A new species of the genus Temnosewellia (Platyhelminthes, Temnocephalida, Temnocephalidae), Temnosewellia vietnamensis n. sp., is described from Quangnam Province, Vietnam, ectosymbiont on the external carapace of Villopotamon thaii Dang & Ho, 2003 (Crustacea, Decapoda, Potamidae). Records of temnocephalans from the region of Southeast Asia are superficial despite a great number of potential host species. Temnosewellia semperi (Weber, 1889), is the only known species and has been recorded from Aru, Java, Sumatra, Philippines, Malaysia, Thailand and China. In 2003, temnocephalans were reported for the first time from Vietnam which led to the current description of a new species based largely on characters from adults and eggs. The new species has five anterior tentacles; black pigment confined to the eyes; an ejaculatory sac; a cirrus straight that is not dilated and which has an obvious distal unspined region; anterior parenchyma with only two Haswell cells; yellow, slightly reniform eggs, that lack a peduncle and which are deposited on host carapace. The opercular plates of the eggs form a ring of seven almost rectangular shapes. The fracture plane of the operculum is parallel to the long axis of the egg. The great number of potential host species available in the Southeast Asia is taken as evidence that the region may harbour many other, yet undescribed, temnocephalan species.
Platyhelminthes, “Turbellaria”, Temnocephalidae, Temnosewellia, Southeast Asia, Vietnam, new species.