MINIATURISATION OF COLONY MEMBERS IS NOT UNIVERSAL

One of the more general insights into the evolution of colonial animals is that colony members tend to be smaller than related solitary organisms. For example, polyps of colonial scleractinian corals are smaller than solitary corals and colonial tunicates have smaller zooids than their solitary relatives. But this insight is derived from sessile colonies only, ignoring pelagic colonies such as siphonophores, doliolids, pyrosomes, and graptolites. We find that members are generally larger than their solitary relatives in pelagic colonies. We propose that this difference in scaling can be attributed to habitat-specific functional requirements with feeding and motility structuring the evolution of cooperation among zooids. Smaller zooids preferentially evolve in sessile suspension feeders, either actively transporting water by ciliary action (in bryozoans) or passively as in sessile cnidarians. Larger zooids relative to solitary and sessile colonial relatives occur in motile colonies that are too large for ciliary propulsion.