Declining populations of Astacus astacus drag European Branchiobdella parasita (Annelida: Clitellata) ectosymbionts to serious biodiversity loss

Branchiobdella parasita is specific ectosymbiont of European freshwater crayfish, including endangered noble crayfish (Astacus astacus). Although numerous information on distribution, biology or ecology were reported, genetic structure of B. parasita in European crayfish populations was not studied truthfully. The available molecular data include only cytochrome oxidase subunit I (COI) barcodes of B. parasita coexisting with noble crayfish in Alpine freshwaters in Austria, as well as stone crayfish (Austropotamobius torrentium) and white-clawed crayfish (Austropotamobius pallipes) in Balkan region. Here we report first genetic diversity and haplotype richness analysis of B. parasita populations on a pan-European scale. Specimens (n = 41) were collected from female noble crayfish from three lakes in northwest Poland (Kołtki Małe, Rosko Duże, Skrzynki Małe) and Elkenrother Weiher Lake in southwest Germany, high-quality COI barcodes were obtained and analysed against existing genetic data. The analysed populations were haplotype rich (h = 23) and had high COI haplotype diversity (Hd = 0.8988, I = 1.4554). Our results indicate that diversification rate of European B. parasita populations highly depends on habitat quality, its connection with surface waters and hosts species and mobility. We revealed that Central European populations inhabiting noble crayfish show significant genetic relationship, presenting common ancestral haplotypes and at the same time high genetic divergence from Balkan populations. Based on our results we suggest that ‘parasita-complex’ of phylogenetically related species with similar body shape and structure of chitin jaws, but with differentiating anatomical structure details are present across the continent.