes are separate), broadcast spawning bivalve 

with external fertilization (Figure 1). The fall and summer periods of gametogenesis 

usually last 1-2 months although Parchaso (1993) reported animals in San Pablo and 

South Bays to be almost continuously reproductively active in 1989-1990 during the 

1987-1992 drought. 

Females produce 45,000-220,000 oocytes. The oocytes (Table 1) are not buoyant and 

are fertilized near the bed (Nicolini and Penry 2000). Males produce sperm that are 

active for up to 12 hours in 15oC water. Non-mobile, non-feeding trochophore larvae 

develop within 24 hours of fertilization. Suspension feeding veliger larvae develop 

24 hours later and swim actively to day 7; thereafter they swim intermittently until 

they settle at day 17-19. 

Juvenile clams become reproductively active at about 5mm in length (Parchaso and 

Thompson 2002) which can occur within 2 months of recruitment. Corbula lives 2- 

2.5 years at most locations and commonly reproduce twice a year but can 

continuously reproduce if conditions permit. Recruitment variability is a function of 

the availability of adults to produce larvae and site specific environmental conditions. 

Thus recruitment timing varies within the estuary. Recruitment usually occurs in the 

western Delta (Chipps Island) in fall, in the northern estuary (Grizzly bay) in early 

spring through fall, and in San Pablo Bay in late spring/early summer (Figures 2a, b, 

and c). Recruitment occurs over several months (Figure 2d) and a second recruitment 

period may occur at all locations although it is most common in the Grizzly 

Bay/Suisun Bay populations (Parchaso and Thompson 2002). 

Corbula populations have wide ranges in abundance within years and between years 

(Table 2). The maximum Corbula abundance, observed thus far was in San Pablo 

Bay, and the maximum biomass was seen in Carquinez Strait. The examples shown 

in Figure 3 of Corbula abundance and biomass at two locations in SFE are 

representative of three types of variability observed in the estuary: (1) The lack of a 

predictive relationship between biomass and abundance is shown at the freshest 

channel station (near Chipps Island) in Figure 3a; note the similarity in biomass in the 
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drought years (1987-1992) and the most recent years (1999-2006) despite much 

higher abundance in recent years. The relative increase in biomass during the drought 

when phytoplankton biomass was uniformly low (Alpine and Cloern 1992) is driven 

by different year class structure (older year classes during the drought) and changes in 

clam condition (weight for a specific length of clam was also greater during the 

drought). (2) Corbula populations in the shallow Grizzly Bay water (Figure 3b) have 

the most consistent seasonal patterns with biomass peaking in late fall/winter and 

abundance peaking in summer/early fall. (3) The relatively steady biomass in Grizzly 

Bay during the drought years (1987-1992) occurred because of the near continuous 

recruitment of Corbula during this period. 

III.Distribution 

A genetic study of Corbula in SFE (Duda 1994) found differentiation among 

populations to be low with all populations showing a high degree of variability. This 

is a genetic characterization of a species equipped to adapt to a large number of 

environments which we have found is true. Based on the few physiological studies 

that have been completed, the primary limiting environmental factor on Corbula’s

distribution is likely to be salinity. Neither distribution nor reproduction appears to be 

temperature limited within the range of temperatures commonly found in the estuary. 

If the species identification is accurate, Corbula’s native range is from 22o to 55o N 

latitude in the western Pacific and thus it is unlikely that the temperature regime in 

SFE would be limiting. The salinity range for adult animals, 0.1-32 (Werner and 

Hinton 2000 and Carlton et al. 1990), and recruits, 2-30 (Nicolini and Penry 2000), is 

sufficiently broad to encompass most of the northern estuary in most seasons and 

years (Figure 4a and 4b). 

Corbula’s distribution in the estuary has changed very little since its initial invasion 

with the exception of a broader distribution in the southern estuary (Thompson et al. 

2008). The similarity in Corbula’s distribution in the spring of drought (1988) and 

wet (1995) years (Figure 4a) is indicative of the intrusion of some salt beyond the 

confluence of the rivers most years. Corbula’s distribution in fall is more variable 

and shows saltwater intrusion up the rivers and into the western Delta with the 

reduced flow during the drought (1990) and during the latter part of 2006. The latter 
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observation shows the variability of salinity between the seasons as 2006 was a wet 

year but with very low freshwater flow in late summer and fall. 

