Bacterial symbiosis in Loripes lucinalis (Mollusca: Bivalvia) with comments on reproductive strategy

Abstract

Loripes lucinalis, a littoral bivalve harbouring endocellular sulphur-oxidizing bacteria within its gills, was studied between September 1991 and September 1992 at the Moulin Blanc beach (Brittany, France). The general macro-anatomy of the gill consists of bacteriocyte channels and cylinders made up of two cell types, namely bacteriocytes and intermediate cells. In addition to the usual assortment of cell organelles, the bacteriocytes contain bacterial cells, electron-dense inclusions and large granular bodies. The bacteria are numerous within the bacteriocyte zone, representing 55±8% of the gill cross-sectional surface area. Acid phosphatase activity is detectable within the gill tissue both in proximity to and within the bacterial cells. The electron-dense inclusions and large granular bodies also stain positively for acid phosphatase suggesting that these bodies represent different phases of endosymbiont digestion. Bacterial lysis may be an important means of providing L. lucinalis with its nutritional requirements. The granular bodies stain positively for acid polysaccharides and the percentage of the gill cross-sectional area occupied by lysed granular zones varied as a function of sampling period. The percentage of total body weight represented by the gill was significantly lower in March compared to September, October and January, all other months being intermediate. The decrease in gill percentages and the levels of gill lysis correspond remarkably well to gamete development.