Abstract
Octangium Looss, 1902 is a taxonomically challenging genus of microscaphidiid trematodes, with members harmful at the adult stage to their reptile hosts, the turtles. In Arthur Loss’s collection preserved at the Swedish Museum of Natural History, we found mounted slides of O. sagitta (Looss, 1899) and of O. hasta Looss, 1902, and vials containing Octangium spp. preserved in ethanol from a host and locality matching those listed in Looss (1899), suggesting that this is the part of type material. Unfortunately, several specimens were dried. We attempted herein to rehydrate them, make whole stained mounts, and investigate the material. Additionally, we examined several specimens of O. sagitta and O. hasta from the Invertebrate collections at the SMNH and provided illustrated redescriptions. In light of the available data, also discuss records of O. sagitta, O. hasta, and previous species synonymized with O. sagitta and provide a key for the identification of Octangium spp. We herein reinstate Octangium takanoi Kobayashi, 1921 and O. hasta, and amend Octangium based on a re-examination of adult specimens of the type material of O. sagitta [type species] and of O. hasta, infecting the intestine of the green sea turtle Chelonia mydas. We demonstrate that O. takanoi differs from O. sagitta by being twice smaller, by the shape of the oral sucker (rectangular vs. rounded), and especially by the extent of vitellaria, with their anterior end situated at the level of the anterior margin of the testes in O. takanoi, while they extend to the intestinal bifurcation in O. sagitta. Additionally, they can be easily distinguished by more anterior placing of the testes in O. takanoi. Similarly, O. hasta can be easily distinguished from O. sagitta by being significantly smaller in all body measurements. More importantly, O. sagitta and O. hasta can be easily distinguished by the shape of the oral sucker. We refute thus the synonymies between O. sagitta and the two species O. takanoi and O. hasta, and we reinstate the two latter species as valid species. We summarized and mapped the distribution of current valid species of the genus Octangium.