Excessive feeding induces development of a colony-like body plan in paratomous flatworms

Colonial animals rely on agametic reproduction to generate repeated colony modules (zooids). Although zooid development has been extensively studied in fully colonial species, such as cnidarians, bryozoans, or tunicates, it provides limited insight into how and why animals transition from solitary asexual fission to colony formation. Here, we fill this gap by studying the dynamics of asexual development in the microscopic flatworm Stenostomum, which can alternate between asexual fission and colony-like linear chains, composed of several zooids connected tail-to-head. Combining ecological, developmental, physiological, and transcriptomic analyses, we demonstrate that in four species of Stenostomum, chain formation can be triggered by manipulating food availability. This ecological input changes the balance between somatic longitudinal growth and head morphogenesis rate, generating a transient modular organism without requiring a new developmental program. We found no evidence for division of labor among zooids or for enhanced predation avoidance of the worms in chains, indicating that chain formation might be a developmental byproduct without obvious adaptive value. Together, these findings suggest that a neutrally evolving developmental byproduct of food-modulated allometric growth may provide a mechanistic route from asexual reproduction to coloniality.