Effect of flow regime on the morphology of a colonial cnidarian

. Phenotypic plasticity is the ability of some organisms to exhibit different phenotypes in response to environmental conditions. Many sessile marine invertebrates are morphologically plastic. In colonial cnidarians, compact morphologies are often associated with high‐velocity flow regimes, whereas elongated morphologies are associated with calmer water. This ability to alter morphology in response to flow regime likely represents an adaptive strategy: these morphologies may permit efficient suspension feeding and gas exchange while reducing the risk of dislodgment in a particular flow regime. Which flow‐related factors (e.g., CO 2 accumulation, drag forces, prey delivery) actually signal a colony to alter its morphology are unclear. In this study, we test the hypothesis that differences in flow regime or some correlate of flow regime (in the absence of differences in prey delivery) signal a colonial cnidarian to change its morphology. To separate prey delivery from water flow, hydroid ( Bougainvillia muscus ) colonies were fed equivalent amounts in still water, regardless of the flow regime treatment to which they were exposed the rest of the time. Our results show that, regardless of prey delivery, colonies grew in ways characteristic of calm water (with a higher percentage of tall pedicels and secondary hydranths, and fewer basal stolon branches) and of high flow (with more hydranths, free stolons, and a denser basal stolon network) environments. This work suggests that, for this hydroid, prey flux is not a proximate cue mediating morphological plasticity in response to flow regime.