Under optimal conditions, bivalves tend to filter the ambient water at a maximum rate but under suboptimal environmental conditions, including low or very high algal concentrations, the filtration rate is reduced. The upper algal concentration at which the blue mussel, Mytilus edulis, exploits its filtration capacity over an extended period of time was identified by stepwise raising the algal (Rhodomonas salina) concentration in steady-state experiments above the threshold for continuous high filtration rate. The duration time before incipient saturation reduction decreased with increasing algal concentration, and the threshold concentration for incipient saturation reduction of filtration activity was found to be between about 5,000 and 8,000？cells mL？1, equivalent to 6.3 and 10.0？μg chl a L？1, respectively. Reduced filtration rate was related to total number of algal cells ingested previous to incipient saturation and found to be cells. Video-microscope recordings of pseudofaeces production revealed that the trigger threshold concentration for formation of pseudofaeces was about 12,000？cells mL？1. Faeces produced by saturated mussels consisted of closely packed undigested algal cells, indicating severe overloading of the digestive system caused by high algal concentrations which mussels are not evolutionary adapted to cope with. 1. Minireview of the Current State of the Art 1.1. Physiological Regulation of Feeding There is still no general agreement regarding physiological control of water pumping by suspension-feeding bivalves in response to changes in concentrations of organic and inorganic particles in the ambient water. Under optimal conditions, suspension-feeding bivalves filter the ambient water at a maximum rate, but under suboptimal environmental conditions, including low or very high concentrations of algal cells, the valve gape is reduced, and the mantle edges retracted [1–14]. However, it has for a long time been a matter of discussion whether the filtration rate in suspension-feeding bivalves is physiologically regulated [14–23], or if it should be conceived as a basically autonomous process [2, 5, 24–27]. More recently, MacDonald and Ward [28] suggested that the blue mussel Mytilus edulis may have “adopted a “maximal clearance rate” strategy, clearing particles from suspension at a rate that is independent of seston quality.” The valve-opening response to the absence or the presence of algal cells has been thoroughly studied in Mytilus edulis and other suspension-feeding bivalves by, for example, J？rgensen [29], Riisg？rd and Randl？v [1],
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