Histology, histochemistry and enzyme biochemistry in the digestive system of the endosymbiont-bearing bivalve Loripes lucinalis (Lamarck)

Abstract

The structural and functional aspects of the digestive gland in the endosymbiont-bearing bivalve Loripes lucinalis were examined. Specimens were collected over a one year period (September 1991 to September 1992) 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 secondary ducts possess no ciliated tracts and present a definite brush border. The structural aspects of the cells lining the ducts, in addition to the presence of abundant acid phosphatase activity, suggest that these cells are actively involved in absorption and intracellulgr digestion of food material. Lipid and glycogen are abundant throughout the digestive diverticula suggesting that this organ is the site of significant energy stores. The tubules contain the two classical cell types, 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 vacuoles suggest an extensive process of tubule autophagy and regeneration. The intestine is lined with cilia and microvilli. The cytological features of the intestinal cells, together with an abundant acid phosphatase activity, suggest that these cells are capable of absorption and intracellular digestion. The wet weight of the gonad-digestive gland complex varied throughout the year between 12 and 61.2 mg with an average wet weight of 31.8 ± 8 mg. Following spawning, total wet weights dropped by 50%. The α-amylase activity was measured and an average β-amylase to protein ratio of 5.34 ± 1.09 U/g protein was obtained. It was observed that amylase activity expressed per g protein may vary with the animal's physiological status. The temporal evolution of absolute α-amylase activity throughout the sampling period revealed that the variations in activity were dependent on a combination of environmental factors, including temperature and food levels. Digestive activity, however, did not seem to correspond to reproductive efforts.