Morphological and molecular characterization of cucullanid nematodes including Cucullanus opisthoporus n. sp. in freshwater fish from the Brazilian Amazon.

Molecular and morphological characterization of two species of Cucullanidae from freshwater fish in Brazil are provided, one of which represented an undescribed taxon. Cucullanus opisthoporus n. sp. was collected in Cichla melaniae from River Xingu, State of Pará, and in C. pinima from River Jamarí, State of Rondônia. Nematodes referable to Cucullanus grandistomis were collected in Oxydoras niger from River Xingu. The new species has an appendage in the tail tip, ventrally covered by small spines, which is an exclusive feature of Cucullanus tucunarensis. However, C. tucunarensis differs from C. opisthoporus n. sp. based upon the relative position of deirids and the excretory pore, which are more posterior from the oesophageal end in the new species. Observations of C. tucunarensis type specimens also revealed features that were wrongly or not reported in the original description. Type specimens of C. grandistomis were observed, although they were poorly preserved. After evaluation of newly collected specimens of C. grandistomis, features unreported in the original description were observed for the first time, including the presence of an intestinal caecum. Thus, C. grandistomis was transferred to Dichelyne. Sequences of the 18S and 28S rRNA genes revealed high genetic similarity between the specimens of C. opisthoporus n. sp. from the two different hosts as well as their genetic distance from Dichelyne grandistomis n. comb. Phylogenetic reconstructions using representatives of Cucullanidae suggested the artificiality of the current morphological system adopted to separate the genera, since most genera were not monophyletic, although the availability of genetic data is still fragmented.