Corbula is capable of living in brackish marshes as shown by its presence in Suisun 

Marsh in 1990 (Figure 4b). Its distribution in the marsh declines with increased 

freshwater flow as seen in the fall 1995 distribution. It is unknown if Corbula can 

live within Egeria densa beds; limited sampling within the beds by the U.S.G.S. in 

2001 and 2003 revealed only Corbicula fluminea (U.S.G.S. unpublished data). 

However the salinity tolerances of Corbula and Egeria densa overlap, so it is possible 

that they could co-occur. 

Corbula has been observed in all habitats except epifaunal (i.e. attached to hard 

substrate) habitats in the SFE. It prefers mid-intertidal to subtidal ranges but large 

populations can be found in the high intertidal. Individuals have been collected in silt, 

clay, hard-pack clay, sand, gravel, peaty mud, and shell hash. They maintain their 

position in hard-pack clay or high velocity areas with a single byssal thread that 

passes through the anterior end of the shell and attaches to a piece of debris in the 

sediment. Animals live with one-half to two-thirds of their shell exposed which is 

verified by the presence of live barnacles on the posterior end of many shells. 

IV. Ecology 

Corbula spends most of its 2-3 year life in the sediment and spends about 3 weeks as 

a larva in the water column (Figures 1 and 5) when it can disperse and expand the 

population distribution. Thus environmental conditions, predators, and food quantity 

and quality vary throughout the life cycle of the bivalve. 

A. Ecological Influences on Corbula’s Life Cycle 

1. Gametogenesis 

The salinity range needed for the initiation of gametogenesis (0.1-28) is quite wide 

(Parchaso and Thompson 2002) and thus most of the adult bivalves present in the 

northern estuary are reproductively active. Almost 100% of the adult bivalves in a 

population are reproductive at the initiation of gametogenesis in each year. 

Gametogenic activity has been linked to food availability by Parchaso and Thompson 

(2002). Although chlorophyll a concentrations (used as a measure of phytoplankton 
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biomass) were quite low during the Parchaso and Thompson (2002) study, they did 

not observe non-reproductive populations; this fact is most likely a reflection of the 

lack of data on other food sources used by Corbula such as bacteria (Werner and 

Hollibaugh 1993). 

2. Spawning and Fertilization 

Corbula can be induced to spawn in the laboratory by withholding food and stressing 

the animals (Nicolini and Penry 2000) but it is unknown if these triggers apply to field 

animals. One of the few identified environmental limits on Corbula’s reproduction 

and thus potentially on its distribution, is the salinity limit (5-25) at which spawning 

and fertilization can occur (Figures 5 and 6). It should be noted that although 

Nicolini and Penry (2000) found animals did not spawn at 2 and 32, they did not test 

salinities >2 and <5, and salinities >25 and <32. Within the 5-25 salinity range, both 

eggs and sperm are quite resilient and can tolerate a 10 step salinity change. 

Similarly, two hour old embryos that develop in a salinity of 15, tolerate salinities 

ranging from 10-30 in laboratory tests. 

Like all broadcast spawning animals with external fertilization, successful 

reproduction is a result of a favorable chemical and physical environment at the time 

of spawning and fertilization (Figure 6). Nicolini and Penry (2000) found that the 

eggs settled to the bottom on release and that the sperm were free swimming, so it is 

unclear what percentage of fertilization occurs at the bed and what percentage of 

fertilization occurs in the water column with resuspended eggs. In either case, the 

near-bed hydrodynamics and currents will be a major determinant of reproductive 

success and the geographic position of the embryo as it develops into a larva. 

3. Pelagic Larvae 

Corbula has two pelagic larval stages with broad salinity tolerances (Figure 5, 2-30). 

The short lived trochophore larvae are non-swimming and non-feeding and are thus 

vulnerable to transport and mixing into non-hospitable water (Figure 6). The veliger 

larva is not a dynamic swimmer but can move vertically in the water column and thus 

may be able to control some of its environment. Veliger larvae of this size mostly eat 

cyanobacteria and small autotrophic flagellates (Raby et al. 1997). Predators on both 

larval forms are likely to be the same as those on other zooplankton in the estuary. 
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Transport of embryos and larvae with the currents is a factor in the successful 

maintenance and spread of this species’ populations and is most likely a factor in the 

timing of recruitment in the northernmost estuary (Figures 2a and 3). Even if the 

salinity is acceptable in spring, the residual flow during the spring outflow period is 

down-estuary throughout spring of most normal freshwater outflow years. Thus the 

largest number of larvae settles into this reach of the estuary during fall when both 

currents and salinity are favorable. In contrast to this pattern, Thompson et al. (2008) 

show that the 2-3 weeks that Corbula larvae are pelagic are sufficient for larvae 

produced in the northern estuary to transit the 40+km and settle in the southern 

estuary. Thus, it is probable that populations throughout the estuary co-mingle and 

populations that are eliminated by mortality and inhospitable water quality conditions 

during one season are re-established by larvae from non-local populations. Because of 

the hydrologic variability in the estuary, the elimination of populations in the shallow 

water in most years (Thompson 2008), and the species short life span, the 

repopulation of an area by non-local larvae is a critical component of the success of 

this species in the estuary. 

