Modularity is the mother of invention: a review of polymorphism in bryozoans

Modularity is a fundamental concept in biology. Most taxa within the colonial invertebrate phylum Bryozoa have achieved division of labour through the development of specialized modules (polymorphs), and this group is perhaps the most outstanding exemplar of the phenomenon. We provide a comprehensive description of the diversity, morphology and function of these polymorphs and the significance of modularity to the evolutionary success of the phylum, which has >21000 described fossil and living species. Modular diversity likely arose from heterogeneous microenvironmental conditions, and cormidia (repeated clusters of associated modules) are an emergent property of the cue thresholds governing zooid plasticity. Polymorphs in a colony have, during phylogeny, transitioned into associated non-zooidal structures (appendages), increasing colonial integration. While the level of module compartmentalization is important for the evolution of bryozoan polymorphism, it may be less influential for other colonial invertebrates.