Effect of zooid spacing on bryozoan feeding success: is competition or facilitation more important?

Most Recent bryozoan species are encrusting sheets, and many of these colonies have densely packed feeding zooids. In this study, I tested whether tight packing of feeding zooids affects food capture. Colonies of a bryozoan with an encrusting sheet form (Membranipora membranacea) were dissected to produce individuals whose feeding zooids were (1) closely packed, (2) more widely spaced, or (3) isolated. For each type, rates of particle ingestion were measured in still water and in a freestream velocity of 2.7 cm s(-1). Ingestion rate increased when zooids were closest together, probably because of reduced refiltration and increased feeding current strength farther from the lophophores. The mean incurrent velocity within 0.02 cm above the center of the lophophore was 0.28 cm s(-1) regardless of zooid spacing; however, when the incurrent velocity was measured more than 0.1 cm from the lophophores, zooids that were close together or spaced one zooid's width apart had significantly faster incurrent velocities than single zooids. Flow visualization suggests that isolated zooids and those spaced far apart refilter more water than zooids that are close together. These results along with the observed trend of increased zooid integration over evolutionary time suggest that the benefits of increasing coordination outweigh the consequences of intrazooid competition.