MEPS 295:191-199 (2005)  -  doi:10.3354/meps295191

ABSTRACT: In order to resolve outstanding problems in the understanding of mechanisms of particle processing in the oyster gill, the technique of mucocyte mapping was used to quantify mucocyte types and fine-scale distributions on the frontal ciliated tracts of the ordinary filaments comprising the gill plicae. The frontal tracts were divided into 1 frontal median tract (FMT) and 2 frontal lateral tracts (FLT) per filament, based on relative cilia lengths. Mucocyte counts within each tract were performed on periodic acid Schiff (PAS)–alcian blue-stained histological sections under oil immersion, and densities were recorded per linear µm (l µm) of tract width, after evaluation of artefactual shrinkage for exact dimensions of tracts. Total mucocyte volumes were obtained using mean mucocyte volumes. Two types of mucocytes were most frequently observed: acid mucopolysaccharide (AMPS) containing mucocytes, and mixed (acid + neutral) mucopolysaccharide (MMPS) containing mucocytes. Neutral mucopolysaccharide (NMPS) containing mucocytes were rare. The apical filaments contained over twice the mucocyte densities and total mucocyte volumes, compared to the lateral filaments of the plicae (p ≤ 0.001), indicating that the apical filaments are the principal particle processing site of the plicae. The lateral filaments may accomplish functions more related to the establishment of the plicae and ensuing consequences (hydrodynamic particle capture, physical separation of the dorsal and ventral gill particle groove functions, and enlargement of filtering area). On the apical filaments, the FMT showed much greater AMPS containing mucocyte densities and total mucocyte volumes, compared to the FLT. This, together with the respective cilia length differences, indicates that the FMT is the principal particle transport tract. Such a specialization was not observed on the lateral filaments of the plicae, again supporting the conclusion that the apical filaments are the active site of particle processing on the plicae. FLT appear to be uninvolved in particle transport, and probably participate in the establishment and maintenance of the dorsalward current flow in the pallial cavity.

KEY WORDS: Crassostrea gigas · Oyster · Gill · Ciliated tracts · Mucocytes · Feeding