Microbiome Diversity, Persistence, and Potential Symbiont Dynamics in Placobdella Leeches from American Alligators

Placobdella leeches are common ectoparasites of reptiles that rely on microbial symbionts to supplement the nutritional limitations of blood feeding. Although the primary symbiont Reichenowia is documented, little is known about the broader microbial community or ecological factors that influence Placobdella microbiome assembly. Here, we characterized the whole-body microbiomes of seven Placobdella clades from American alligators (Alligator mississippiensis) across the southeastern United States. Sampling revealed four Placobdella lineages not previously reported from alligators, including several undescribed species. Placobdella microbiomes, based on 16S rRNA gene sequencing, were dominated by Actinobacteriota, Alphaproteobacteria, Bacteroidota, and Pseudomonadota (the former Betaproteobacteria). Although microbiome composition was not strongly structured by leech clade, geography, or habitat type, location-specific bacterial signatures were observed. Captivity experiments resulted in significant declines in microbial diversity, and persistence of four bacterial taxa. Comparisons between leech microbiomes and microbial communities on alligator skin revealed limited overlap. Additionally, while a distinct Reichenowia lineage was generally associated with each Placobdella species, multiple low-abundance Reichenowia variants were detected within individuals, and an identical Reichenowia variant occurred in sympatric Placobdella species. Together, these results identify a conserved candidate core microbiome in Placobdella and demonstrate that microbiome assembly in blood-feeding leeches reflects persistent symbiotic interactions and ongoing environmental acquisition.