Abstract Evolutionary transitions between individual and colonial organisms remain enigmatic. Siphonophores, abundant pelagic cnidarians, exhibit a complex colony structure composed of repeated individual (zooid) clusters called cormidia. Many siphonophores release their posterior-most cormidia as independent fragments known as eudoxids, ensuring sexual reproduction. However, the mechanisms of eudoxid production and its evolutionary origins are unknown. Using live imaging, immunohistochemistry and pharmacological inhibition we provide a mechanistic understanding of eudoxid formation. We demonstrate that eudoxid release is controlled by a dedicated muscle and involves tissue remodeling, leading to the formation of an integrated dispersive unit with specific behaviors and a different buoyancy. We show that eudoxids and parental colonies often have different spatial or temporal distributions, suggesting niche partitioning. We infer that eudoxids evolved once through the concomitant evolution of multiple cormidium subparts. This study reveals how the acquisition of an individual-like dispersal stage, through muscle evolution and colony modification, contributed to the ecological success of a primary carnivore in marine planktonic ecosystems. Teaser Co-option of a muscle in siphonophores enabled life cycle complexification and ecological diversification

The evolution of an individual-like dispersive stage in colonial siphonophores