Invasive Alien Species in Northern Ireland

Corbicula fluminea, Asian clam

Corbicula fluminea (Müller, 1774)
This freshwater clam can occur in large numbers where there are clean clays, sands and gravels in lakes, rivers and streams and it can survive in the upper region of estuaries where salinities are low. This is a highly invasive species and it is likely to cause significant impacts to aquatic bottom-living species.
It has a globular, solid amber to brown shell and distinctive rounded concentric ridges on the outer shell surface. The species may rarely attain more than 50mm in shell length but shells are normally up to 20-25mm. Shell height to shell length ratios may vary and there is some indication that there are two forms in Europe classified by some as separate species.
Country of origin
The species is native to eastern Asia, eastern Russia, Korea and China.
Current distribution
The species has extended its range to both North and South America. It arrived in Portugal in the 1970s and has since spread across the continent to Romania. It is also known to occur in south-eastern Britain.
Location in Ireland
It is not known to be present in Ireland. However, the species is known to expand its range rapidly and its presence in south-eastern Britain is likely to result in it becoming more widespread over the forthcoming decade and this increases the risk of its arrival to Ireland.
Life cycle
This species is a filter feeder and can also remove organics from sediments using its extendable foot. It can occur in great abundance with densities that can exceed 1000m-2. It is tolerant of water temperatures ranging from 2-34°C and salinities of up to c.5psu, but can withstand short periods of exposure of up to c.14psu. It can survive over winter in lakes covered in ice.
It is a hermaphrodite capable of both cross-fertilisation and self-fertilisation and it releases broods of non-swimming pediveligers when these are 200μm length. Once they attain 6-10mm, from an age of about three months, they can reproduce once temperatures are of about 16°C. They may produce more than one brood each year with releases from late spring to autumn. Seventy thousand pediveligers can be produced by an individual in a year. This stage produces byssal threads that can allow it to be dragged by water currents. Juveniles and adults can be dispersed in the same way by producing tacky mucus strings. The species is tolerant of aerial exposure for weeks if maintained under damp conditions but it is intolerant of low oxygen levels. The species may often survive only a single year but under certain conditions can live to seven years. Growth is variable. Within a population there are normally two or more size-frequency modes. Should they become eliminated from an area they may quickly recolonize.
Asian clams require well-oxygenated water and are normally found on muddy to sandy sediments, but they will also occur on gravel and cobbles in lake shallows, rivers and streams. They may be found in irrigation and drainage canals. They have been found in nutrient-poor to nutrient-rich waters.
Wildlife and habitat impacts
Asian clams compete with other filter feeding bivalves and, because it can use its foot to collect organics in sediments, it can compete with snails. When present in great numbers their high densities can alter the food web within the ecosystem it occupies.
Human impacts
Individuals become entrained in abstraction and drainage piping causing changes to water flow and blockages. Shells may lodge within the narrow piping used in the condensers of power plants. Dead clams can become buoyant and reduce water flows by collecting on water intake screens. Should they die in pipe systems they can affect water quality.
Key vectors
It arrived in Portugal most probably with discharges of ships’ ballast water. It was subsequently spread by a number of dispersal modes elsewhere. The rapid expansion to its range within both North America and Europe would suggest that there is more than one vector process involved.
Clams are likely to arrive by:
  1. Transfer across seas and oceans with ships’ ballast water
  2. Used as an aquarium species and then possibly released to the wild.
  3. It may also be spread in the following ways:
    1. Used as an angling bait and surplus bait may be disposed
    2. May enter ornamental ponds and subsequently release/escape to the wild
    3. Spread with water craft movements
    4. Stock movements of fish
    5. Movements of river gravels
    6. Natural dispersal by bird and other animal movements
    7. Dispersal with water currents.

      What you can do as an individual
      Unless found at an early stage it is unlikely that the species will be eliminated. Should you find this species in the wild you should directly notify the Fisheries Division of the Department of Agriculture and Rural Development, The Natural Heritage Division of the Environment and Heritage Service of Northern Ireland and/ or CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email[at]
      Advice to specific groups relevant to this species
      The clam is an attractive species and so it is likely to be considered as an aquarium or garden pond ornamental. Should it occur in an aquarium, changed water should be poured on soil to avoid any larval or juvenile stages entering a water culvert. Stocked fish occurring in areas where there are Asian clams should have the contained water filtered or otherwise treated to prevent further spread. It should not be used as angling bait.
      Management measures
      Clams may be controlled using oxydising molluscicides, or by the flushing of water at more than 40°C. Should they occur in pipes they may be removed by using “pigs” (which are wads passed through the pipe under pressure to remove and purge fouling organisms) or by killing them by stopping the water flow through pipes. Screens and shell traps reduce impacts. The use of chlorine or bromine can kill juveniles and adults at low concentrations, but this should be done regularly otherwise their shells will continue to cause reduced flow and blockages. Legislation and control of its import and use may be successful for islands.
      Management groups
      The presence of this species is of importance to wildlife managers and those that abstract water. Development of legislation to prevent entry of Asian clams, and of other species that are known to have some unwanted impacts in the aquarium and ornamental trade, or its use as an angling bait would be advised.
      Further information
      Conditions in Northern Ireland are suitable for this species. It is likely to occur in unpolluted sediments free of silt and the species normally avoids sluggish waterbodies, yet it has been found in the Broads waterways in Britain. Because it is capable of self-fertilization it is theoretically capable of producing a new population based on the introduction of a single individual. Within its native range it is eaten and used as a food source for domestic fowl. For this reason there is the possibility that some ethnic groups may consider an introduction as a human food, as may have happened in North America. The species was present in Britain during the previous interglacial period.
      Britton, J.C. and Morton, B. (1977). Corbicula in North America: the evidence reviewed and evaluated. Proceedings, first International Corbicula Symposium. Fort Worth, Texas pp. 249-287.
      Jenner, H.A., Whitehouse, J.W., Taylor, C.J.L. and Khalanski, M. (1998). Cooling water management in European power stations: biology and control of fouling. Hydroecologie Applique Tome  19 Vol 1-2. 225pp.
      Karatayev, A.Y., Burlakova, L.E., Kesterson, T.and Padilla, D.K. (2003). Dominance of the Asiatic clam, Corbicula fluminea (Muller), in the benthic community of a reservoir. Journal of Shellfish Research  22 (2):  487-493.
      McMahon, R.F. (2000). Invasive characteristics of the freshwater bivalve Corbicula fluminea. In: R. Claudi and J Leach (eds) Non-indigenous Freshwater Organisms: Vectors, Biology and Impacts, pp. 315-343. Lewis Publishers. Boca Raton, Florida.
      Morgan, D.E., Kesar, M., Swenarton, J.T. and Foertch, J.F. (2003). Population dynamics of the asiatic clam, Corbicula fluminea (Muller) in the Lower Connecticut River: Establishing a foothold in New England. Journal of Shellfish Research 22 (1): 193-203.
      Swinnen, F., Leynen, M., Sablon, R., Duvivier, L. and Vanmaele, R. (1998). The Asiatic clam Corbicula (Bivalvia: Corbiculidae) in Belgium. Bulletin de L'Institut Royal des Sciences Naturelles de Belgique Biologie 68: 47-53.
      Vincent T, Brancotte V. (2002). Present distribution and spreading modes of Corbicula spp. in France. Bulletin de la Societe Zoologique de France.  127(3) < 241-252.
      See the Invasive Species Ireland web site for further information -
      Text written by:
      Dan Minchin, MOI, Ballina, Killaloe, Co Clare