INGESTION AND ASSIMILATION OF CARBON FROM CELLULOLYTIC BACTERIA AND HETEROTROPHIC FLAGELLATES BY THE MUSSELS GEUKENSIA-DEMISSA AND MYTILUS-EDULIS (BIVALVIA, MOLLUSCA)

We C-14-labeled cellulolytic bacteria and 3 species of bacterivorous n anoflagellates and fed these cultured organisms to 2 species of intert idal mussels, Geukensia demissa and Mytilus edulis, using a pulse-chas e experimental design under controlled laboratory conditions. Ingestio n and assimilation of C from these microheterotrophs by mussels were c alculated from measured rates of defecation, respiration, excretion, a nd tissue incorporation. The proportion of available C ingested by G. demissa did not differ significantly among bacteria (39%), heterotroph ic flagellates (58%), or the unicellular algae Isochrysis galbana (66% ), which was used as a reference diet. In contrast, M. edulis ingested a significantly lower proportion (19%) of the small bacteria (<1 mu m in diameter) than the larger (3 to 5 mu m diameter) heterotrophic fla gellates (58%). The efficiency with which G. demissa assimilated C fro m I. galbana (77%) was significantly greater than that from either bac teria (42%) or heterotrophic flagellates (44%). M. edulis assimilated bacterial C with significantly lower efficiency (21%) than C from hete rotrophic flagellates (62%). These results indicate that heterotrophic flagellates can contribute to the C requirements of both G. demissa a nd M. edulis; however, only G. demissa is capable of assimilating a co nsiderable amount of C from bacteria.