VON DASSOW, M; Univ. of California, Berkeley: Influences of flow and feeding on colony organization in a bryozoan

The bryozoan Membranipora membranacea has been an excellent system for studying the mechanistic basis for integration in colonial animals. Colonies of M. membranacea feed by pumping seawater through the colony and capturing food particles from the current of seawater. The seawater flows back out of the colony at regularly spaced openings (chimneys) that are produced by the morphological modification of many individuals, some of which degenerate and stop feeding. Previous results indicate that fluid flow controls the position of chimneys as they form: chimney formation is induced by increased excurrent flow speed at the colony edge. However, once formed, the chimneys do not appear to respond to modifications to the flow, suggesting that fluid flow may influence chimney morphology over a narrow developmental window. Excurrent flow speed could influence chimney formation directly, but it might also influence chimney formation indirectly by affecting the local feeding rate. Preliminary results suggest that food concentration influences chimney size, but not chimney spacing. Colonies grown at either high or low food concentrations produced larger chimneys than colonies grown at an intermediate food concentration, but chimney spacing was similar in all three treatments. These results suggest that both excurrent flow speed at the colony edge, and food concentration are important in organizing these colonies, but that flow is more important in determining the pattern of colony integration than feeding.