What makes diatoms attractive for suspensivores? The organic casing and associated organic molecules of Coscinodiscus perforatus are quality cues for the bivalve Pecten maximus

A major question in the study of seston particle dynamics is the properties of particles which induce selection in suspension feeders. In order to determine whether the diatom organic casing and associated organic molecules (herein termed ‘perifrustular envelope’) influences selection by the Great Scallop Pecten maximus L., a mixed culture containing intact and naturally dead (empty frustules) Coscinodiscus perforatus was presented to normally feeding scallops. The presence of the envelope on all cells of the culture was verified using scanning electron microscopy. Proportions of intact and dead cells were determined via endoscopy-assisted sampling at strategic points in the particle processing chain. The use of C. perforatus as a food item was verified by microscopic examination of faeces at the end of the experiment. Selection indices were calculated to quantify eventual enrichment of empty, dead cell proportions at the rejection sites, in relation to the previous steps in the particle-processing chain. No significant difference was observed in the proportions of intact and dead cells at any of the sites, nor in the selection indices, and the large P-values suggest that both cell types were treated in the same manner by these actively feeding scallops (P = 0.760 and 0.839 for proportions and selection indices, respectively). Together with previous studies which have shown that empty, cleaned (perifrustular envelope absent) diatom frustules provoke rejection; these results strongly support the hypothesis that the presence of the external organic components is a major selection cue. This has implications for phytoplankton dynamics, notably with respect to the potential fates of species captured by this suspension feeder.