Corbula amurensis
(Amur River clam)

Pictures

Picture	Title	Caption	Copyright
Title Small shells Caption Small clam shells in hand, showing various periostracum layers. Copyright U.S. Geological Survey, Menlo Park, CA.	Small shells	Small clam shells in hand, showing various periostracum layers.	U.S. Geological Survey, Menlo Park, CA.

Identity

Preferred Scientific Name

• Corbula amurensis (Schrenck, 1861)

Preferred Common Name

• Amur River clam

Other Scientific Names

• Potamocorbula amurensis Coan (2002)

International Common Names

• English: Amur River corbula; Asian bivalve; Asian clam; brackish-water corbula; Chinese clam

Local Common Names

• Germany: Nordpazifik-Venusmuschel
• Japan: numakodaki

Summary of Invasiveness

C. amurensis is considered an invasive species due to its rapid spread in the San Francisco Estuary (SFE) and its reduction of phytoplankton biomass to critical levels (Alpine and Cloern, 1992; Kimmerer, 2002). The first SFE specimen of C. amurensis was reported in late 1986 and within 2 years it was the dominant bivalve in the estuary.  Carlton et al. (1990) hypothesize ballast water as the mode of introduction. Its success in the estuary is due to its ability to occupy most habitats (sediment and water depths) in the system (Carlton et al., 1990), its pelagic larvae, and its broad physiological tolerance of salinity as adults (Werner et al., 2003) and as larvae (Nicolini and Penry, 2000). It is on the alert list for ISSG (Invasive Species Specialist Group) where it is listed as among the 100 World’s Worst Invaders and is listed as a Pest by NIMBUS (National Introduced Marine Pest Information System) (Hewitt et al., 2002).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Mollusca
•             Class: Bivalvia
•                 Subclass: Heterodonta
•                     Order: Myoida
•                         Unknown: Myoidea
•                             Family: Corbulidae
•                                 Genus: Corbula
•                                     Species: Corbula amurensis

Notes on 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). The species name was provisionally selected at that time as Dr. Matsukuma was starting a revision of the genus. Coan (2002) in his analysis of Eastern Pacific Corbulidae revised the identification to Corbula amurensis but warned that it may be changed again, once the Asian Corbula group is revised. Morphologically similar species include C. laevis (Hinds 1843 as reported in Carlton et al., 1990), C. ustulata (Reeve, 1894 as reported in Carlton et al., 1990), and Potamocorbula rubromuscula (Zhuang and Cai, 1983 as reported in Carlton et al., 1990). Common names have included the Amur River clam, Asian clam or bivalve, Chinese clam, and the overbite clam; the latter name is a common name given to C. amurensis in California by media and resource managers due to confusion with Corbicula fluminea which is also called the Asian clam.

Description

As described by Coan (2002): “Ovate, thin; right valve decidedly larger than left valve; beaks anterior to midline (approximately 41% from anterior end); anterior end sharply rounded; posterior end sharply rounded…Shell white exteriorly and interiorly…right valve with a narrow tooth, attached to shell wall below hinge-line; left valve with a long, projecting chondrophore that is conspicuously divided and with a very small tooth on its posterior end…Pallial line with a small sinus.”  Specimens in SFE have included shells that could be described as subtrigonal and ovate-elongate. An exterior posterior keel is apparent on left valve.  The periostracum can be thick, range in color from tan to dark brown, or it can be very thin on individuals living in high velocity sandy habitats. The maximum length is 19.7mm. The type locality shell (CAS 121534, Carquinez Strait, San Francisco Bay, CA, USA) is available at California Academy of Sciences, San Francisco, CA. C. amurensis was originally described by Schrenck (1861, as reported in Coan, 2002) as Potamocorbula amurensis. Photos of veliger larvae are available in Nicolini and Penry (2000).

Distribution

The confusion in the taxonomy of this group makes a definitive geographic distribution difficult.  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 P. 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.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

Carlton et al. (1990) hypothesize that C. amurensis was introduced into the SFE by ballast water, most likely originating from an Asian port. The first specimen was reported in late 1986, when juveniles were reported in the northern estuary. Within a year it 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 is a common and frequently dominant bivalve (Thompson, 2005).

Introductions

Risk of Introduction

Embryos can survive a 2 to 30 salinity range at an age of 24 hours (Nicolini and Penry, 2000), the larvae have a long pelagic phase (2-3 weeks), and it can survive anoxic conditions (McEnnulty et al., 2001) which increases the risk of a successful ballast water introduction of C. amurensi. This is particularly true when ballast water less than 14 days old is released in estuaries. Intra-coastal introductions by ballast water in the Eastern Pacific or by recreational boating through incidental introduction of adults on anchors and in bait boxes seems most likely. However, so far C. amurensis has not been reported in other Eastern Pacific estuaries.  Australia considers C. amurensis to be a sufficient threat to trigger an emergency response and New Zealand has classified it as one of “six exotic high impact species” included in an early detection surveillance system (Global Invasive Species Database, 2005).

Habitat

C. amurensis have been observed in all habitats except epifaunal (ie. attached to hard substrate) habitats in the SFE. They prefer 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. When found in hard-pack clay or high velocity areas, a single byssal thread 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. Large populations have been found near the freshwater endpoint in the estuary and in the southern estuary where salinities are similar to ocean salinities.

