The mussel filtration rate (= clearance rate, CR)intercalibration exercise conducted by Petersen et al.(2004) is a laudable initiative and a turning point onthe stony road to reliable CR data. It is an expression oftrue scientific spirit when the participants in thecalibration are ready to accept the consequences of theintercalibration, requesting a critical re-evaluation ofthe literature, including some of their own earlierworks. Objective discussions based on scientific factsmay hopefully soon replace the barren and polariseddiscussion climate of recent years (Jorgensen 1996,Bayne 1998, 2001, Riisgard 2001a,b,c,d, Widdows2001, Cranford 2002).The intercalibration exercise is an important step inthe right direction. Especially the flow-through cham-ber method used for measurement of CR has causedtrouble over the last 25 yr, because the same flawswere repeated over and over again (Riisgard 2001a). Itis encouraging to note that the self-appointed Dutch,Danish and French intercalibration teams ‘call for athorough examination of previous investigations’—although it is doubtful whether it is possible to correcterroneous CR for suboptimal chamber flow-throughrate and/or poor chamber design (Larsen 2001).Petersen et al. (2004) correctly state that a prerequi-site for obtaining reliable CR data is that the applied ex-perimental methods are technically valid so that—within well-defined conditions—the different methodsactually measure the true CR. However, it is regrettablethat the participants in the calibration exercise did notreport on the degree of opening of the mussels; musselgape must be determined, to ensure that the CR mea-sured by means of the different methods were actuallycomparable. Prior to the calibration experiments, mus-sels were ‘acclimated’ to the grazing chambers withwater flow-through for ‘at least 1 h’. This exposure timemay not have been sufficiently long to ensure that themussels attained their clearance capacity, whichshould be the reference in this type of intercalibration.The degree of valve opening should have been care-fully monitored before and during the CR measure-ments. This well-intentioned comment is further sub-stantiated below in order to draw attention to the im-portance of the opening degree of bivalves in future CRexperiments. The thick shells of mussels make them robust tohandling, but this characteristic does not imply thatmussels are insensitive to the experimental conditions.Actually, mussels are very sensitive, not only to mech-anical disturbances, but also to the presence of sus-pended food particles in the ambient water (Jorgensen1975, 1990, Riisgard & Randlov 1981, Riisgard et al.2003). This is reflected in the gape of the valves. Thus,valve-opening behaviour ranges from closed valvesand retraction of mantle edges, to fully open valvesand extended mantle edges (Jorgensen et al. 1988, seeFig. 1 therein, Riisgard et al. 2003, see their Fig. 9).Under optimal conditions, mussels filter the ambientwater at a maximum rate; under suboptimal environ-mental conditions, including low or (very) high con-centrations of phytoplankton, the valve gape is re-duced and the mantle edges are retracted.The sensitive opening-closing phenomenon andresponse times in 3 species of bivalves,