Specimens of Spinitectus osorioi Choudhury and Pérez-Ponce de León, an intestinal nematode species previously considered to be specific to Chirostoma spp and endemic to some lakes in the Pacific drainage in Michoacán, were collected from the freshwater fish Atherinella alvarezi (Díaz-Pardo) (Atherinopsidae) of the Michol River near Palenque, Chiapas, Southern Mexico, which belongs to the Atlantic drainage system. Studies using light and scanning electron microscopy revealed some taxonomically important, previously unreported or erroneously reported features of S. osorioi, such as the location of the vulva, the actual number and distribution of postanal papillae and phasmids and the presence of a short median cuticular ridge anterior to the cloacal opening (in addition to two long subventral ridges). The recorded somewhat shorter spicules (420-465 and 105-111 μm) and mostly smaller eggs (33-36 × 18-20 μm) as compared to the original species description may be due to a different type of host, geographical region or generally smaller body measurements of these specimens. These biometrical differences are considered to be within the limits of the intraspecific variability of S. osorioi. A key to species of Spinitectus parasitizing freshwater fishes in Mexico is provided.

Philometra katsuwoni Petter & Baudin-Laurencin 1986, nematóide parasito do ovário de Katsuwonus pelamis (Linneaus 1758), originalmente descrito no Golfo da Guiné, é referido pela primeira vez na zona costeira do Estado do Rio de Janeiro, Brasil. Comparados com a descrição original, os machos do material brasileiro são um pouco maiores e possuem o espículo direito e o gubernáculo mais longos. A fêmea subgrávida é descrita pela primeira vez. Ela se caracteriza pela região anterior do esôfago inflada e pela presença de dois lobos caudais distintos.

The nematode Philometra katsuwoni Petter & Baudin-Laurencin, 1986, a gonad-infecting parasite of Katsuwonus pelamis (Linneaus 1758), originally described from the Gulf of Guinea, is reported for the first time from the coastal zone of Rio de Janeiro State, Brazil. As compared with the original description, the males of the Brazilian material are generally somewhat larger and their right spicule and the gubernaculum are longer. The subgravid female is described for the first time. It is characterized by the anterior inflation of the esophagus and by the presence of two distinct caudal lobes.

Camallanus tridentatus is redescribed on the basis of the examination of specimens obtained from the stomach, caeca and intestine of the naturally infected arapaima Arapaima gigas (Schinz) from the Mexiana Island, Amazon River Delta, Brazil. Data on the surface morphology of adults inferred from confocal laser scanning and scanning electron microscopical observations are also provided. The study revealed some taxonomically important, previously unreported morphological features in this species, such as the presence of the poorly sclerotized left spicule and deirids. C. tridentatus distinctly differs from other congeneric species parasitizing freshwater fishes in South America mainly in the structure of the buccal capsule and the female caudal end. C. maculatus Martins, Garcia, Piazza and Ghiraldelli is considered a junior synonymm of Camallanus cotti Fujita.

A new nematode species, Capillostrongyloides arapaimae sp. n., is described from the intestine and pyloric caeca of the arapaima, Arapaima gigas (Schinz), from the Mexiana Island, Amazon river delta, Brazil. It is characterized mainly by the length of the spicule (779-1,800 µm), the large size of the body (males and gravid females 9.39-21.25 and 13.54-27.70 mm long, respectively) and by the markedly broad caudal lateral lobes in the male. It is the third species of genus Capillostrongyloides reported to parasitize Neotropical freshwater fishes.