VON DASSOW, M.; Univ. of California, Berkeley: Chimney Formation and Plasticity in a Bryozoan

How does fluid flow affect the growth of fluid transport systems? The sheet-like colonial bryozoan Membranipora membranacea is a good system for studying this question because it has a simple geometry and is amenable to experimental manipulation. Crowns of ciliated tentacles (lophophores) form a canopy over M. membranacea colonies. The lophophores capture food particles from water currents generated by their cilia. Excurrent water flows under the canopy, exiting at openings in the canopy (chimneys) or at the canopy edge. Lophophores adjacent to a chimney tilt away from the chimney. Previous studies have suggested that fluid flow through the colony affects chimney formation and that gaps in the canopy due to grazing or overgrowth become chimney-like. I conducted experiments to determine if chimneys form where the pressure under the canopy is greatest, or if grazed areas (expected to act as new sites of excurrent flow) become chimney-like. Chimneys formed at the canopy edge, suggesting that they do not form where the pressure under the canopy is greatest. Holes of different sizes were made in colonies to simulate grazing. At 3 days after the holes were made, neighboring lophophores closed the gap in the canopy over small holes, but gaps in the canopy were present over large holes. Lophophores around the gaps leaned away from the gaps after 7 to 11 days. These results suggest that chimneys do not form either where there are small gaps in the canopy or where the pressure under the canopy is highest, but may form at sites of persistent excurrent flow such as at large gaps in the canopy and at the canopy edge.