Feeding responses of the gastropodCrepidula fornicatato changes in seston concentration

We investigated the response of the suspension-feeding activity of the prosobranch gastropodCrepidula fornicata(L.) to changes in seston abundance and composition, characteristic of the natural particle variability experienced by suspension-feeding organisms as a consequence of tidal resuspension. The experimental conditions were characterized by seston concentrations (12.2 to 196.0 mg l–1) inversely related to the organic content of particulate matter (56 to 6%). The organic fraction was composed of 2 benthic diatoms,Entomoneis pseudoduplex(Smith) andNitzschia aurariae(Cholnoky). Inorganic matter was composed of 2 to 3 μm diameter kaolinite particles. The functional unit used for determination of ecophysiological responses was a chain of 6 individuals with mean chain dry tissue weight of 1.05 g. The efficiency of particle retention as a function of particle size increased with increasing particle size from 2 to 4 μm (equivalent spherical diameter). Above 4 μm, all particles were cleared from suspension with 100% efficiency, but retention efficiencies of smaller particles significantly declined with increasing seston loads. A constant clearance rate (CR) was found throughout the seston range, with a mean of 0.76 ± 0.05 l h–1g–1(±SE). Filtration rate increased linearly with seston concentration from a minimum value of 10 mg h–1g–1to a maximum of 42.8 mg h–1g–1at the highest concentration. The total biodeposition rate (sum of pseudofaeces and faeces) followed a similar linear trend, with an increase from 3.8 to 41.6 mg h–1g–1at the highest concentration. Absorption rate was very stable throughout the range of suspended particulate matter concentrations tested with a mean of 4.9 ± 0.5 mg h–1g–1(±SE). It was not possible to ascertain pre-ingestive selection, even though the organic content of biodeposits and diet was very similar. The low retention efficiency of small inorganic particles at high seston loads was the sole mechanism that was clearly identified in this study as counteracting food dilution by inorganic material, through the enrichment of the organic content of filtered matter. The absence of pre-ingestive selection inC. fornicatawould increase the physiological significance of this process in turbid environments.