No vagina, one vagina, or multiple vaginae? An integrative study of Pseudaxine trachuri (Monogenea, Gastrocotylidae) leads to a better understanding of the systematics of Pseudaxine and related genera

The presence/absence and number of vaginae is a major characteristic for the systematics of the Monogenea. Three gastrocotylid genera share similar morphology and anatomy but are distinguished by this character:PseudaxineParona & Perugia, 1890 has no vagina,AllogastrocotyleNasir & Fuentes Zambrano, 1983 has two vaginae, andPseudaxinoidesLebedev, 1968 has multiple vaginae. In the course of a study ofPseudaxine trachuriParona & Perugia 1890, we found specimens with structures resembling "multiple vaginae"; we compared them with specimens without vaginae in terms of both morphology and molecular characterisitics (COI barcode), and found that they belonged to the same species. We also investigated the male copulatory organ (MCO) of this species, the accuracy of the original description of which is known to be a matter of debate. We found that the genital atrium is armed with 12 hooks arranged as a single circle and a central hollow stylet which is probably involved in traumatic insemination. We redescribedPseudaxine trachuribased on newly collected specimens from off the coast of Algeria and Museum specimens from off France. Specimens from the type-host,Trachurus trachurus,were found to be similar, for both molecular sequences and morphology, to those found onBoops boops. We can therefore confirm, for the first time with molecular evidence, thatB. boopsis a host of this parasite. We consider thatPseudaxinoideswas erected on the basis of an erroneous interpretation of structures which are not vaginae and, consequently, propose the transfer of most of its species toPseudaxine,asP. australis(Lebedev, 1968) n. comb.,P. bychowskyi(Lebedev, 1977) n. comb.,P. caballeroi(Lebedev, 1977) n. comb.,P. cariacoensis(Nasir & Fuentes-Zambrano, 1983) n. comb., andP. vietnamensis(Lebedev, Parukhin & Roitman, 1970) n. comb. We also proposeAllogastrocotyle dillonhargisorumnom. nov. forPseudaxine bivaginalisDillon & Hargis, 1965 to avoid a secondary homonymy.