Developmental, regenerative, and behavioral dynamics in acoel reproduction.

Acoel worms are an enigmatic and understudied animal lineage. Sparse descriptions suggest a diversity of reproductive anatomies across acoels, and likely a corresponding behavioral diversity. Here, we study the reproductive life history of the acoel Hofstenia miamia, an emerging lab-tractable model system. We describe H. miamia's reproductive organs, identifying structures previously unknown in acoels. Following worms from zygotes to adulthood, we find that their reproductive organs emerge in a stereotyped sequence as a function of increasing body size. These organs regenerate in a similar sequence after major injuries and are lost in the opposite sequence during starvation-induced de-growth, suggesting that organ growth may be regulated by a single, size-associated program in all contexts. Studying egg-laying behavior, we find that H. miamia lay their eggs through their mouths after loading them into their pharynges. Worms lay eggs for months after a single mating, suggesting long-term sperm storage despite lacking a storage organ. They can also lay viable eggs without mating, indicating a capacity for self-fertilization. Finally, worms assess past and present environmental features during egg-laying decisions, frequently laying eggs in communal clutches. Together, our work establishes foundational knowledge for the study of reproductive development, physiology, and behavior in acoels.