Availability of dissolved organic matter oVsets metabolic costs of a protracted larval period for Bugula neritina (Bryozoa)

For nearly a century researchers have inves­ tigated the uptake and utilization of dissolved organic matter (DOM) by marine invertebrates, but its contribu­ tion to their growth, reproduction, and survival remains unclear. Here, the beneWt of DOM uptake was assessed for the marine bryozoan Bugula neritina (Linnaeus 1758) through performance comparisons of individuals in the presence and absence of DOM. The experiments were performed using B. neritina collected from Xoating docks in Beaufort, NC, USA from July to September 2004. Seawater was subjected to ultraviolet irradiation to reduce naturally occurring DOM, and then enriched with either 1 �M of palmitic acid or a mixture containing 1 �M each of glucose, alanine, aspartic acid and glycine. Larvae in DOM-enriched and DOM-reduced treat­ ments were sampled and induced to metamorphose fol­ lowing 1, 6, 12, and 24 h of continuous swimming at 25°C. Sampled larvae were assessed for initiation of metamorphosis, completion of metamorphosis, and ancestrular lophophore size to determine the extent to which energy acquired from DOM uptake could oVset the metabolic costs of prolonged larval swimming. DOM treatment had no signiWcant eVect on initiation of metamorphosis, but did have a signiWcant eVect on comple­ tion of metamorphosis and lophophore size. Larvae swimming in DOM-enriched treatments for 24 h experi­ enced a 20% increase in metamorphic completion rate, compared to larvae swimming for 24 h in the DOMreduced treatment. In addition, larvae in the amino acid and sugar mixture for 24 h had a signiWcantly larger lophophore surface area and volume (23 and 31%, respectively), compared to larvae in DOM-depleted sea­ water. To ensure that the increases in performance found in larvae with access to DOM were not due to a decrease in metabolic activity, the respiration rates for these larvae were compared to those of larvae in DOMdepleted seawater. There were no signiWcant diVerences between these treatments, indicating that the increases in performance were due to the energy acquired from DOM. These results clearly show that for B. neritina, DOM uptake results in increased metamorphic success and in the size of the feeding apparatus following an extended larval swimming duration.