NEOLEBOURIA MULLINEAUXAE N. SP. (TREMATODA: DIGENEA) AND ANOTHER OPECOELID FROM DEEP-SEA HYDROTHERMAL VENT FIELDS OFF CENTRAL AMERICA AND PAPUA NEW GUINEA, WITH SPECIES KEYS AND A COMPARISON TO MESOBATHYLEBOURIA.

Neolebouria mullineauxae n. sp. (Digenea: Opecoelidae), exhibiting remarkable morphological variation (i.e., 2 distinctive morphotypes), is described from the intestine of the zoarcid eelpout or pink vent fish, Thermarces cerberus, collected from deep-sea hydrothermal vent fields along the East Pacific Rise (EPR) off the west coast of Central America. It can be distinguished from its congeners by having a unique diagnostic combination of features including a small body and gonads that are smaller relative to body size, a prebifurcal genital pore, a cirrus pouch that extends posteriorly as far as the posterior margin of the ventral sucker while the anterior margin of the latter is either close to or overlaps the intestinal bifurcation, confluent vitelline fields within the posttesticular space, and vitelline follicles and eggs that are not as dense and few to moderate in number, respectively. This report introduces a new family (Zoarcidae) and genus (Thermarces) of fish infected by the genus Neolebouria, a new host record in that this is the first taxonomic description of a digenetic trematode from T. cerberus; a new geographic locality for Neolebouria (sensu stricto)-the East Pacific Ocean off the coast of Central America-and a life cycle of the new species within the hydrothermal vent biome is postulated, including the sporocyst stage which utilizes the glass limpet, Eulepetopsis vitrea, as a first intermediate host. Two immature digeneans (cf. Opecoelidae) are described herein, collected from vent fields off Papua New Guinea, and represent a new host record in that this is the first report of a digenetic trematode from the zoarcid eelpout, Pyrolycus manusanus. Based on molecular analysis of the 28S gene, N. mullineauxae n. sp. was 99.92% (1,218/1,219 base pairs [bp]) similar to Neolebouria georgiensis. It was genetically closer to a previously described vent digenean, Buticulotrema thermichthysi, than to Mesobathylebouria lanceolata, despite the new species sharing morphological characteristics with the genus Mesobathylebouria. The 2 distinctive morphotypes of N. mullineauxae n. sp. were genetically identical in the 28S and ITS2 barcoding regions, supporting their identity as a single species. Given the similarity of N. mullineauxae n. sp. to members of Mesobathylebouria morphologically and ecologically, a thorough comparison of both genera is presented, as well as new keys to species, and a plea offered for a more efficacious diagnostic suite of morphological characters to distinguish both genera.