HISTOLOGY, HISTOCHEMISTRY AND ENZYME BIOCHEMISTRY IN THE DIGESTIVE-SYSTEM OF THE ENDOSYMBIONT-BEARING BIVALVE LORIPES LUCINALIS (LAMARCK)

The structural and functional aspects of the digestive gland in the en dosymbiont-bearing bivalve Loripes lucinalis were examined. Specimens were collected over a one year period (September 1991 to September 199 2) from the Moulin Blanc beach, Bay of Brest, (Brittany, France). The two apertures leading from the stomach into the digestive diverticula are very large as are the digestive ducts and tubules. The main and se condary ducts possess no ciliated tracts and present a definite brush border. The structural aspects of the cells lining the ducts, in addit ion to the presence of abundant acid phosphatase activity, suggest tha t these cells are actively involved in absorption and intracellular di gestion of food material. Lipid and glycogen are abundant throughout t he digestive diverticula suggesting that this organ is the site of sig nificant energy stores. The tubules contain the two classical cell typ es, namely the secretory and digestive cells. Also present within the tubules are extremely large acid polysaccharide-filled vacuoles which originate from the lysis of digestive and/or secretory cells. These va cuoles suggest an extensive process of tubule autophagy and regenerati on. The intestine is lined with cilia and microvilli. The cytological features of the intestinal cells, together with an abundant acid phosp hatase activity, suggest that these cells are capable of absorption an d intracellular digestion. The wet weight of the gonad-digestive gland complex varied throughout the year between 12 and 61.2 mg with an ave rage wet weight of 31.8+/-8 mg. Following spawning, total wet weights dropped by 50%. The alpha-amylase activity was measured and an average alpha-amylase to protein ratio of 5.34+/-1.09 U/g protein was obtaine d. It was observed that amylase activity expressed per g protein may v ary with the animal's physiological status. The temporal evolution of absolute alpha-amylase activity throughout the sampling period reveale d that the variations in activity were dependent on a combination of e nvironmental factors, including temperature and food levels. Digestive activity, however, did not seem to correspond to reproductive efforts .