Gill function and mucocyte distribution in Placopecten magellanicus and Mytilus edulis (Mollusca: Bivalvia): the role of mucus in particle transport

In order to elucidate the role of mucus in particle transport on the gill in suspensionfeeding bivalves, the mucocyte distribution was examined on the frontal surfaces of 2 gill types known to differ in structure and function: the heterorhabdic plicate gill of Placopecten magellanicus (Pectinidae) and the homorhabdic gill of Mytilus edulis (Mytilidae). Microscopic counts of mucocyte abundance were performed both on whole mounts of gill segments and on histological sections stained with alcian blue and Periodic acid-Schiff (PAS). A very clear separation of mucocyte types was observed on the P. magellanicus gill: mixed-secretion mucopolysaccharide (MPS) mucocytes were found mainly in the principal filament troughs, while acid MPS mucocytes were found mainly on the crests of the ordinary filament plicae. This distribution corresponds to the functional specialization of these 2 sites as revealed by direct endoscopic observation: feeding (principal filament) and cleaning (ordinary filaments). A more uniform distribution and a broader range of MPS types was found for the M. edulis gill, corresponding to the absence of anatomical specialization for the separation of feeding and cleaning functions in this species. Although M. edulis gill functioning is characterized by the presence of a substantial mucus cord in the ventral groove, no mucocytes are located in this groove. Implications of these and further results are discussed in terms of theories of particle transport on different gill types.