Taxonomy

The genus Rhabdias Stiles et Hassall, 1905 was erected without any diagnosis, just the type species, R. bufonis (Schrank, 1788), was indicated. The emended diagnoses for the genus were published by Travassos (1930), Yamaguti (1961) and Baker (1978).

Pereira (1927) discovered the new species which he placed into a new genus Acanthorhabdias Pereira, 1927. The differentiation from Rhabdias Stiles et Hassall, 1905 was based on the differences in cephalic end morphology: from 8 to 10 cone-shaped circumoral protuberances found in A. acanthorhabdias Pereira, 1927 which have never been observed in Rhabdias spp.

Travassos (1930) proposed the new genus Entomelas Travassos, 1930 for 3 species previously belonging to Rhabdias Stiles et Hassall, 1905: E. entomelas (Dujardin, 1845), E. dujardini (Maupas, 1916) and E. chameleonis (Skrjabin, 1916). The differentiation of the two genera was based on the differences in buccal capsule size, small in Rhabdias spp. and enlarged in Entomelas spp. Taken into account were the absolute dimensions of buccal capsule, without respect to the body size of the worms. E. entomelas (Dujardin, 1845) was indicated as type species for the genus. In the same paper Travassos (1930) also gave brief diagnoses for the genera Rhabdias Stiles et Hassall, 1905, Acanthorhabdias Pereira, 1927 and Entomelas Travassos, 1930.

Yamaguti (1943) described the species Rhabdias horigutii Yamaguti, 1943 and proposed to place it into the subgenus Ophiorhabdias Yamaguti, 1943. Differentiation from the nominative subgenus Rhabdias was based on the absence of buccal capsule in R. (Ophiorhabdias) horigutii Yamaguti, 1943 and its specificity to snakes. However, the differentiation was not absolutely correct since subgenus Rhabdias was regarded as including only amphibian parasites. The species R. fuscovenosa (Railliet, 1899), R. vellardi Pereira, 1928, R. ophidia Goodey, 1924 and R. labiata Pereira, 1928, all possessing the buccal capsules, were not mentioned by Yamaguti (1943). Later Sharpilo (1976) elevated the rank of the subgenus Ophiorhabdias Yamaguti, 1943 to generic level.

Sharpilo (1964) transferred the species Hexadontophorus ophisauri Kreis, 1940 from Strongylidae to Rhabdiasidae. The similarities between H. ophisauri Kreis, 1940 and Entomelas spp. were emphasised, however, the generic position of the species was left unchanged and the differential diagnosis for the genus Hexadonthophorus Kreis, 1940 was not emended.

Ballantyne and Pearson (1963) transferred Pneumonema tiliquae Johnston, 1916 from Rictulariidae to Rhabdiasidae reffering to the close morphological and biological affinities between the species and other rhabdiasids. The genus Pneumonema Johnston, 1916 was differentiated from all genera of Rhabdiasidae on the base of presence of cuticular spines in P. tiliquae Johnston, 1916.

Szczerbak and Sharpilo (1969) proposed the new genus Kurilonema Szczerbak et Sharpilo, 1969 for the species K. markovi Szczerbak et Sharpilo, 1969. The genus was distinguished from Rhabdias Stiles et Hassall, 1905 by notably larger buccal capsule, and from Entomelas Travassos, 1930 by the absence of teeth.

Singh and Ratnamala (1975) described a new rhabdiasid species Shortia shortii Singh (1975) which they placed into the new genus Shortia Singh et Ratnamala, 1975. The new genus was differentiated from Rhabdias Stiles et Hassall, 1905 based on the presence of hypodermal glands - "areoles" in S. shortii Singh, 1975.

Sharpilo (1976) proposed the new genus Paraentomelas Sharpilo, 1976 for two species, P. dujardini (Maupas, 1916) and P. kazachstanica (Sharpilo et Vakker, 1972) previously belonging to the genus Entomelas Travassos, 1930. The author believed that Paraentomelas species differed from Entomelas ones in possession of 3 onchia (vs 6 onchia in E. entomelas), swollen body cuticle and needle-like tail tip.

