The role of mucus in particle processing by suspension-feeding marine bivalves: unifying principles

Contemporary research on bivalve suspension-feeding has revealed a diversity of particle processing mechanisms depending on the anatomy and functioning of the pallial organs involved. On the biochemical level, however, some evidence of homogeneity has emerged concerning the types of mucopolysaccharide associated with particle processing. The present study uses both previous data and original research combining video endoscopy and mucocyte mapping to further explore the relationships between pallial organ topography, functional correlates, direction of current flow, and mucocyte secretion type. Five species representing five different families and all four major gill types are represented: Mytilus edulis, Placopecten magellanicus, Crassostrea virginica, Mya arenaria, and Spisula solidissima. Viscous acid or acid-dominant mucopolysaccharides are used when particle transport occurs on an exposed surface, or on a structure leading directly to such a surface, counter to the prevailing current flow. Associated functions are indiscriminate transport in gill ventral particle grooves and rejection of pseudofeces. Lower-viscosity mixed mucopolysaccharides are used when particle transport is on an enclosed or semi-enclosed surface, leading to other such surfaces, and with the current flow. Associated functions are transport of particles destined for ingestion, and ingestion itself. Low-viscosity neutral mucopolysaccharides are found in regions where reduction of mucus viscosity is important, such as the areas of the labial palps responsible for fluidization of the high-viscosity mucus-particle cord of the gill ventral particle groove prior to particle extraction. There thus appears to be a specialization of mucus type corresponding to functional specialization of the various pallial organs in suspension-feeding marine bivalves.