Corbula amurensis
(Amur River clam)

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).

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.

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).

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.

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).

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).

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).

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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

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.

16/02/08 Original text by:

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