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 g ill in suspension-feeding bivalves, the mucocyte distribution was exam ined on the frontal surfaces of 2 gill types known to differ in struct ure and function: the heterorhabdic plicate gill of Placopecten magell anicus (Pectinidae) and the homorhabdic gill of Mytilus edulis (Mytili dae). Microscopic counts of mucocyte abundance were performed both on whole mounts of gill segments and on histological sections stained wit h alcian blue and Periodic acid-Schiff (PAS). A very clear separation of mucocyte types was observed on the P. magellanicus gill: mixed-secr etion mucopolysaccharide (MPS) mucocytes were found mainly in the prin cipal filament troughs, while acid MPS mucocytes were found mainly on the crests of the ordinary filament plicae. This distribution correspo nds to the functional specialization of these 2 sites as revealed by d irect endoscopic observation: feeding (principal filament) and cleanin g (ordinary filaments). A more uniform distribution and a broader rang e of MPS types was found for the M edulis gill, corresponding to the a bsence 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 v entral groove, no mucocytes are located in this groove. Implications o f these and further results are discussed in terms of theories of part icle transport on different gill types.