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   Potamocorbula amurensis (mollusc)     
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      Photo: Ministry of Fisheries, NZ - Click for full size   Corbula amurensis from San Francisco Bay (Photo: U.S. Geological Survey) - Click for full size   Corbula amurensis from San Francisco Bay. Note the dark fouling on the upper one-third of the shell that had protruded from the mud (Photo: Andrew N. Cohen www.exoticsguide.org) - Click for full size   Dried shells of Corbula amurensis from San Francisco Bay. Note the “overbite” of the larger valve that shows clearly in the clam at the center right (Photo: Andrew N. Cohen www.exoticsguide.org) - Click for full size   A Corbula amurensis shell, showing the overbite (Photo: Andrew N. Cohen www.exoticsguide.org) - Click for full size   Photo: Machiko Yamada - Click for full size   Corbula amurensis from San Francisco Bay with a young barnacle (Balanus improvisus) attached to the hind part of the shell in the lower right part of image (Photo: Luis A. Solórzano www.californiabiota.com) - Click for full size
    Taxonomic name: Potamocorbula amurensis (Schrenck, 1861)
    Synonyms: Corbula amplexa (for P. ustuala) Adams, 1862, Corbula frequens (for P. ustulata) Yokoyama, 1922, Corbula labiata (for P. ustulata) Reeve, 1844, Corbula pustulosa (for P. ustulata) Yokoyama, 1922, Corbula ustulata (for P. ustulata) Reeve, 1844, Corbula vladivostokensis (for P. ustulata) Bartsch, 1929, Potamocorbula amurensis Reeve (now considered to be, 1861), Potamocorubala ustulata separate spp by Carlton, 1999., 1844
    Common names: Amur river clam (English), Amur river corbula (English), Asian bivalve (English), Asian clam (English), brackish-water corbula (English), Chinese clam (English), marine clam (English), Nordpazifik-Venusmuschel (German), Numakodaki (Japan)
    Organism type: mollusc
    The suspension-feeding clam, Potamocorbula amurensis is native to Japan, China and Korea in tropical to cold temperate waters. Known as the Asian or Chinese clam, it has been designated as a major bilogical disturbance with significant ecological consequences in the San Francisco Bay area of California where large populations have become established.
    Description
    The Asian clam Potamocorbula amurensis grows to around 2-3cm in length. It is usually white, tan or yellow in colour with no markings on the external valves. The valves are thin and smooth, with one shell slightly longer than the other. There is a prominent external keel on the top of the left valve, which extends slightly down the shell. Older specimens appear wrinkled on the shell surface. The inhalent and exhalent siphons are brown in colour and short in length. P. amurensis buries into sediments on the sea floor and exposes 1/2 to 2/3 of its shell above this sediment in order to feed (NIMPIS, 2002a).
    Similar Species
    Potamocorbula laevis, Potamocorbula rubromuscula, Potamocorbula ustulata

