Neuromuscular Architecture of the Siphonophore Colony

Siphonophores are colonial hydrozoans with unprecedented differentiation and specialization, in which individual zooids are transformed into functional organs rather than autonomous polyps capable of feeding. As a result, the entire colony acts as a single, modular individual with the highest level of coordination and integration, from development through behavior. Deciphering these integrative mechanisms requires understanding the microanatomical organization of the nervous and muscular systems in all elements of the colony. Here, using two immunohistochemical markers (anti-tubulin and anti-RFamide antibodies), we systematically characterize the neuromuscular organization across the entire Nanomia colony, encompassing pneumatophore, stem, and all zooid classes (nectophores, gastrozooids, palpons, male and female gonophores, and protective zooids). The use of two neural markers enables visualization of distinct neural subpopulations, some of which are not revealed by a single marker. We provide evidence of neuroanatomical interactions within all elements of the colony, including contributions of giant axons, stem polygonal nets, and RFamide-ir neural rings at the base of each zooid, as well as describe different subpopulations of neural nets and muscles elements in the body of various zooids. The presented mapping facilitates identification of novel conductive and signaling pathways for future analysis of the cellular basis of behavioral integration within decentralized, broadly distributed networks and non-neuronal elements of these unique superorganisms.