Mercenaria mercenaria

Phylum: Mollusca
Class: Pelecypoda
Order: Veneroida
Species name: Mercenaria mercenaria (Linnaeus 1758)
Synonyms: Venus mercenaria L.
Common name: American hard-shelled clam, little-neck clam, quahog, cherry stone clam.
Date of introduction and origin
Mercenaria mercenaria originates from the east-coast of N. America (Nova Scotia, Canada to Yucatan, Mexico. The first live specimen was found in the Humber in 1864 and last recorded from Cleethorpes in 1907 (Heppell 1961). It was successfully introduced from the USA, possibly the New York area, in 1925 to Southampton Water (Mitchell 1974). There were earlier introductions but none resulted in the establishment of self-sustaining populations.
Method of introduction
There was deliberate commercial introduction a barrel of live clams was imported from the USA to be tried as eel bait. Some were laid in the River Test arm of Southampton Water in the cooling water discharge from the former town power station (Mitchell 1974). The point of introduction is now under the Eastern Docks extension. Some were introduced into the Fleet, Dorset, in the early to mid-1960s, where they persisted for some time but do not appear to have bred (D.R. Seaward pers. comm.). It is considered that some clams may also have been thrown overboard into Southampton Water from transatlantic liners using the port, but there is no direct evidence for this unlikely event (Ansell 1963).
Reasons for success
The population has apparently increased since the 1950s, possibly due to occupying the niche of the soft-shelled clam Mya arenaria, which was eliminated from the estuary by the cold winters of 1947 and 1962/63 (Mitchell 1974). The Mya arenaria population has never recovered. Favourable physical conditions are likely to be the prime reason for the original colonisation. There were ideal estuarine conditions available i.e. lowered salinity and soft substrata with temperatures elevated by power station cooling water discharges from Southampton power station and later Marchwood Power Station (opened in 1957) almost opposite the original site of introduction. Further heating of Southampton Water and the eastern Solent has occurred through industrial cooling water discharges from ESSO Fawley and the Fawley Power Station (Mitchell 1974, 1976). Furthermore, it appears that this species has become physiologically adapted to be able to spawn at 3-4·C lower (i.e. 18-19·C) than populations in its area of origin (Mitchell 1974, 1976). Hibbert (1976) recorded some spawning at 17·C.
Rate of spread and methods involved
Since the original introduction to upper Southampton Water in 1925, the population has spread naturally by larval dispersal along the eastern side of Southampton Water and into Portsmouth and Langstone Harbours in The Solent. Some dumping from the fishery has probably helped to augment the populations in Portsmouth and Langstone Harbours. A substantial number have also been introduced to Newtown Harbour, Isle of Wight, and the Blackwater Estuary, Essex. Specimens obtained from the north Isle of Wight are never smaller than 80 mm suggesting that this population is not breeding (J. Light & I. Killeen pers. comm.).
The extent of its current occurrence is the eastern side of Southampton Water, Portsmouth and Langstone Harbours, sporadically between Newtown Harbour and Ryde Pier along the north coast of the Isle of Wight (J. Light & I. Killeen pers. comm.) and the Blackwater Estuary, Essex. In Europe populations exist in The Netherlands and France but it is not known whether these are self-sustaining.
Factors likely to influence spread and distribution
Elevated estuarine temperatures through heated industrial discharges are likely to favour the spread of this species in British waters, especially where the temperature required for reproduction is reached. Dumping and deliberate introduction attempts by fishermen and fishery scientists also influence its spread.
Effects on the environment
It filled the niche left by the cold weather die-off of the soft-shelled clam Mya and thus prevented they re-establishment of Mya. Digging and dredging for this clam has a significant effect on the environment, particularly eel grass Zostera beds (Cox 1991; Anon. 1992). The populations of Mercenaria in the Solent are now very low (MAFF pers. comm.).
Effects on commercial interests
No commercial interest is known to have been adversely affected by the arrival of this species. Instead it has supported a thriving fishery from the 1960s to the present. Latterly the fishery has been severely depleted, primarily due to poor spatfall (MAFF pers. comm.), but possibly due also to the large numbers taken and physical damage to the environment.
Control methods used and effectiveness
The species is not controlled although the population has been severely depleted by the fishery.
Beneficial effects
See above.
The history of Mercenaria mercenaria in England has shown that deliberate introductions can work commercially.
Anon. 1992. An experimental study on the impact of clam dredging on soft sediment macroinvertebrates. (Contractor: Southern Science, Hampshire Laboratory, Otterbourne, Hants.) Unpublished report to English Nature. (English Nature Research Report, No. 13.)
Ansell, A.D. 1963. Venus mercenaria L. in Southampton Water. Ecology, 44: 396-397.
Cox, J. 1991. Dredging for the American hardshell clam - the implications for nature conservation. Ecos. A Review of Conservation, 12: 50-54.
Hayward, P.J., & Ryland, J.S. eds. 1990. The marine fauna of the British Isles and north-west Europe. 2 vols. Oxford, Clarendon Press.
Heppell, D. 1961. The naturalization in Europe of the quahog, Mercenaria mercenaria (L.). Journal of Conchology, 25: 21-34.
Hibbert, J.H. 1976. Production studies of a bivalve population on an intertidal mudflat, with particular reference to the energy budget of Mercenaria mercenaria (Linne). PhD Thesis, University of Southampton.
Mitchell, R. 1974. Aspects of the ecology of the lamellibranch Mercenaria mercenaria (L.) in British waters. Hydrobiolgical Bulletin, 8: 124-138.
Mitchell, R. 1976. A possible relationship between rate of river flow and recruitment in an estuarine bivalve population. In: Freshwater on the sea, ed. by C.M. Skreslet, 203-209. Oslo. Association of Norwegian Oceanographers.
Dr R. Mitchell, English Nature.