An advance in the understanding the systematics of Echinochasmus Dietz, 1909 and Stephanoprora Odhner, 1902 (Digenea: Echinochasmidae), with the description of a new species of Echinochasmus from the Neotropical region of Mexico.

Echinochasmids are a group of globally distributed digeneans, and the adults are found in the intestines of birds, mammals and reptiles. In the Neotropical region of Mexico, adult specimens were obtained from seven fish-eating bird species in six localities, whereas specimens of Stephanoprora aylacostoma were obtained experimentally in Argentina. Morphologically, the new specimens from the Neotropical region of Mexico were identified as Stephanoprora uruguayense and an undescribed species of Echinochasmus. Sequences for two nuclear (large subunit (28S) and internal transcribed spacer from DNA ribosomal (ITS1-5.8S-ITS2)) molecular markers were generated and analysed together with other sequences downloaded from GenBank. The phylogenies obtained with each molecular marker indicated that Echinochasmus is paraphyletic and agreed with previous phylogenetic studies. The first cluster included the type species (E. coaxatus, which has 24 head-collar spines) plus three congeneric species. The second cluster contained species of Echinochasmus plus Stephanoprora, including the species analysed herein, S. uruguayense, S. aylacostoma (with 22 head-collar spines) and Echinochasmus sp. (with 20 head-collar spines), which formed three independent subclades, allowing us to recognize a lineage that was described morphologically as a new species. Echinochasmus ostrowskiae n. sp. can be distinguished from its congeners by having a head collar with 20 spines in a single row, seven spines on each edge and three angle spines, and a pharynx with an irregular edge and by the body, egg and collar spine sizes. Additionally, new host and locality records for S. uruguayense are presented, expanding its geographical distribution range in the Americas.