Morphological and Molecular Evidence for a New Rhabdochonid Species (Nematoda: Rhabdochonidae) Parasitizing Profundulus oaxacae (Profundulidae) in the State of Oaxaca, Mexico

The genus Rhabdochona Railliet, 1916 is one of the most speciose group of nematodes, parasites of freshwater fishes, with approximately 103 species described worldwide. Twenty-two species have been recorded in the Americas, 14 of them in Mexico. In this paper we describe a new species of Rhabdochona on the basis of light and scanning electron microscopy and molecular tools. Rhabdochona adentata n. sp. was recovered from the gallbladder of the freshwater Oaxaca killifish, Profundulus oaxacae (Meek, 1902) (Profundulidae) captured in the Río Grande, State of Oaxaca, Mexico. Rhabdochona adentata n. sp. differs markedly from its congeners by possessing an unusual prostom lacking anterior teeth, small simple deirids, and the location of the excretory pore at the level of the union of the muscular and glandular esophagus. Sequences of cytochrome c oxidase subunit I (COI) from mitochondrial deoxyribonucleic acid and the D2 + D3 domains of the large ribosomal subunit (28S) were obtained from 3 specimens and were analyzed using maximum likelihood and Bayesian inference analyses. Phylogenetic analyses using COI and 28S genes recovered 1 new lineage of Rhabdochona. The new species is described on the basis of a detailed morphological study. This parasite represents the first species of Rhabdochona without prostomal teeth and with a different site of infection, the gallbladder.

Redescription of Rhabdochona papuanensis (Nematoda: Thelazioidea), a parasite of rainbow fishes (Melanotaenia spp.); the first record of the species of Rhabdochona in Australia

Numerous specimens of the parasitic nematode Rhabdochona papuanensis Moravec, Riha et Kuchta, 2008 (Spirurida: Rhabdochonidae) were collected from the intestines of the Australian endemic freshwater fish (eastern rainbow fish) Melanotaenia splendida (Peters) (Melanotaeniidae, Atheriniformes) in the Behana Creek, North Queesland during November of 2015. Although many species of Rhabdochona Railliet, 1916 are known to be common parasites of fishes in other continents, the present finding of R. papuanensis represents the first record of the species belonging to this genus from the Australian mainland. Light and scanning electron microscopical examinations of these newly collected specimens made it possible to redescribe in detail this nematode species, originally incompletely described from a congeneric host in Papua New Guinea. Fully developed, filamented eggs of R. papuanensis and the conspecific fourth-stage larva are described for the first time. The present finding of R. papuanensis in M. splendida from Australia represents new host and geographical records.

The trophic vacuum and the evolution of complex life cycles in trophically transmitted helminths

Parasitic worms (helminths) frequently have complex life cycles in which they are transmitted trophically between two or more successive hosts. Sexual reproduction often takes place in high trophic-level (TL) vertebrates, where parasites can grow to large sizes with high fecundity. Direct infection of high TL hosts, while advantageous, may be unachievable for parasites constrained to transmit trophically, because helminth propagules are unlikely to be ingested by large predators. Lack of niche overlap between propagule and definitive host (the trophic transmission vacuum) may explain the origin and/or maintenance of intermediate hosts, which overcome this transmission barrier. We show that nematodes infecting high TL definitive hosts tend to have more successive hosts in their life cycles. This relationship was modest, though, driven mainly by the minimum TL of hosts, suggesting that the shortest trophic chains leading to a host define the boundaries of the transmission vacuum. We also show that alternative modes of transmission, like host penetration, allow nematodes to reach high TLs without intermediate hosts. We suggest that widespread omnivory as well as parasite adaptations to increase transmission probably reduce, but do not eliminate, the barriers to the transmission of helminths through the food web.

New data on the morphology of some Far-Eastern species of Rhabdochona (Nematoda: Rhabdochonidae), as revealed by SEM observations

Specimens of the nematode genus Rhabdochona Railliet, 1916 (Rhabdochonidae) were collected during helminthological examination of four species of cyprinid fishes in two rivers of the Amur River basin in the Russian Far East (Primorsky Region) in June 2011. Detailed light microscopical (LM) and scanning electron microscopical (SEM) examinations (the latter used for the first time for the reported nematode species) of the available material revealed the presence of three inadequately described nominal species of this genus: R. (Rhabdochona) denudata (Dujardin, 1845) from the spotted steed Hemibarbus maculatus Bleeker (Gobioninae), and R. (Rhabdochona) longispicula Belous in Roytman, 1963 and R. (Globochonoides) coronacauda Belous, 1965 from Culter alburnus Basilewsky (Cultrinae) in the Ilistaya River. Detailed morphological study of these worms, especially SEM examination, made it possible to reveal some previously unreported morphological features (e.g., the presence of sublabia or the character of ventral precloacal ridges) and to confirm other taxonomically important characters such as the shape of deirids, number of anterior prostomal teeth, number and situation of lateral preanal and postanal papillae or the detailed structure of the crown-like formation on the female tail tip in R. coronacauda. Unidentifiable Rhabdochona (Rhabdochona) gravid females were recorded from the humpback Chanodichthys dabryi (Bleeker) (Cultrinae) in the Ilistaya River and from the Amur minnow Rhynchocypris lagowskii (Dybowski) (Leuciscinae) in the Komissarovka River.

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.