A new family for Cephalotrema elasticum (Digenea) based on molecular data, with notes on the status of the definitive host population.

The host diversity, complex life cycle and distribution of digeneans underline their value as bioindicators of ecosystem health. We examined the intestinal digenean trematode Cephalotrema elasticum of insectivorous mammals, sampled from Gammarus balcanicus (Crustacea: Amphipoda) as second intermediate hosts. The species is particularly relevant as part of the helminth species complex associated with protected mammals. Our study is based on an intensive survey of helminths in 8920 gammarids collected from 38 localities in Slovakia, Poland, and France. DNA sequences for three markers (18S rDNA, 28S rDNA, and CO1 mtDNA) were analysed using Bayesian inference. Molecular data suggest that C. elasticum represents a distinct genetic lineage closely related to Cryptotropidae. Rare anatomical traits, such as the marginal or submarginal genital pore arrangement, may influence mating success and reproductive isolation within Microphalloidea. Genetic and morphological evidence supports establishing a new family, Cephalotrematidae, within Microphalloidea, to include Cephalotrema, Pseudocephalotrema, and Opisthioparorchis. A family diagnosis and a key to the genera are provided to facilitate further taxonomic studies. The geographic distribution of C. elasticum highlights its utility as a biological indicator, reflecting the decline of water shrew populations due to climate-induced ecological changes, particularly in the French Pyrenees.