EFFICIENT GRAZING AND UTILIZATION OF THE MARINE CYANOBACTERIUM SYNECHOCOCCUS SP BY LARVAE OF THE BIVALVE MERCENARIA-MERCENARIA

Efficient grazing by marine bivalve larvae has been thought to be limited to particles larger than 4 mu m in diameter, thereby eliminating photosynthetic and non-photosynthetic picoplankton as contributers to larval diets. Documentation of ingestion, carbon retention and growth of laboratory-reared larvae of the bivalve Mercenaria mercenaria L. on Synechococcus sp. (WH7803), a small unicellular cyanobacterium 1 mu m in diameter, was facilitated using C-14-labelled cells in pulse/chase experiments and growth of larvae on diets of cell mixtures of both Synechococcus sp. and the haptophyte Isochrysis aff. galbana (TISO). Clearance rates on Synechococcus sp. ranged between 2 and 23 mu l larva(-1) h(-1) depending on ambient cell concentration and larval age. Retention efficiency of cell carbon after gut evacuation was about 55% for both prey species. Growth rates of larvae fed on monocultures of Synechococcus sp. at typical summer concentrations in coastal waters (1x10(5) cells ml(-1), similar to 29 mu g C l(-1)) was two-fold lower than on monocultures of Isochrysis galbana at 1x10(4) cells ml(-1) (similar to 120 mu g C l(-1)). Larval growth was inhibited and atrophy of the digestive gland was observed when Synechococcus sp. was offered at concentrations at or exceeding 8.6x10(5) cells ml(-1). Larval growth was enhanced, however, in the presence of Synechococcus sp. (5x10(4) cells ml(-1)) when Isochrysis galbana was limiting. During the diurnal study of Synechococcus sp. population dynamics conducted by Waterbury et al. (1986) in Vinyard Sound, Massachusetts, the abundance of bivalve larvae was sufficient to account for 12 to 24% of the calculated grazing activity on Synechococcus sp. When nanoplankton are scarce, invertebrate larvae may exert considerable grazing pressure on Synechococcus sp. and derive benefit from ingestion of these cyanobacteria.