Can bacterivory sustain survival and growth in early juveniles of the bryozoan Bugula neritina, the polychaete Hydroides elegans and the barnacle Balanus amphitrite?

Since phytoplankton abundance can constitute a highly unpredictable source of food for suspension feeders, we examined whether early juvenile Bugula n eritina, Hydroides elegans, and Balanus amphitrite can use bacteria to achi eve extended survival or growth when phytoplankton is scarce. At bacterial concentrations of similar to 2 to 3.5 x 10(6) cells ml(-1), within the rang e of values observed in the field, early juvenile B, neritina and H. elegan s were effective bacterivores, obtaining enough energy and nutrients from b acteria to sustain survival and growth. When provided with bacterial concen trations of similar to 3 x 10(6) cells ml(-1), early juvenile B, neritina g rew as rapidly as individuals provided with abundant phytoplankton, and com pleted the development of a second zooid slightly earlier. At bacterial con centrations of similar to 1 x 10(6) cells ml(-1), however, early juvenile B , neritina fared only slightly better in terms of growth and survival than in controls where bacterial concentrations were kept to a minimum. Growth o f early juvenile H. elegans at similar to 2 x 10(6) bacteria ml(-1) was sig nificantly faster than in the controls, but was nevertheless slower than in the phytoplankton treatment. Finally, Balanus amphitrite did not obtain an y detectable benefit from bacteria, despite being provided with bacterial c oncentrations of similar to 9 x 10(6) cells ml(-1), well in excess of value s observed in the field. These results indicate that early juveniles of som e species are effective at using bacteria as an alternative food source, an d may use bacteria as a food supplement or even as a sole source of particu late matter, if necessary, to support survival and growth, Starvation may t herefore be a minor natural cause of mortality, if a cause at all, in early juvenile H, elegans and B, neritina which can use bacteria and possibly ot her alternative food sources when phytoplankton is scarce.