4. Recruitment 

As the veliger larva begins its transformation to become a benthic organism it swims 

less frequently and depends on the turbulence to transport it to the bed. Once there it 

has a short period of time to attach and burrow or to remain passive and be moved 

back into the water column (Crimaldi et al. 2002). There are several factors that 

determine the larva’s transition from pelagic to benthic organism. First the water 

quality has to be acceptable and the salinity must fall in the 2-30 range. For this 

reason the abundance of recruits at a location can be related to the position of X2 in 

this estuary, as it is the bottom salinity that determines the success of the settling 

larvae (Figures 6 and 7). Second the physical environment may be such that the time 

to attach to the bottom is too short before the turbulence re-suspends the larvae. One 

factor shown to affect the time for attachment is the density of adult Corbula at a 

location (Crimaldi et al. 2002). In addition, adult Corbula may limit their own 

success by filtering their larvae out of the water column in a manner similar to that 

described for copepod nauplii (Kimmerer et al. 1994). The combination of these 

factors results in a threshold of adult Corbula beyond which Corbula recruits are less 

likely to settle. Our data show that at one location (shallow water Grizzly Bay) adult 
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densities above about 2000 m-2 greatly limit the success of recruits (Figure 8a). If 

conditions allow the estuarine amphipod Ampelisca abdita or some other surface 

tube-dweller to invade, we see a threshold effect on the Corbula recruits as was seen 

with adult Corbula. Nichols and Thompson (1985) reported a similar relationship 

between the bivalve Macoma balthica (now recognized as M. petalum) and A. abidita

in the southern estuary. Although high abundances of adult Corbula can co-occur 

with A. abdita, recruits appear to be limited either by the flow disturbance of the tubes 

(Friedrichs et al. 2000) or the feeding activities of the suspension-feeding amphipod 

(Figure 8b). 

The number of recruits is highly variable and, in addition to the intra- and 

interspecific competition noted above, predation and environmental variability are 

likely responsible for much of the extreme variability (Figure 6). We have observed 

that the number of recruits in the shallows of North Bay is only reduced by freshwater 

outflow when we have prolonged, high outflow events as were seen in 1995 and 1998 

(Figures 3a and 3c). When a high outflow event is followed by extremely low 

freshwater flow in summer and fall (e.g. 2006) we have observed near record number 

of recruits (reflected in the abundance in Figure 3). Such patterns may be related to 

the removal of most benthic organisms during these flood events, leaving a 

depauparate benthic community that is therefore available to an opportunist like 

Corbula (Nichols et al.1990). 

Although there have been few studies on the predation on juvenile Corbula, we 

observed in the 1980’s that a remarkable number of clams 10+mm in length from 

Carquinez Straits showed signs of being handled but not successfully eaten by a 

crustacean (Carlton et al 1990). Smaller clams did not show the chips along the edge 

of the valve, and we interpreted that as a sign that if they were picked up by the 

predator they were successfully consumed. We have since learned that Cancer 

magister, the Dungeness crab, occurs in this part of the estuary and preys on Corbula

(Stewart et al. 2004). Other likely predators on juvenile Corbula include demersal 

fish, diving ducks in the shallow water, shrimp, and the exotic opisthobranch Philine 

auriformis in the more saline portion of the estuary (e.g. San Pablo Bay, Gosliner 

1995). 
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Once settled to the bottom recruits continue to feed on smaller sized pelagic food 

sources due to their small size (Table 1). We have observed growth to be more than 

5mm in length within two months of recruitment in the southern estuary (Thompson 

et al. 2008), so the recruits likely graze on an increasing variety and size of algae and 

bacteria within a month of settling. Achieving reproductive size (Figure 9) is 

dependent on growth rate that varies between years and is likely a result of 

environmental stressors and food quantity and quality. Preliminary estimates of 

growth rate of Grizzly Bay Corbula range from a peak of 3-4 mm/month in spring to 

<1-2 mm/month in fall (analyses by J. Thompson). 

