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Observations on the measurement and interpretation of clearance rate variations in suspension-feeding bivalve shellfish

Mussels Mytilus edulis (35 ± 2 mm shell length) were fed on cultured Isochrysis galbana in a flow-through system, and measures were undertaken to quantify separate effects of inflow cell concentration, outflow cell concentration, percentage reduction (i.e. recycling), and flow rate and/or feeding history on clearance rate measures (CR; l h -1 ). Findings identify the following features of physiological regulation: (1) a lag phase in the feeding response to changes in food concentration, of around 30 min; (2) a trigger level in algal cell (chlorophyll (chl) a) volume/concentration (4000 cells ml -1 , 0.5 µg l -1 chl a) below which filtering in most mussels ceases; (3) saturation reduction, or satia- tion, resulting in reduced CR and valve closure after feeding for over 2 h at 30 000 or more cells ml -1 (~6 µg l -1 chl a); and (4) at food levels between Features 2 & 3, CR increases to maximal rates (ca. 2 to 2.5 l h -1 ind. -1 or 6 to 7.5 l -1 h -1 g -1 dry weight). Our findings also help resolve uncertainties associ- ated with the measurement and interpretation of clearance rate variations. These uncertainties are associated with: differences that result from analyses of feeding responses to different components of the available seston; whether variations in seston availability are physiologically relevent; analyses of individual shellfish as compared with average responses computed for >1 ind.; and comparisons between short-term and integrated measures. We think it is important to consider whether measures of feeding are really required per individual in real time, and we stress that population averaging and temporal integration are required for the scaling up of results when simulating ecological interrela- tions between shellfish populations and their environments.