Diplomonorchis fallax n. sp. (Digenea: Monorchiidae) from the northern Gulf of Mexico with evaluation of sympatric congeners.

Diplomonorchis micropogoni Nahhas & Cable, 1964 was considered a junior subjective synonym of Diplomonorchis leiostomi Hopkins, 1941 in 1969. Diplomonorchis leiostomi has since been widely reported from the coastal Western Atlantic between Delaware Bay and southern Brazil. Until now, taxonomically verifiable DNA sequence data for D. leiostomi has been available from an individual worm collected from the northern Gulf of Mexico. We generated a partial sequence of the 28S rRNA gene from D. leiostomi from the spot croaker, Leiostomus xanthurus Lacepède (type-host) from Beaufort, North Carolina, USA (type-locality) that differed at 31 of 1,246 bases from the available 28S sequence. This prompted a reevaluation of Diplomonorchis spp. identities from the northern Gulf of Mexico. We found D. leiostomi and D. micropogoni distinguishable by testes shape and size, and to a lesser degree by relative caecal length. Museum specimens of D. leiostomi, identified from the Gulf of Mexico represent a species complex containing D. leiostomi, D. cf. micropogoni and, a new species of Diplomonorchis. The sequences previously identified as D. leiostomi in GenBank (AY222137 & AY222252) are herein identified as D. cf. micropogoni. The new species is described from newly collected material herein. Phylogenetic analysis of 28S rRNA sequences from the species complex plus 46 species from the Monorchioidea Odhner, 1911 indicated all three Diplomonorchis spp. are closely related and form a clade with some species of Lasiotocus Looss, 1907. With the addition of the new species, and acceptance of D. micropogoni, there are currently 14 valid species in Diplomonorchis.