Observations on two nematode species parasitizing freshwater fishes in Thailand,including <Emphasis Type="Italic">Spinitectus thaiensis</Emphasis> sp. nov. (Cystidicolidae) from <Emphasis Type="Italic">Pseudomystus siamensis</Emphasis> (Bagridae)

One new and one already known species of parasitic nematodes are described from the intestine of freshwater fishes in Chiang Mai Province, northern Thailand: Spinitectus thaiensis sp. nov. (Cystidicolidae) from the catfish Pseudomystus siamensis (Regan) (Bagridae, Siluriformes) in the Fang Brook, a tributary of the Kok River (the Mekong River basin), Fang District and Oceanicucullanus chitwoodae Le-Van-Hoa et Pham-Ngoc-Khue, 1971 (Cucullanidae) from the cyprinid Mystacoleucus marginatus (Valenciennes) (Cyprinidae, Cypriniformes) in the Ping River (the Chao Phraya River basin), Muang District. The new species, S. thaiensis, is very similar to S. petrowi Belous, 1965, differing from it, in addition to some biometrical differences, mainly in having simple cuticular spines (instead of transversely oriented peg-like spines with rounded ends) on the ventral surface of the female tail; the spicules are 156–171 and 66–72 μm long, the vulva is situated at 74% of female body length, and the entire oesophagus including the vestibule represents 14–15% of body length in the male and 12% in the female. The latter species, O. chitwoodae, was studied for the first time by scanning electron microscopy, which enabled to recognize some previously unrecorded morphological details in this species. The finding of O. chitwoodae in M. marginatus in Thailand represents new host and geographical records for this nematode.