Although the structure and ultrastructure of the digestive tract of the hydrothermal vent mytilid Bathymodiolus thermophilus conform to those of other bivalves, enzymological data are lacking. To address this question, digestive enzyme assays and histoenzymological tests were performed on different regions of the digestive tract: labial palps, oesophagus, stomach, digestive gland, intestine, and rectum. Carbohydrases, mainly present in the digestive gland and the stomach, were the most active of the 33 enzymes studied. These enzymes would allow substantial digestion of particles from the immediate environment as well as those descending from the photic zone. Acid phosphatases present in all the compartments of the digestive tract indicate intracellular digestion, whereas alkaline phosphatase activity, mainly in the digestive gland and the stomach, demonstrates absorption phenomena. We conclude that, in addition to the nutrient supply furnished by chemoautotrophic bacteria in the gill bacteriocytes, the digestive tract is functional and provides at least some of the nutritive requirements of this species.
Posterior Respiratory Apparatus of Inia geoffrensis and Sotalia fluviatilis: Structure and Ultrastructure