Microbiome dynamics across the life cycle of Platynereis dumerilii

Microbial symbioses are increasingly recognized as key contributors to animal development and evolution. The marine annelid Platynereis dumerilii is a widely used model in evolutionary developmental biology, yet its associated microbiome has remained unexplored. Here, we present the first longitudinal characterization of the microbiome of laboratory-cultured P. dumerilii across its entire life cycle, including major culture subenvironments. Using 16S rRNA gene sequencing, we describe the composition, diversity, and temporal dynamics of hostassociated and environmental microbial communities. We identify a persistent hostassociated core microbiome supported by temporal continuity of specific taxa, alongside strong sample type-specific microbial signatures. Shifts in culture conditions triggered a pronounced microbiome restructuring event, marked by high taxonomic turnover and the collapse and reassembly of microbial interaction networks. Source-tracking and diversity analyses indicate that the worm microbiome is primarily maintained through temporal persistence, with transient environmental contributions from the microbial mat during feeding stages. Network analyses further reveal structurally central taxa, such as Vibrio, Fluviicola, CL500-3 (family Phycisphaeraceae), Algimonas, and members of the family Flavobacteriaceae, suggesting functional organization despite compositional variability. Comparative analyses with the sibling species Platynereis massiliensis and other P. dumerilii cultures highlight substantial microbiome divergence, reflecting both species-specific traits and environmental context. Overall, our results establish P. dumerilii as a tractable holobiont model and provide a foundation for integrating host-microbe interactions into evo-devo research and experimental reproducibility frameworks.