Agrachanus Tikhomirova 1971

Genus Agrachanus Tikhomirova, 1971 emend

Diagnosis [based on Tikhomirova (1971), emended]. Skrjabillanidae, Skrjabillaninae. Head end arch-shaped. Four external cephalic papillae, four internal cephalic papillae; amphids large. Buccal capsule well sclerotized, small, with three long esophageal teeth and three long teeth-like protrusions of basal part of capsule (onchi). Esophagus muscular, external esophageal gland well-developed. Tail of female with three small terminal processes. Caudal end of males with well-developed copulatory bursa and unpaired sclerotized copulatory plate. Five pairs of postcloacal genital papillae present, of which three pair sessile and two pairs pedunculated.

Type species: Skrjabillanus scardinii Molnár, 1966; the valid binomen is Agrachanus scardinii (Molnár, 1966) Tikhomirova, 1971.

The taxonomic status of the Esocineminae and the systematic position of the genus Mexiconema should not be reviewed before molecular data for the type-genus of the family Daniconematidae, Daniconema Moravec & Køie, 1987, have been obtained.

The superfamily Dracunculoidea belongs to the group Spirurina incertae sedis in De Ley and Blaxter (2002, 2004). However, the data for SSU rDNA sequences as well as analysis of the complete mitochondrial genome support Dracunculoidea as a sister taxon to the Camallanoidea (Wijová et al. 2006; Nadler et al. 2007; Černotíková et al. 2011; Laetsch et al. 2012; Choudhury & Nadler 2016; Sokolov & Malysheva 2017b; Zou et al. 2017), which corresponds to the traditional views on the systematics of rhabditid (secernentean) nematodes (e.g. Chitwood 1937; Ivashkin et al. 1971; Chabaud 1974; Maggenti 1981). In the system by developed by De Ley & Blaxter (2004), the Camallanoidea Railliet & Henry, 1915 belongs to the infraorder Spiruromorpha De Ley & Blaxter, 2002. Laetsch et al. (2012) placed Dracunculoidea into the separate infraorder Dracunculomorpha Laetsch, Heitlinger, Taraschewski, Nadler & Blaxter, 2012. Roberts et al. (2013) shared this point of view, assigning Camallanoidea to the infraorder Camallanomorpha Roberts, Janovy & Nadler, 2013. In our opinion, the Dracunculoidea and the Camallanoidea can be combined into the infraorder Camallanomorpha. The Camallanomorpha concept is consistent with that of suborders Camallanata sensu Ivashkin, Sobolev & Khromova, 1971 and Camallanina sensu Chabaud, 1974, adjusted for angillicollids. According to modern data, anguillicolids have an affinity for gnathostomatoid nematodes (Wijová et al. 2006; Černotíková et al. 2011; Laetsch et al. 2012; Choudhury & Nadler 2018).

In conclusion, we would like to note that molecular data from many freshwater nematodes of the genera Philometra and Philometroides are still lacking. Moreover, even families such as Guyanemidae, Lucionematidae and Steinertetanonematidae remain underrepresented in this respect. This information would be vitally important both for reconstructing the phylogeny of this group and for solving taxonomic problems associated with the polyphyly of the skrjabillanids and the polyphyletic origin of the genera Philometra and Philometroides.