The effect of food supply on feeding strategy in sessile female gastropods Crepidula fecunda

Sessile females of the gastropod Crepidula fecunda are exclusively suspension-feeders. Our previous work showed that particles removed from suspension are entrained on the gill and continuously accumulate as a mucous string on the distal margin of the lamella, although some are transferred by a ciliary tract to the food pouch, where they form a mucous ball. The mucous string is transferred from the gill margin to the neck canal, where it forms a mucous cord which is transported to the mouth. The mucous ball and the mucous cord are ingested with the aid of the radula. In this study we examined the response of the system to a wide range of cell concentrations of the flagellate Isochrysis galbana. As the particle concentration increased from 5,000 to 200,000 cells ml(-1), the volume of the mucous ball almost doubled, as did the number of radular extrusions required to ingest it. The total volume of mucous balls produced and ingested increased very slightly as particle concentration rose from 5,000 to 100,000 cells ml(-1), but the volume of mucous cord material ingested increased four to five times over the same range. At higher food concentrations the mucous cord increased in thickness, but not in length, which is constrained by the length of the gill margin. At all particle concentrations the mucous cord accounted for 80-95% of the total material (mucous cords plus mucous balls). Ingestion rate reached a maximum at 140,000 cells ml(-1) for mucous balls and mucous cords. At higher concentrations the production of pseudofaeces increased from all sources ('type-I' pseudofaeces from rejected mucous balls; 'type-II' pseudofaeces from rejected fragments of mucous cords; and 'type-III' pseudofaeces, which come from particles trapped in a coarse mucous net at the mouth of the inhalant mantle cavity and are transferred via the lateral ciliary tract to the mantle margin, by-passing the food pouch). Thus C. fecunda has adopted a feeding strategy in which particle entrainment by the gills is continuous, even at very high ambient particle concentrations (200,000 cells ml(-1) for I. galbana), and excess material which cannot be ingested is rejected by one or more of three routes for the elimination of pseudofaeces.