Juvenile and adult Corbula are capable of being transported as bedload and within the 

water column. Small clams 5-8mm have been “sampled” using zooplankton nets in 

the deep water of the estuary (William Bennett pers. Comm., University of California 

Davis). These individuals had a byssal thread still attached and we believe that like 

other bivalves they use the byssal thread for lift and subsequent transport (Beukema 

and Vlas 1989). Transport can result in individuals being both removed and deposited 

at a location (Figure 9). 

5. Adult Corbula 

Adult Corbula (>5mm in length) are remarkably tolerant of salinity changes and their 

wide distribution in the Estuary reflects that tolerance (Table 1, Figure 10). Corbula 

is an osmo-conformer that can adapt to changes in salinity within 48 hours. Adults 

can tolerate very low salinities (0.1) in the laboratory but die with prolonged 

freshwater exposure (Werner et al. 2003). Corbula pays a metabolic price for this 

tolerance. Pagannini et al. (in press) observed that Corbula had 40% lower oxygen 

consumption rates (VO2) at salinities fluctuating between 4 and 14 than the VO2 

observed at high (28) and low (2) salinities. Glycogen (Werner et al. 2003) and lipid 

stores (Richman and Lovvorn 2004) are generally low in Corbula relative to many 

bivalves. Therefore it appears Corbula can not depend on energy stores during 

periods of stress. Pagannini et al. (in press) find that Corbula’s metabolic rate and 

filtration rate is higher at higher salinities (28). They suggest that Corbula’s high 

malate dehydrogenase activity, in combination with a high respiration rate, allows 

them to rapidly oxidize food when it becomes available. Canuel et al (1995) in 

examining the fatty acids in Corbula also suggested that they are “capable of rapid 
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assimilation and incorporation of algal-derived organic matter into their tissues during 

periods of high phytoplankton abundance.” 

Although the data are not shown in their report, McEnnulty et al. (2001) report that 

Corbula “has a high tolerance to low oxygen and is found in polluted or eutrophic 

areas”. 

Corbula is a suspension feeder that can filter and assimilate both phytoplankton and 

bacteria (<1.2 μm) from the water column (Werner and Hollibaugh 1993). Their high 

secondary production in areas with low phytoplankton biomass (Thompson 2005) 

suggests it depends on carbon sources other than phytoplankton. Retention efficiency 

for natural bacteria is lower (28% and 13 % for 1 and 2 cm long clams) than for 

phytoplankton (100%). Bacterial gross assimilation rates (73-75%: Decho and 

Luoma 1991, Werner and Hollibaugh 1993) are somewhat lower than measured for 

phytoplankton (78-90% for a range of phytoplankton species; Schlekat et al. 2002). 

Bacterial carbon also appears to be more quickly metabolized than phytoplankton 

carbon (Werner and Hollibaugh 1993). Net assimilation rates have been estimated at 

45% for bacteria and ≈85% for phytoplankton (Werner and Hollibaugh 1993, 

Schlekat et al. 2002). 

Known aquatic consumers of adult Corbula in the SFE include the Dungeness crab 

(Cancer magister, Carlton et al. 1990, Stewart et al. 2004), the Sacramento Splittail 

(Pogonichthys macrolepidotus, Deng et al. 2007), and the White Sturgeon (Acipenser 

transmontanus, Urquhart and Regalado, 1991, Kogut in press). It is likely that any 

bottom feeding fish with a sufficiently large mouth that feeds in areas occupied by C. 

amurensis consume some quantity of the bivalves due to their large population 

numbers. Birds known to consume adult Corbula include the Greater and Lesser 

Scaup (Aythya marila and A. affinis, Poulton et al. 2002) and the Surf Scoter 

(Melanitta perspicillata, Hunt et al. 2003). Conchiolin layers, organic laminae in the 

shells that occur in bivalves in the Corbulidae family, act both to increase the strength 

of the shell against predation from mechanical crushing and to deter gastropods from 

drilling through the bivalves shell (Kardon 1998). 
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Populations of Corbula are reduced and in some cases locally decimated by migratory 

birds in fall and winter in all but the deep channel areas of the SFE (Poulton et al. 

2002, Thompson et al. 2008). Therefore the shoal areas are mostly dependent on the 

adjacent deep water populations to supply recruits each year. 

The combination of Corbula’s ability to respond to food quickly to initiate gamete 

production and to produce gametes and embryos that are tolerant of large changes in 

salinity is likely to be a good strategy for this species to maintain its present 

distribution, as well as to expand its distribution during seasonal salinity changes in 

the estuary. Individuals become reproductively active within their first year of life and 

in many cases, spring recruits are capable of reproducing by summer and fall, so 

adults are present during most months of the year to respond to a cue to begin 

gametogenesis. 