The revision of the genus Entomelas Travassos, 1930 and related genera has been performed by Baker (1980). The author synonymized the genera Kurilonema Szczerbak et Sharpilo, 1969, Hexadontophorus Kreis, 1940 and Paraentomelas Sharpilo, 1976 with the genus Entomelas Travassos, 1930, and the genera Ophiorhabdias Yamaguti, 1943 and Shortia Singh, 1975 with the genus Rhabdias Stiles et Hassall, 1905. The species Entomelas chameleonis (Skrjabin, 1916) Travassos, 1930 was transferred to the genus Rhabdias Stiles et Hassall, 1905; three species: P. dujardini (Maupas, 1916), P. kazachstanica (Sharpilo et Vakker, 1972) and H. ophisauri Kreis, 1940 were synonymized with E. entomelas (Dujardin, 1845).

The synonymizations were based on the peculiarities of late parasitic development of two Rhabdias species (Baker, 1979) (the data were extrapolated to Entomelas spp. development) and author's attitude towards significance or insignificance of some morphological characters (e. g. presence or absence of teeth or buccal capsule, number of teeth, size of buccal capsule, etc.). As the result, only 4 generic names were left valid within Rhabdiasidae Railliet, 1915:

1. Acanthorhabdias Pereira, 1927
2. Entomelas Travassos, 1930
3. Pneumonema Johnston, 1916 and
4. Rhabdias Stiles et Hassall, 1905.

This system was accepted in the "CIH Keys to nematode parasites of vertebrates, No. 9" (Anderson, Bain 1982).

Later, Hasegawa (1989) described a new rhabdiasid species with quite an aberrant head end morphology and proposed the new genus Neoentomelas Hasegawa, 1989 for it. The genus was differentiated from Entomelas Travassos, 1930 (sensu Baker 1980) by the presence of dorsal and ventral pseudolabia and markedly enlarged head end bearing posterior lobes.

Lhermitte-Vallarino, Bain, Deharo et al., (2005) erected a new genus Chabirenia Lhermitte-Vallarino, Bain, Deharo et al., 2005 for one species, Chabirenia cayennensis, found in buccal cavity of South-American teiid lizards. In addition to abnormal localization, the new species possesses longitudinal cuticular ridges similar to the synlophe of trichostrongylid nematodes.

Recently, Tkach et al. (2014) revised the system of Rhabdiasidae based on molecular phylogenetic studies. The new genus Serpentirhabdias Tkach, Kuzmin and Snyder, 2014 was erected for the species parasitizing snakes. This group within Rhabdiasidae was shown to be a monophyletic and well separated from other rhabdiasids. The phylogenetic data supported the validity of Rhabdias (including the species parasitic in amphibians and lizards), Entomelas and Pneumonema. The phylogenetic relationships within the Entomelas clade confirmed the validity of E. ophisauri, E. dujardini and E. kazachstanika; the taxa Hexadontophorus and Paraentomelas were shown to be polyphyletic.

1. Phylogenetic tree of Rhabdiasidae showing separation of Serpentirhabdias (after Tkach et al., 2014).

2. Phylogenetic tree of Rhabdiasidae showing monophyly of Entomelas, Pneumonema and Rhabdias (after Tkach et al., 2014).

Based on morphological and molecular data, the following system of Rhabdiasidae is proposed here as the working one:

1. Genus Rhabdias Stiles et Hassall, 1905
2. Genus Acanthorhabdias Pereira, 1927
3. Genus Chabirenia Lhermitte-Vallarino, Bain, Deharo et al., 2005
4. Genus Entomelas Travassos, 1930
5. Genus Kurilonema Szczerbak et Sharpilo, 1969
6. Genus Neoentomelas Hasegawa, 1989
7. Genus Pneumonema Johnston, 1916
8. Genus Serpentirhabdias Tkach, Kuzmin et Snyder, 2014

In our opinion, the validity of Acanthorhabdias and Neoentomelas is to be confirmed by (molecular) phylogenetic studies.