Horizontal transmission of symbiotic bacteria and host selective sweep in the giant clam Tridacna crocea.

Giant clams, with their significant ecological importance, depend on associated bacteria for their health and development, yet the transmission modes and succession of community dynamics of these bacteria remain poorly understood. This study employed 16S rRNA gene sequencing and microscopy to investigate the transmission and community dynamics of symbiotic bacteria in the giant clam Tridacna crocea during early developmental stages (fertilized eggs, blastocyst, D-larvae, and pediveliger larvae). Fluorescence in situ hybridization and transmission electron microscopy did not detect internal symbiotic bacteria in fertilized eggs and adult gonad gametes, but scanning electron microscopy revealed microbial structures on egg surface microvilli, suggesting their role as microbial carriers. 16S rRNA sequencing confirmed microbial presence in fertilized eggs, indicating bacterial acquisition via external vertical transmission (adherence to microvilli) or horizontal transmission. Given the lack of internalized bacteria in reproductive organs, we prefer to classify the symbiotic bacteria acquisition as horizontal transmission. Microbial community analysis showed that T. crocea acquired a significant portion of its microbiome from seawater throughout its development. Before reaching the pediveliger stage, the bacterial community composition closely resembled that of the surrounding seawater, primarily featuring the family Rhodobacteraceae. As T. crocea matured, the host's selective pressure increased (e.g. deterministic assembly), which simplified the microbial community and reduced diversity. During the pediveliger stage, the genus Endozoicomonas became dominant, forming a large proportion of the bacterial community within the gonads. This highlights the ecological significance of host-microbe interactions in maintaining biodiversity and driving ecosystem stability through dynamic community assembly processes.