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    Occurs in:
    estuarine habitats, marine habitats
    Habitat description
    Potamocorbula amurensis is a highly tolerant species. It is found from almost freshwater areas to high salinity areas. It exists from tropical to cold temperate waters, mostly subtidally, but it has been found in the intertidal zone. It occurs in all sediment types: mud, peat, clay, sand and is most abundant on a variety of mixed mud-sand bottoms (NIMPIS, 2002a). With its ability to survive in polluted environments, this salinity-tolerant bivalve has a distinct advantage in invading a variety of geographic areas and habitats.
    General impacts
    The suspension-feeding clam Potamocorbula amurensis has been designated as a major biological disturbance with significant ecological consequences. NIMPIS (2002a) states that "the introduction of P. amurensis to the San Francisco Bay in California has resulted in dramatic changes to the soft sediment communities of the area. It is thought to be responsible for the collapse of some commercial fisheries in addition to the decline in the diversity and abundance of many benthic species in the area. The clam consumes large amounts of phyto- and zooplankton and therefore changes many of the existing community dynamics, resulting in many benthic species being unable to obtain enough food for growth. The clam is also a dominant species in the bay, accounting for 95% of the biomass in some areas. This reduces the amount of available space for other species to grow and reproduce".
    Notes
    The largest specimen of Potamocorbula amurensis collected in San Francisco Bay as of August 1988 was 25.1mm in length (NIMPIS, 2002b).
    Geographical range
    Native range: Japan, China and Korea.
    Known introduced range: San Francisco Bay in the USA.
    Introduction pathways to new locations
    Ship ballast water: The Asian clam's initial introduction to San Francisco Bay was as veliger larvae transported in ballast water by trans-Pacific cargo ships. P. amurensis larvae have the ability to tolerate substantial changes in salinity. Studies in the San Francisco Bay area show that spawning and fertilization can occur at salinities from 5 to 25 psu, and eggs and sperm can each tolerate at least a 10-psu step increase or decrease in salinity. Embyros that are 2 hr old can tolerate salinities from 10 to 30 psu, and by the time they are 24 hr old they can tolerate the same range of salinities (2 to 30 psu) that adult clams can. This ability supports the strong potential of P. amurensis larvae to survive incomplete oceanic exchanges of ballast water and subsequent discharge into receiving waters across a broad range of salinities (Nicolini and Penry, 2000).
    Management information
    Preventative measures: Strategies to decrease the risks of future introductions involve ballast water management. Ballast water management regulations have been put in place in countries like the USA, Australia and New Zealand. Oxygen deprivation, which has been trialed as a treatment in ballast tanks, is found to be unsuccessful in the case of the Asian clam. It has a high tolerance to low oxygen and is found in polluted or eutrophic areas and hence, low oxygen is unlikely to be successful unless hypoxic conditions can be maintained for a long time (McEnnulty et al., 2001).

    In Australia's National List of Invasive Marine Species, the Asian clam has been classified as one of the species whose incursion can trigger an emergency response. Regulations have been put in place for the management of internationally sourced ballast water, a known vector for the Asian clam (DAFF, 2004). In New Zealand, surveillance systems have been put into place for the early detection of any incursions of P. amurensis, classified as one of 6 exotic high impact species (MAF, 2008).

    Physical: The Asian clam is preyed upon by birds, fish and crabs. However, options for its large-scale control are limited (Department of Fisheries, WA). Dredging, beamtrawling and mopping as control options have been found to be unsuccessful in the case of the Asian clam. Dredging is unlikely to succeed as a control option due to very high densities and the small size of this species. P. amurensis is a comparatively thin, fragile shell more subject to breakage.

    Nutrition
    Potamocorbula amurensis is a suspension feeder that can consume large amounts of phytoplankton and zooplankton per day. It consumes many species, the main ones appearing to be diatoms and copepods (NIMPIS, 2002a). The bivalve buries itself in subtidal sediments, exposing half to three-quarters of its shell above the sediment-water interface and anchoring itself by byssal threads which adhere to small rocks or other hard objects in the sediment. A current of water passing in through the bivalve’s inhalant siphon is filtered by gills to extract oxygen and microscopic plants, such as diatoms, which are passed on to its mouth. The water current then flows out of the bivalve’s gill chamber through the posteriorly-located exhalant siphon. Very little was known about the reproduction, growth and feeding of P. amurensis. However, significant research is being undertaken in California following its invasion of San Francisco Harbour (Department of Fisheries, 2000-2001).
    Reproduction
    Studies indicate that some populations of Potamocorbula amurensis spawn throughout the year and that newly settled individuals become reproductive within a few months. San Francisco North Bay populations spawn in Spring and Fall, while South Bay populations are reproductive all year round. In Korea spawning occurs twice a year from May - June and Sept – Oct. Males and females are separate. Spawning is induced by physical stress, heat shock, rough handling or placing in filtered water. Newly settled clams become reproductive within a few months. Females can produce from 45,000 to 220,000 viable oocytes, the number produced is, however, independent of female size. The development of larval stages appears to be influenced by water temperature (Nicolini & Penry (2000) in NIMPIS, 2002b).
    This species has been nominated as among 100 of the "World's Worst" invaders
    Compiled by: IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Wednesday, 9 November 2005


ISSG Landcare Research NBII IUCN University of Auckland