A new species of Hedruris Nitzsch, 1821 (Nematoda: Hedruridae) parasitic in eel Synbranchus marmoratus Bloch (Synbranchiformes: Synbranchidae) from Argentina, Neotropical Region.

During a parasitological survey, specimens belonging to the genus Hedruris (Nematoda: Hedruridae) were found in the stomach of Synbranchus marmoratus Bloch from the Paraná Guazú River, Paraná River basin (Argentina). Comparative morphological studies with its congeners showed that these nematodes belonged to a new species, herein described as Hedruris anguila sp. nov. The new species can be distinguished from remaining Hedruris species by the pseudolabia size, the morphology and size of spicules, the length of female caudal hook, the shape and size of eggs and the particular morphology of the male tail. Hedruris anguila sp. nov. closely resembles Hedruris bifida Rossin & Timi, 2016 and H. suttonae Brugni & Viozzi, 2010, both also described from fish hosts, in the arrangement of caudal papillae and morphology of eggs, but differ in the size of female hook and spicules, and the number of caudal papillae in males. The main morphological features, host range and geographical distribution of all species in the genus Hedruris species are analyzed and discussed. This is the first report of a representative of the family Hedruridae parasitizing a species of Synbranchidae and the eleventh report of the genus Hedruris for a fish host. The number of valid species of Hedruris described was increased to 26, including the new species; 11 for the Neotropical Region and represent the third species found in fish host from Argentina. Based on morphological analysis, the speciation process probably occurred first in Gondwana and the non-mammillated eggs could be an ancestral morphological character. The diet of Synbranchus marmoratus and values of prevalence and mean intensity could indicate that this host species is a true definitive host for this nematode.