B. Corbula’s Control of Phytoplankton Biomass 

The SFE is a high nutrient, high turbidity estuary that has low primary production due 

to a combination of light limitation and bivalve grazing (Cloern 2001, Thompson et 

al. 2008). The northern estuary has always had low primary production, but declines 

in primary production following the invasion of Corbula lead many researchers to 

conclude that suspension feeding by Corbula resulted in the decline in phytoplankton 

biomass and the elimination of the annual phytoplankton bloom in the estuary (Alpine 

and Cloern 1992, Kimmerer 2002). The annual primary production has been reduced 

to <20 g C m-2

 yr

-1 from ≈100 C m-2

 yr

-1 (Alpine and Cloern 1992, Jassby et al. 2002). 

 

Two copepods (Acartia spp, Eurytemora affinis), a rotifer (Synchaeta bicornis), and a 

mysid shrimp (Neomysis mercedis) concurrently declined with the phytoplankton, 

presumably due to food limitation (Kimmerer 2002, Feyrer et al. 2003). Some of the 

declines in copepods may also be due to direct filtration of juveniles by Corbula

(Kimmerer et al. 1994). Despite what appeared to be a collapsing foodweb within 5 

years of the invasion of Corbula, most of the species involved remain in the estuary 

today. The robustness of the foodweb may due to (1) its complexity and therefore 

weak links (Kimmerer 2002), (2) the importance of microzooplankton and small 

phytoplankton that are not presently accounted for in the data, or (3) the migration of 

one important, competing filter feeder, the Northern Anchovy (Engraulis mordax), 
15

out of the northern estuary following the reduction in phytoplankton biomass 

(Kimmerer 2006). Although the cause of a recent decline in pelagic fish in the 

ecosystem (Sommer et al. 2007) is probably at least partially due to changes in the 

pelagic foodweb, there is no conclusive evidence that food is the primary or only 

cause for the decline of the fish species. 

Corbula’s pumping rate (a physiologically controlled rate), estimated in a flume with 

varying velocities, was found to range from 100-575 L (g dry weight)-1 d-1 with a 

mean of ~400 L (g dry weight)-1 d-1 or 1-5 L d-1 clam-1 for the phytoplankton 

Chroomonas salinay (Cole et al. 1992). Clearance rates (a physiologically controlled 

rate that varies with filtrate character) estimated in beaker experiments are similar: 4 

L d-1 for a 1 cm clam {filtration rate = -40+199 x shell length (cm)} for phytoplankton 

(Isochrysis glabana) and 45 ml hr-1 clam-1 (independent of shell size) for bacteria 

(Werner and Hollibaugh 1993). We can estimate the effect of Corbula’s grazing rate 

(our estimate of a field rate) in two ways. First, Corbula’s maximum effect on the 

phytoplankton biomass can be estimated by using pumping rates and assuming a 

vertically well mixed water column. The second method estimates a minimum effect 

of clam grazing on the phytoplankton by (a) assuming that a concentration boundary 

layer (CBL) forms within a bed of clams and (b) that not all clams feed at once (66% 

active clams at a time). In this case the phytoplankton removed by the bivalves is not 

replaced by vertical mixing which results in a vertical gradient in phytoplankton 

concentration (i. e.. the clams downstream of other clams have less food). The CBL 

method and a more thorough explanation of the estimation technique can be found in 

Thompson et al. (2008). Corbula’s consumption of phytoplankton is a function of 

environmental factors such as temperature and suspended sediment in addition to the 

biomass of bivalves and vertical mixing rates (Figure 11). The effect of many of the 

environmental factors is not yet well defined. Table 3 shows grazing rates using the 

two methods. Note that the median grazing rate varies by a factor of 4 between 

stations for both methods. This range reflects the larger biomass of Corbula observed 

in the deep water (Table 2). Vertical turnover time, or the time that the clams need to 

filter the water column over them, assuming the water column is vertically well 

mixed, is a reflection of the potential impact that Corbula has on the phytoplankton 

standing stock and its capacity to achieve a bloom biomass. Turnover time using the 

median grazing rate is about 2-4 days for the shallow water population in Grizzly Bay 
16

and 4-8 days for the channel populations. These turnover times are shortest in fall 

when biomass and grazing rate peak; Corbula could theoretically clear the water 

column once every 8-16 hours in Grizzly Bay and every 8-24 hours in the channel 

during this period. Biomass and therefore minimum grazing rates have consistently 

occurred in spring and early summer at all shallow water locations and most channel 

stations; the biomass time series shown in Figures 3b and 3d are representative of this 

seasonality. 

