Consequences of coloniality: influence of colony form and size on feeding success inthe bryozoan Membranipora membranacea

Abstract

Colonial animals can grow in a wide variety of shapes and sizes, but certain forms have apparently evolved several times in different taxa. Using adaptive arguments to explain the evolution of colony form is tempting, but when testing the functional advantage of different colony forms, con- trolling for species differences is difficult. In this study, 1 species of bryozoan, Membranipora mem- branacea (Linnaeus, 1767), was grown into different colony shapes and sizes to test how colony form affected feeding performance. Clearance rate was measured for 3 colony forms (encrusting sheet, erect sheet, erect tree), and zooid ingestion rate was measured for 5 colony sizes (comprising of 1, 8, 16, 125, or 975 zooids). The 2 erect forms had higher clearance rates than did the encrusting sheet form at a freestream velocity of 3 cm s -1 . At this velocity, erect colonies experienced less refiltration and yet benefited from feeding in a downstream wake. For encrusting colonies the change in inges- tion rate per zooid with colony size was non-linear. Overall, zooid ingestion rate increased from 1 to 16 zooids, decreased for small colonies (with ~125 zooids), and then increased again for medium colonies (with ~975 zooids). Colonies with 8 or more zooids had higher feeding success than single zooids, suggesting a feeding advantage to zooids in a colony. Conversely, small and medium colonies had lower mean ingestion rates than colonies with 8 or 16 zooids, suggesting a feeding cost for increasing beyond a relatively small size.