Metabolic responses to sulfur in lucinid Bivalve

SYNOPSIS. Lucinid bivalves occur widely in habitats ranging from subtidal to deep ocean basins. The evidence reported to date supports the contention that all lucinids contain intracellular, sulfur-oxidizing bacterial symbionts which contribute substantially to the nutrition of the intact association. These burrowing bivalves are found in both high and low sulfide habitats. Examinations of sulfur compounds in the hemolymph of lucinids reveal that thiosulfate may be a key intermediate in the metabolism of sulfide in all members of this family. The presence of free sulfide in the hemolymph of both freshly collected and experimental clams suggests that the total sulfide oxidation/detoxification capacity of the lucinids may be substantially lower than that found in other sulfur bacteria/invertebrate symbioses. Intracellular granules catalyze the oxidation of sulfide in L. floridana, one apparent line of sulfide toxicity defense in this species. These electron-dense granules occur in high densities in the bacteriocytes (bacteria-containing gill cells) of all lucinids. Thiosulfate produced during this detoxification may be available for further oxidation by the abundant symbionts housed in these same cells. Hemolymph thiol and elementalsulfur levels in clams from low sulfide habitats suggest that these animals have evolved mechanisms for sulfur acquisition. In sulfur-free seawater, sulfur-starved Lucina floridana are infrequent, periodic ventilators, but in the presence of sulfur show a significant increase in ventilation frequency. This periodic ventilation may be a trait of all lucinids that plays a substantial role in reducing metabolic costs during low nutrient availability.