Patterns and Consequences of Whole Colony Growth in the Compound Ascidian Polyclinum planum.

Abstract

The size and shape of colony-forming modular animals can convey ecological advantages, but many patterns and consequences of colony-level growth are not well understood. I carried out a longitudinal study on an intertidal population of the pedunculate ascidian Polyclinum planum to determine the patterns and consequences of its colony growth. I found that each P. planum colony is a nonfragmenting genet, and that colony size is limited by water-flow forces and reproductive state. P. planum mitigates the effects of water-flow forces by having an attenuating pattern of growth and by producing a laterally flattened, zooid-bearing lobe atop its tough flexible peduncle. Growth slows as the colony nears a size limit set by the environment and as it becomes reproductively active. The laterally flattened lobe allows colonies to increase their surface-to-volume ratio, to house increased numbers of zooids (thereby increasing reproductive potential), and to minimize the effects of the acceleration reaction of water. P. planum's growth pattern fits predictions for colonies living in wave- or surge-impacted environments. The growth of P. planum provides insight into how indeterminate modular growth conveys ecological and reproductive advantage, even amidst a physically stressful environment.