Habitat List

Biology and Ecology

Genetics

Reproductive Biology

Physiology and Phenology

Nutrition

Associations

C. amurensis is a euryhaline species (able to adapt to rapidly changing salinity) that is also able to maintain a dominant position in relatively salinity-stable portions of estuaries. Its abundance can exceed 10,000/m² (Carlton et al., 1990) and biomass can exceed 200 g dry tissue wt/m² (Thompson, 1999). It is not surprising therefore to find it living with a variety of euryhaline species in some locations and with species with much narrower salinity tolerances in other locations. C. amurensis occurs among many non-indigenous species in the benthic community of the SFE (Cohen and Carlton, 1998). All species listed in this discussion as co-dominants with C. amurensis in SFE are cryptogenic (likely to be non-indigenous) or have been established as non-indigenous. Based on the wide geographic range of vectors for non-indigenous species in SFE, it is not surprising that the benthic species associations of C. amurensis in SFE are quite different than those in its native habitat. Unless otherwise stated the information for SFE is from Thompson et al. (2007).

Climate

Water Tolerances

Notes on Natural Enemies

Known aquatic consumers of C. amurensis 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 Regalada, 1991; Kogut, in press). It is likely that any bottom feeding fish with a sufficiently large mouth, that feeds in areas where C. amurensis is found consumes some quantity of the bivalves due to their large population numbers. Birds that are known to consume C. amurensis 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, laminae in the shells that are of an organic nature, which occurs 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).

Means of Movement and Dispersal

Natural Dispersal (Non-Biotic)

Vector Transmission (Biotic)

Accidental Introduction

Pathway Causes

Pathway Vectors

Economic Impact

The SFE is the source of freshwater for over 25 million people and over 500,000 ha of farmland in the state of California, which supports a multi-billion dollar agriculture industry. Most of the water in the state is retained in the northern state, initially as snow, and then as snowmelt in reservoirs that are operated by the state and federal governments. A very elaborate canal and pumping system then transports the water throughout the state in addition to releasing some of the water into the estuary where it is used for agricultural irrigation in addition to its use in the ecosystem.  

The precipitous decline of several fish species, including one federally-designated threatened, and recently petitioned to be endangered, species has caused resource managers to alter the flow of water to water users beginning in 2007. One of the conceptual models being tested as the cause of the fish decline is a possible shift in the food web due to overgrazing of phytoplankton by C. amurensis (Sommer et al., 2007). There has been a reduction in zooplankton and mysid shrimp concurrent with the invasion of C. amurensis, both of which are important prey for larval and adult fish.

Striped bass (Morone saxatilis) is the most important sport fishery in the SFE and it has been steadily declining since the 1960s. A sharper decline in abundance of this species has been seen since 1999, concurrent with the decline of other fish species (Sommer et al., 2007). This fishery was estimated to bring US $47 million into the San Francisco Bay area in 1985 when the striped bass abundance was estimated at about a half million. In 2001, the striped bass abundance had fallen to less than 50,000; no revenue estimate has been made for present day conditions (California Department of Fish and Game, 2001).

Resource managers in California have invested hundreds of millions of dollars to restore habitat and purchase water for environmental use due to the decline in pelagic fish, mysid shrimp, and zooplankton. One ecosystem restoration effort in the SFE, funded through a combined state and federal program (CALFED) has spent US $335 million. Part of this funding has been directed towards restoration of primary producers to levels seen prior to C. amurensis introduction.

Environmental Impact

Impact on Habitats

Impact on Biodiversity

Threatened Species

Risk and Impact Factors

• Proved invasive outside its native range
• Has a broad native range
• Abundant in its native range
• Highly adaptable to different environments
• Is a habitat generalist
• Pioneering in disturbed areas
• Highly mobile locally
• Fast growing
• Has high reproductive potential
• Gregarious
• Has high genetic variability

• Altered trophic level
• Damaged ecosystem services
• Ecosystem change/ habitat alteration
• Modification of natural benthic communities
• Modification of nutrient regime
• Modification of successional patterns
• Negatively impacts aquaculture/fisheries
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Competition - monopolizing resources
• Filtration
• Herbivory/grazing/browsing
• Interaction with other invasive species
• Predation
• Rapid growth

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

Similarities to Other Species/Conditions

The only eastern Pacific corbulid that might be confused with C. amurensis is Corbula luteola, which occurs south of the present range of C. amurenis (the San Francisco Estuary; Coan, 2002). Young juveniles may be confused with juveniles of other inequivalve bivalves such as Mya arenaria and Cryptomya californica; the original specimen in SFE was initially misidentified as a member of the Myidae. As they age, the “smaller, flatter left valve is drawn into the larger and more swollen right valve” of C. amurensis (Carlton et al., 1990). M. arenaria has a deep pallial sinus compared to the shallow pallial sinus of C. amurensis and C. californica is less ovate and lacking the posterior keel (Carlton et al., 1990).

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

The only management strategy that has been proposed for containing C. amurensis has been to stop it before it arrives by controlling ballast water releases. Given the wide salinity tolerance of adult and juveniles (Nicolini and Penry, 2000; Werner et al., 2003) and their apparent low oxygen tolerance (McEnnulty et al., 2001) this will be difficult unless ballast water is either totally exchanged or rendered non-biotic in some manner.
 

Monitoring and Surveillance

Ecosystem Restoration

Gaps in Knowledge/Research Needs

The largest research needs include resolution of the taxonomic questions in the family and 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. 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 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.

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Distribution References

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Links to Websites

Contributors

16/02/08 Original text by:

Janet Thompson, U.S. Geological Survey, 345 Middlefield Rd. MS-496, Menlo Park, CA 94025, USA

Distribution Maps