Biological and morphological aspects of Branchiobdella italica (Annelida: Clitellata) in a native crayfish population of central Italy

The present study aimed to improve the knowledge of the biology of Branchiobdella italica, a symbiontic worm on the whiteclawed crayfish Austropotamobius pallipes, with particular attention to the feeding strategy, infestation and colonization ability, and morphology. Branchiobdellids occurred most frequently on adult crayfish rather than juveniles, due to the larger size of the former that permit a higher colonization by epizoic organisms, and to the high moult rate of smaller crayfish. We did not observe massive infestations of branchiobdellids, and the few specimens found do not seem to influence crayfish health. The higher occurrence of branchiobdellids took place on the A. pallipes carapace where worms find trophic resources. Gut content analysis of B. italica showed a generalist feeding strategy. The presence of fungal hyphae found within the gut of B. italica could lead us to infer that branchiobdellids could contribute to limit the fungal growth on the exoskeleton or on eggs of their host. Histological analysis of reproductive traits does not show structural differences from those referred to previous descriptions; the observation of one or two ciliated funnels (not always two), and the absence of hooks or spines on the penis surface, represent the main exceptions. Additional studies are needed to assess the true nature of the symbiosis between branchiobdellids and their hosts.