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 limi ted to particles larger than 4 mu m in diameter, thereby eliminating p hotosynthetic and non-photosynthetic picoplankton as contributers to l arval diets. Documentation of ingestion, carbon retention and growth o f 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 Syn echococcus sp. and the haptophyte Isochrysis aff. galbana (TISO). Clea rance rates on Synechococcus sp. ranged between 2 and 23 mu l larva(-1 ) h(-1) depending on ambient cell concentration and larval age. Retent ion efficiency of cell carbon after gut evacuation was about 55% for b oth prey species. Growth rates of larvae fed on monocultures of Synech ococcus 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 t o 120 mu g C l(-1)). Larval growth was inhibited and atrophy of the di gestive gland was observed when Synechococcus sp. was offered at conce ntrations at or exceeding 8.6x10(5) cells ml(-1). Larval growth was en hanced, however, in the presence of Synechococcus sp. (5x10(4) cells m l(-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 larv ae was sufficient to account for 12 to 24% of the calculated grazing a ctivity on Synechococcus sp. When nanoplankton are scarce, invertebrat e larvae may exert considerable grazing pressure on Synechococcus sp. and derive benefit from ingestion of these cyanobacteria.