The rapid turnover of the water column by the shallow water bivalves, in conjunction 

with the relatively slow phytoplankton growth rate (0.25 d-1 in the shallow water, 

Cloern et al. 1985) highlight the importance of shallow water Corbula as one of the 

factors determining system-wide phytoplankton biomass. The high grazing rates in 

the channel may also influence phytoplankton biomass, but the negative growth rate 

of phytoplankton in the channel due to the poor light climate (i.e. limited illumination 

in the water as determined by incident light, absorption, scattering, and turbidity) 

(Cloern et al. 1985) is likely to be the primary factor controlling phytoplankton 

biomass in the deep water. Therefore the only locale in the northern estuary where 

the light climate allows phytoplankton to grow is in the shallows. Historically, a slow 

growing, 4-5 month long phytoplankton bloom began in the shallows and spread to 

the channels in the northern estuary (Cloern et al. 1985). Today, high Corbula

grazing rates in conjunction with relatively high water column vertical mixing rates 

continually challenge the potential for phytoplankton biomass increase in these 

shallow systems. The phytoplankton consumption rates reported here for Corbula in 

the northern estuary will affect adjoining areas through tidal dispersion. Therefore the 

Corbula filtration in the northern estuary may have a significant effect on the 

phytoplankton in the western Delta as reported by Jassby et al. (2002). 

C. Negative Influence of the Trophic Transfer of Contaminants 

The near-surface growth position of Corbula makes them more available to predators 

than the deep-burrowing bivalve that previously dominated the northern estuary 

(Macoma petalum, previously known as Macoma balthica). The caloric content of 

the two bivalves is similar (Richman and Lovvorn 2004). Thus the invasion of 

Corbula might be an advantage to bottom feeding predators if not for the propensity 

of Corbula to accumulate selenium at near toxic levels for consumers (Stewart et al. 
17

2004). Since its arrival in the estuary, predators on Corbula that now have selenium 

concentrations in their liver in excess of the toxicity threshold include the Sacramento 

Splittail (Pogonichthys macrolepidotus), the Dungeness crab (Cancer magister), the 

White Sturgeon (Acipenser transmontanus) (Stewart et al. 2004) and diving ducks 

(Scoter: Melanitta perspicillata and Scaup: Aythya spp) (White et al. 1987, 1988, 

1989, Urquhart and Rigelado 1991, Linville et al., 2002). Although Corbula also 

bioaccumulates heavy metals (Brown and Luoma 1995), hydrocarbons (Pereira et al. 

1992), butyltins (Pereira et al. 1999), and pesticides (Gale et al. 2003), there is no 

literature on the effects of these toxins on Corbula’s predators. 

D. Constraints on Restoration 

Corbula invaded the SFE after a 100 yr flood event that greatly reduced the diversity 

and abundance in the benthic community at the invasion location (Nichols et al. 

1990). Although Corbula then invaded other parts of the estuary in the following two 

years, and did so within a normally diverse community (Thompson 2005), its ability 

to monopolize the depauperate benthic community in the northern estuary signifies its 

likely response to newly restored habitat. This opportunistic ability, in combination 

with Corbula’s prodigious grazing rates and ability to accumulate and transfer 

contaminants may limit our restoration options in the northern estuary. 

The decline of fish that prey on zooplankton and shrimp, which depend on primary 

producers, has resulted in a primary restoration goal of increasing the production of 

phytoplankton. Because Corbula grazing can restrict the net growth rate of 

phytoplankton, it has been suggested that control of Corbula biomass in some select 

areas might allow phytoplankton to grow and be transported out of these areas. As 

the major control on Corbula distribution is salinity, it has been discussed if 

increasing freshwater outflow during peak reproductive periods would limit their 

distribution. The low salinity stress on the eggs, embryos and pelagic larvae in 

combination with their transport out of the area with high flows (large flood events 

like 1995 and 1998) might reduce the population size and distribution of Corbula in 

localized areas for short time periods. It takes very large, prolonged outflow events to 

reduce the estuary-wide population (Figure 3) over an extended time period, as local 

and non-local adults can respond quickly and produce new larvae as long as food is 

available. Increased outflow periods would need to be maintained for this to be a 
18

long term solution as depauparate periods such as was seen in 2006, can be followed 

by an increase in the population size of Corbula during subsequent years with normal 

salinity distributions. Therefore, sustained reduction in grazing would require the 

water for controlled floods most if not all years. 

