PRATT, M.C.: Evolution of colony morphology in bryozoans: Do encrusting sheets have an advantage?

Colonial animals show a wide diversity of growth forms that can be categorized into six basic growth forms: runners, vines, sheets, mounds, trees, and plates. Each of these growth forms is represented in the phylum Bryozoa; however, the encrusting sheet growth form dominates the phylum. Encrusting sheets consist of a two-dimensional layer of tightly packed feeding structures called lophophores. Mathematical models disagree on whether packing lophophores closely together in a two-dimensional sheet is advantageous or disadvantageous to food capture. In this study, I test directly whether or not tight packing of lophophores affects food capture. Colonies of a bryozoan with an encrusting sheet growth form were dissected to produce individuals with closely packed and more spaced lophophores. The particle capture success of these manipulated colonies were measured in a recirculating flow tank at a velocity of 3.5 cm/s. The colonies with closely packed lophophores captured more particles, which suggests an advantage to tightly packing lophophores.