Dynamic filter-feeding responses in fouling organisms

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Filter-feeding responses were studied in 2 ascidians, Ciona intestinalis and Ascidiella aspersa, and a bryozoan Membranipora membranacea to suspensions of Isochrysis galbana at con- centrations between 1000 and 100 000 cells ml -1 , and to a mixture of I. galbana and inorganic silt (0.36 to 36 µg l -1 chlorophyll a; 0.5 to 15 mg l -1 total particulate matter), thus simulating a wide range of food availabilities. Clearance rates (CR) varied significantly, reaching maxima of 4.61 l g -1 h -1 at about 5000 cells ml -1 in C. intestinalis and 5.26 l g -1 h -1 above about 10 000 cells ml -1 in A. aspersa. Between 1000 and 25 000 cells ml -1 , CR in a standardised 1000 mm 2 colony of M. membranacea increased from 0.50 to 1.23 l h -1 , the maximum equating to 0.36 ml zooid -1 h -1 . At 100 000 cells ml -1 , CR in M. membranacea reduced dramatically to 0.11 l h -1 , associated with satiation and the cessation of feeding. Apart from this one observation of satiation in M. membranacea, ingestion rates increased linearly with cell concentration in all 3 species. Assessing the selective retention of particles between 2 and 5.5 µm in diameter in the I. galbana and silt mixture, C. intestinalis showed a significant decrease in retention efficiency for all particles above 4.5 µm, and M. membranacea exhibited posi- tive selection for algal particles over inorganic silt, whereas A. aspersa showed similar retention of all particles. Collective findings indicate that functional dependencies of CR and retention efficiency upon food quantity and quality should be included within dynamic models of feeding and growth in these and other suspension-feeding species.