The influence of suspension density and temperature on the filtration rate of Hiatella arctica

The rate of filtering Phaeodactylum tricornutum and Isochrysis galbana was measured in Hiatella arctica (L.) by the indirect suspension depletion method monitored by optical density measurement. The filtration rate of H. arctica was found to be 1.412×10^–2 l/h/g wet weight at a temperature of 15°C when fed with P. tricornutum, at average cell concentrations up to 3.5×10⁶ cells/ml. The filtration rate dropped almost to zero when the concentration of P. tricornutum reached 11×10⁶ cells/ml. The filtration rate of I. galbana diminished at a much lower cell concentration of 1×10⁶ cells/ml, and almost ceased at 3 to 4×10⁶ cells/ml. In mixed cultures of I. galbana and P. tricornutum, the filtration rate ratio was 0.37 to 1.00, and this was believed to be due to a proportion of the smaller former cells passing through the ostia. However, when resuspended in sea water, I. galbana cells were taken at a rate slightly less than P. tricornutum. The medium in which the I. galbana cells had been grown was inhibitory to the filtering activity of H. arctica, since, when cells of either alga were resuspended in the medium, the filtration rate was considerably reduced. No inhibitory factor existed in either of the original nutrient media. Hence, the importance of using low cell concentrations and of eliminating any inhibitory metabolic products when measuring filtration rates of bivalves is stressed. H. arctica shows a typical activity temperature eurve for a boreo-arctic species, with a steady rise from 0°C to a maximum between 15° and 17°C, and a sharp fall in activity to about zero at 25°C. The rates of filtration of various species at temperatures approaching the optimum were compared after allowance was made for fall in filtration rate with increasing body weight. The results suggested that the Mytilacea had the highest filtration rates and that H. arctica possesses one of the lowest filtration rates recorded.