Localization of qualitative particle selection sites in the heterorhabdic filibranchPecten maximus(Bivalvia: Pectinidae)

Particle-processing, and specifically qualitative particle selection, was studied inPecten maximususing intact and empty versions of the naturally occurring diatomCoscinodiscus perforatus. Endoscope-directed sampling allowed comparison of particle proportions or numbers in the experimental medium, dorsal and ventral gill tracts, and labial palp pseudofeces. The general processing of particles was closely similar to that previously described for the more primitive scallopPlacopecten magellanicus, despite the large phylogenetic distance between these 2 species. The gill and the labial palps ofPecten maximuseach effected particle selection (rejection of empty, cleaned frustules), both in the presence of intactC. perforatusparticles, and when only empty, cleaned frustules were presented. Particle selection operated even when small proportions (approx. 10%) of empty, cleanedC. perforatusfrustules were present in the particle mixture. This represents the first direct experimental demonstration of the sites of qualitative particle selection in a heterorhabdic filibranch bivalve, and shows the process to be located in both the gills and the labial palps. The qualitative selection capacity of the heterorhabdic gill could be the result of either early evolution in turbid environments, or an adaptation which avoids overloading qualitative selection of the palps in this gill type.