VENIT, E.P.; Duke University: Interzooidal connection predicts division of labor in colonial marine invertebrates

Life is organized as a structural hierarchy manifesting division of labor among elements at each level. The evolutionary processes by which this structural hierarchy formed are not well understood. This study shows that the type of inter-zooidal connections within a marine invertebrate colony predicts presence of division of labor in that colony and suggests that division-of-labor plays a standardized role in hierarchy evolution. Higher levels of biological hierarchies form by the coalescence of lower levels: organelles combine to form cells, cells combine to form multicellular organisms, and multicellular organisms combine to form colonies or societies. Examination of transitional forms suggests that division of labor among morphological variants called �polymorphs� plays a crucial role in these coalescences. It is poorly understood why some biological collectives evolve division-of-labor systems, leading to higher hierarchical levels, while others do not. This is particularly perplexing among major colonial marine invertebrate groups, such as Cnidaria, Bryozoa and Urochordata, all of which contain both monomorphic and polymorphic subtaxa. Here a meta-analysis demonstrates that in all taxa, colonies with zooids connected by wide cavities are exclusively monomorphic, colonies with zooids connected by unobstructed tubes are either monomorphic or polymorphic, and colonies with zooids connected by tubes featuring valves or other restrictions are exclusively polymorphic. Thus, there appears to be a phylogenetically independent and �rule-like� correlation between the restrictedness of interzooidal connection and the presence of polymorphism.Given the close relationship between division of labor and biological hierarchies, the evolution of new levels of hierarchy may also behave according to general rules.