Corbula’s ability to accumulate toxins and its place in the food web as prey is also an 

important consideration in restoration plans. Without the elimination of Corbula from 

the estuary, the best that can be done for protecting predators is to identify sources of 

contaminants and try to eliminate their availability to Corbula. Any restoration action 

that increases the concentration of contaminants in Corbula may result in an increase 

in the contaminant concentration in their predators although the predictability of the 

amount transferred is contaminant and species specific. 

Climate change may make Corbula population biomass distribution less consistent 

between years as the frequency of high and low freshwater outflow events are 

predicted to increase. In addition to this “natural” population variability is that caused 

by declining freshwater resources. If there is an overall decline in freshwater 

available to the estuary as the need for freshwater is increased in California for human 

consumption, Corbula will expand into the western Delta as the salinity increases. 

This will further complicate restoration in the western Delta. 

V. Future research 

The immediate research needs include (1) resolution of the taxonomic questions in the 

family, (2) incorporation of biomass estimates in the DWR monitoring program, and 

(3) examination of the physiological characteristics of the species. 

C. amurensis has apparently not spread to intra-coastal ports nor has it been 

introduced to other Eastern Pacific locations. The wide distribution of Corbula 

amurensis in the Western Pacific leads us to question if the species is correctly 

identified. It seems unlikely that adults and larvae have not been included in ballast 

water that has been released in other ports. The only explanations we have at present 

for the lack of spread is that (1) there is something in its physiology that limits its 

transport and or settlement in most situations, (2) the balance of trade today reduces 
19

the number of ships arriving fully in ballast from Asia and thus there has been less 

opportunity for release of ballast water, or (3) it has invaded elsewhere but in 

localities without monitoring programs. All of these explanations would be 

influenced by finding the taxonomy is incorrect. 

The second research need is to incorporate estimates of biomass of the major species 

as part of the routine analysis of DWR’s Environmental Monitoring Benthic Program. 

As seen in Figure 3, our understanding of ecosystem effects cannot be facilitated 

using abundance alone. The inclusion of biomass in the analyses would allow the 

benthos to be defined as carbon, the common currency used in food web analyses. In 

addition, an estimate of biomass would give us the data needed to estimate grazing 

rates on phytoplankton and zooplankton, growth rates (i.e. how much carbon are they 

consuming), and the mass and contaminant content available to predators. 

We cannot confirm many of our assessments of possible controls on the temporal and 

spatial distribution described in this document until basic physiological measurements 

of Corbula are done in the laboratory. To thoroughly understand the distribution 

potential of Corbula in a restored area we need to know how salinity and temperature 

co-vary, i.e. when does temperature narrow Corbula’s salinity tolerance and when 

does salinity limit Corbula’s ability to withstand high temperatures that may occur in 

high-residence time restored embayments. Understanding Corbula’s low oxygen 

tolerance will help explain distribution anomalies in Suisun Marsh where low oxygen 

has been concurrently observed with dead Corbula and help us understand the length 

of time necessary for Corbula to die in low oxygen water. Finally, grazing rates are 

very important if we are to understand Corbula’s effects on the estuary, but we know 

nothing about how its pumping rates are related to suspended sediment concentration, 

temperature, and food concentrations. 

Acknowledgements 

Development of this model was supported the U.S. Geological Survey’s Priority 

Ecosystem Science and Toxic Substances Hydrology programs, CALFED Bay/Delta 

Program, and the CASCADE project (CALFED Project #SCI-050C01-84). The 

model benefitted by the comments of two anonymous reviewers. 

 
20

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VII. Tables 

Table 1. Corbula Life-Stage by Biological Measures. 

 A B C D E F G H J 

Habitat 

(life stage) Dates 

Age Weight Length Optimum 

Temp 

(Celsius) 

Maximum 

Critical 

Temp 

(Celsius) 

Min 

Salinity 

Max 

Salinity 

Min 

DO 

Limits 

W

Vel

(Days post hatch) 

1 Benthic (Eggs/Sperm) Jan- Dec -1 UNK UNK UNK 5 25 UNK U2 

Benthic 

(Fertilization 

of Egg) 

JanDec -1 UNK 60+ μm UNK UNK 5 25 UNK U

3 

Pelagic 

(Trochophore 

Larvae)

JanDec 0-1 UNK 70 μm UNK UNK 2 30 UNK U

4 Pelagic (Veliger 

Larvae

JanDec 1-19 UNK 75-135 μm UNK UNK 2 30 UNK U

5 Benthic (Recruit) Jan- Dec . 19~120 300 μm- 5 mm UNK UNK 2 32+ UNK U6 

Benthic 

(Reproductive 

Adult) 

JanDec Maximum >3 years <4 years >5mm UNK UNK .1 32+ UNK U
Table 2. Corbula Population Characteristics. 

