Studies concerned with the sub-lethal aspects of the toxicity of copper to the mussel have, in many cases, been compromised through the use of static experimental systems (see Davenport & Redpath (1984) for review). Manley, Gruffydd & Almada-Villela (1984), using a through flow system, reported a significant reduction in shell growth at a concentration of 10 μg I-1 added copper with recovery after metal addition had ceased. Strömgren (1982) investigated the effects of a series of concentrations of the metal (amongst others) on Mytilus, but there are discrepancies between his results and the work of others which have been fully discussed elsewhere by Davenport and Redpath.