Classification of pleurodire polystomes (Platyhelminthes, Monogenea, Polystomatidae) revisited with the description of two new genera from the Australian and Neotropical Realms

Polystomatids are platyhelminth parasites that infect mostly amphibian and chelonian hosts. Polystomatid of testudines were, for more than seven decades, classified in the three genera – Neopolystoma Price, 1939, Polystomoides Ward, 1917 and Polystomoidella Price, 1939. The genus delimitation was primarily based on the absence of hamuli in Neopolystoma, the presence of one pair of hamuli in Polystomoidella, and two pairs in Polystomoides. From 2016 to 2020, five new genera were erected - namely Uropolystomoides Tinsley and Tinsley, 2016, Uteropolystomoides Tinsley, 2017, Apaloneotrema Du Preez and Verneau, 2020, Aussietrema Du Preez and Verneau, 2020 and Fornixtrema Du Preez and Verneau, 2020. The generic diagnosis was based not only on the size and shape of morphological characters such as hamulus 1, uterus and eggs, but also on the site of infestation (i.e. urinary bladder, oral cavity or conjunctival sacs). Despite large advancements in polystome classification over the last decade, Neopolystoma was still polyphyletic with some species nested within Polystomoides and others being closely related to the Australian Aussietrema. Regarding the distribution of freshwater turtles of the two suborders Pleurodira (Southern continents) and Cryptodira (distributed worldwide except in Australia), one may wonder whether Australian chelonian polystomes of the genus Neopolystoma may have diverged from species infecting other pleurodires of South America. In the present study based on the analysis of several species selected among all genera, we reveal striking morphological differences within polystomes infecting pleurodiran turtles, which herein led to the proposal of two new chelonian polystome genera, Pleurodirotrema n. g. and Manotrema n. g. Pleurodirotrema n. g. is characterized by the absence of hamuli, presence of latero-ventral vaginae and includes species that infect either the oral region or the urinary bladder of Australian hosts of the Pleurodira. Manotrema n. g. is characterized by the presence of small hamuli, latero-ventral vaginae, deep incisions between suckers, a low genital spine number (<10) and includes species that infect the oral region of South American Pleurodira.