RESUMO
Pseudocorynosoma tepehuanesi n. sp., is described from the intestine of the ruddy duck Oxyura jamaicensis Gmelin, 1789 from single locality from northern Mexico. The new species is mainly distinguished morphologically from the other five described species of Pseudocorynosoma from the Americas (P. constrictum, type species, P. peposacae, P. anatarium, P. enrietti and P. iheringi) associated with waterfowl species by possessing a proboscis with 15 longitudinal rows with 7-8 hooks each, a trunk expanded anteriorly and by having smaller lemniscus. Partial sequences of the mitochondrial gene cytochrome c oxidase subunit I (cox 1) and the large subunit (LSU) of ribosomal DNA including the domains D2+D3 were used independently to corroborate the morphological distinction between the new species and other two congeneric species (P. constrictum and P. anatarium) from North America. The genetic divergence estimated among the new species and the other two species ranged from 15 to 18% for cox 1 and from 3.2 to 4% for LSU. The cox 1 alignment shows 24 sequences from P. anatarium with abnormalities, which were defined as pseudogenes due the presence of insertions, deletions and premature stop codons. Maximum likelihood and Bayesian inference analyses with each data set showed that the acanthocephalans from ruddy duck represent an independent clade with strong bootstrap support and posterior probabilities. The phylogenetic tree inferred with cox 1 gene placed all the pseudogenes from P. anatarium in single clade suggesting that those genes arose after speciation process within genus Pseudocorynosoma. The morphological evidence, plus the monophyly in both phylogenetic analyses indicate that the acanthocephalans collected from intestine of the ruddy duck from northern Mexico represent a new species.