Newly discovered diversity within European branchiobdellids – a story of co-evolution and a need for conservation

Branchiobdellids are small ectosymbiotic annelids living primarily on freshwater crayfish. Because humans frequently translocate crayfish, these symbionts can be co-translocated, with implications for biosecurity and conservation. Despite over a century of study, relationships between European branchiobdellids and their crayfish hosts (basibionts) remain poorly resolved. Therefore, we analysed 215 mitochondrial COI sequences (67 new and 148 from GenBank and BOLD databases) spanning most documented ectosymbiont–basibiont associations. Phylogenetic reconstruction and species delimitation identified seven recognized species and revealed deep intraspecific divergence in five, suggesting the presence of cryptic lineages. Branchiobdella hexadonta and Branchiobdella parasita each contained multiple genetically distinct groups linked to different crayfish species, whereas Branchiobdella astaci and Branchiobdella balcanica showed strong host-specific monophyly. Branchiobdella italica and Branchiobdella pentadonta showed lower divergence, consistent with a younger origin inferred by molecular clock analyses. Divergence times broadly matched crayfish basibiont history, indicating likely co-speciation. Results support an origin of Branchiobdella inhabiting crayfish gill chambers and show that crayfish translocations may blur natural epibiont–basibiont boundaries, thus complicating monitoring of symbiont lineages. This work advances our understanding of co-evolution in freshwater ecosystems and underscores the conservation importance of preserving both native crayfish and their associated symbionts, whose extinction risks may be underestimated due to unresolved taxonomy.