FLEXIBLE DIGESTION STRATEGIES AND TRACE-METAL ASSIMILATION IN MARINE BIVALVES

Pulse-chase experiments show that two marine bivalves take optimal adv antage of different types of particulate food by varying food retentio n time in a flexible two-phase digestive system. For example, carbon i s efficiently assimilated from bacteria by subjecting nearly all the i ngested bacteria to prolonged digestion. Prolonging digestion also enh ances assimilation of metals, many of which are toxic in minute quanti ties if they are biologically available. Detritus-feeding aquatic orga nisms have always lived in environments naturally rich in particle-rea ctive metals. We suggest that avoiding excess assimilation of metals c ould be a factor in the evolution of digestion strategies. We tested t hat suggestion by studying digestion of particles containing different Cr concentrations. We show that bivalves are capable of modifying the digestive processing of food to reduce exposure to high, biologically available, Cr concentrations. The evolution of a mechanism in some sp ecies to avoid high concentrations of metals in food could influence h ow effects of modern metal pollution are manifested in marine ecosyste ms.