CAN BACTERIVORY ALONE SUSTAIN LARVAL DEVELOPMENT IN THE POLYCHAETE HYDROIDS ELEGANS AND THE BARNACLE BALANUS-AMPHITRITE

Although recent studies have found that some invertebrate larvae can m eet part of their metabolic needs through bacterivory, it is unclear t o what extent bacterivory can compensate for reduced phytoplankton abu ndance. The present study determined whether larvae of a polychaete (H ydroides elegans) and a barnacle (Balanus amphitrite) can survive, gro w, and develop to competence solely on a diet of bacteria. In laborato ry experiments, H. elegans larvae provided with bacteria as their sole particulate food source completed larval development, attached, and m etamorphosed into healthy early juveniles. In addition, several of the se juveniles, when provided with phytoplankton after metamorphosis, de veloped to maturity and spawned viable offspring. Bacterial abundances measured in Port Shelter, a bay in Hong Kong waters where organisms f or the present study were collected, would not sustain maximum develop ment rates of H. elegans larvae. However, bacterial abundances were su fficiently high and consistent over time to constitute a reliable food source. Consequently, starvation may not be a direct cause of larval mortality in this species. On the other hand, B. amphitrite larvae pro vided with bacteria did not grow, and death occurred at the same age a s in starved larvae, suggesting they cannot use bacteria as a suppleme ntary food source. Available data suggests that most barnacle recruitm ent in Hong Kong waters occurs when phytoplankton abundance reaches pe ak levels. Recruitment of H. elegans, however, occurs throughout the y ear, suggesting that spawning and successful larval development may be independent of phytoplankton availability, and that larvae largely re ly on alternate food sources such as bacteria.