Some aspects of the taxonomy, biology, possible evolution and biogeography of nematodes of the spirurine genus Rhabdochona Railliet, 1916 [Rhabdochonidae, Thelazioidea]

At present the nematode genus Rhabdochona Railliet, 1916 (Rhabdochonidae) comprises 92 possibly valid species, mostly intestinal parasites of freshwater fishes in all zoogeographical regions. Because of the absence of any phylogenetic studies using molecular methods in this nematode group, an attempt to evaluate affinities among these parasites and their zoogeography on the basis of morphological features and host-parasite relationships has been carried out. Only true definitive hosts should be considered for such evaluation. It appears that, during the evolution of Rhabdochona spp., there occur an increase in the number of anterior prostomal teeth, loss of lateral alae, the right spicule attains a boat-like shape and a dorsal barb develops on its distal tip, and the distal tip of the left spicule attains gradually a more complex structure; the eggs with a smooth surface are considered to be more primitive. Nematodes of this genus most probably originated in the region of present southern Asia at the beginning of Tertiary. Closest to the initial type appear to be members of the subgenus Globochona Moravec, 1972, from which lineages leading to the subgenera Globochonoides Moravec, 1975, Rhabdochona Railliet, 1916 and Sinonema Moravec, 1975 can be derived. The morphology of Rhabdochona spp. indicate a close relationship between the Palaearctic fauna of these nematodes with the Nearctic species, and the Oriental fauna with the Ethiopian fauna. The Neotropical species can be derived from Nearctic members of the subgenus Rhabdochona; the latter probably penetrated, along with leuciscine hosts into North America from Palaearctic Eurasia at the end of Tertiary. The distribution of Palaearctic species of Rhabdochona corresponds to faunistical complexes in the conception of ichthyologists.