Gill ultrastructure and symbiotic bacteria in the tropical lucinid, Linga pensylvanica (Linne)

The cellular organization of the gill filament is described in the lucinid, Linga pensylvanica, whose gill cells host the same species of sulfur-oxidizing bacterial endosymbionts as Codakia orbicularis, another inhabitant of similar tropical seagrass beds. In L. pensylvanica, the ciliated zone is shorter and the intermediary zone is more elongated than they are in the gill filament of Lucinidae described to date. Secretory cells appear to be much diversified, with three secretory cell types; one classical mucous cell type interspersed with bacteriocytes and intercalary cells in the lateral zone, and two secretory cell types located at the limit of the intermediary zone and lateral zone. Bacteriocytes have been observed to have direct contact with pallial sea water even so intercalary cells overlap partly their apical microvilli. Peroxisomes which have not been identified in the gill cells of other symbiotic bivalves are common organelles in the bacteriocytes of L. pensylvanica. To compare gill structures in L. pensylvanica, C. orbicularis, and Lucina pectinata shows that each species has unique features. Significant differences between C. orbicularis and L. pensylvanica gill structure have not received a straightforward interpretation so far.