* Data for D4, D7 and D41A Courtesy of DWR CALFED Biomass Study

A B C D E F G H J K 

Geographic 

Location 

Station 

ID 

Source 

Station 

depth 

at 

MLLW 

(m) 

Maximum 

Biomass 

Maximum 

Abundance 

Abundance Range 

(#/m2) 

gC/m2 mo/yr #/m2 mo/yr max yr min yr max 

1 Colinsville D4* DWR 11 5 4/91 3000 10/00 0- 3000 2000 0 2002 0- 5 3 Chipps Island 4.1 USGS 9 22 10/07 12,000 11/00 400- 12,000 2000 10- 500 1996 2- 22 5 Carquinez Strait 8.1 USGS 17 80 5/88 30,000 4/07 1600- 26,000 2006 10- 800 1998 10- 80 6 Honker Bay 433 USGS 2 4 7/90 5700 10/06 100- 5700 2006 10- 300 1995 0.2- 3 7 Grizzly Bay D7* DWR 2 5 12/05 17,000 6/87 300- 16,000 2006 100- 600 1995 0.2- 5 8 Grizzly Bay 417 USGS <1 3 8/07 15,000 9/06 not enough data 9 San Pablo Bay D41A* DWR 2 16 9/89 39,000 5/00 200- 39,000 2000 400- 1000 1994 0.3- 16 
Table 3. Corbula Grazing Rates 

* using median grazing rate ** Data for D7 Courtesy of DWR CALFED Biomass Study 

 A B C D E F G H I 

Habitat 

(life stage) Dates 

Water 

Column 

Depth 

(m) 

CBL Corrected, 67% Feeding 

(m3 m-2 d-1) 

Pum

(m

mean median max min 

Turnover 

Time 

(day)* 

mean median 

1 

Benthic 

Shallow 

D7 DWR** 

1988-2006 

2 1 0.5 3 0 4 1 1 

2 

Benthic 

Deep 

4.1 USGS 

1988-2006 8 1 1 10 0 8 2 1 

3 

Benthic 

Deep 

8.1 USGS )

1988-

2006 17 3 2 15 0 6 7 4 
28

VIII. Figures 

Figures 6, 9, 10 and 12 are matrices showing transitions from one life stage to the next. 

Negative and positive effects of processes on transition survival are designated by (-) and 

(+). Levels of importance, understanding and predictability of each process are given in the 

key below. 

Low 

Medium 

Importance Understanding Predictability 

Positive response (+) Negative response (-) 
29

Figure 1. Life cycle of Corbula amurensis in San Francisco Estuary. 

(Adapted from Nicolini, MH and DL Penry. 2000. Spawning, fertilization, 

and larval development of Potamocorbula amurensis (Mollusca: Bivalvia) 

from San Francisco Bay, California. Pacific Science: 54(4):377-388 NO 

PERMISSION FOR PUB.) 
30

Figure 2. Distribution of months with peak Corbula recruitment in (a) the 

western Delta, (b) Grizzly Bay, (c) San Pablo Bay. Average recruits 

occurring each month in the western Delta over the period of 1988 through 

2006 (d). Grizzly Bay data courtesy of DWR CALFED Biomass Study. 
31

Figure 3. Density and biomass of Corbula at the western Delta (a, b) and in Grizzly 

Bay (c,d). Grizzly Bay data courtesy of DWR CALFED Biomass Study. 

b 

a 

c 

d 
32

Figure 4. Distribution of Corbula in SFE during (a) spring and summer months and 

(b) fall months. 1990 data from Hymanson (1991). 1995 data courtesy of DWR. 
33

Figure 5. Life Cycle and habitat conceptual model for Corbula. 
34

Figure 6. Transition probability of Corbula gametogenesis to recruit. 
35

Figure 7. Corbula recruit density in cores collected from station located at X2 

km 64 (line on graph). Recruits are limited when the maximum excursion of 

X2 is downstream of the station, i.e. when salinities are less than 2 at the 

station the previous month prior to the sampling. 
36

Figure 8. Corbula recruit density as a function of (a) adult Corbula

density and (b) Ampelisca abdita density. 
37

Figure 9. Transition probability of Corbula recruit to reproductive adult. 
38

Figure 10. Transition probability of Corbula one year old to a two year old or greater. 
39

Figure 11. Transition probability of phytoplankton to bloom status.