Chapter 19: Colonial Microorganisms & Invertebrates

At the peak of this evolutionary trajectory are the colonial hydrozoans, such as siphonophores, which function as "superorganisms" where formerly independent members have been modified into specialized structures resembling organs, such as gas-filled floats for buoyancy, nectophores for jet propulsion, and gastrozooids for nutrient distribution. These complex assemblages demonstrate sophisticated behavioral coordination through shared nervous systems and epithelial conduction, allowing the entire colony to move and react as a unified whole. This developmental process, known as astogeny, mirrors the growth of single organisms, as the entire colony typically originates from a single fertilized egg. Beyond siphonophores, the text examines the adaptive advantages of colonial living, including increased resistance to physical environmental stress, the ability for sessile bottom-dwellers to transition into free-swimming pelagic forms, and enhanced competitive abilities through rapid asexual budding and specialized defensive castes. The chapter also highlights the remarkable convergent evolution between eukaryotic slime molds and prokaryotic myxobacteria, both of which utilize molecular signaling—specifically cyclic AMP—to trigger the aggregation of solitary cells into multicellular fruiting bodies for survival and dispersal. Furthermore, the study of corals and bryozoans reveals a diverse spectrum of colonial integration, ranging from loosely connected individuals to highly polymorphic systems featuring unique specialists like the "avicularia," which act as defensive lids to protect the colony from invaders. Ultimately, these colonial microorganisms and invertebrates represent a critical milestone in natural history, illustrating how life can escape structural limitations by organizing individual organisms into complex, higher-order biological units.