Mononchida is an order of predatory nematodes and includes the suborders Bathyodontina and Mononchina. In this survey, sequences of the 18S rDNA were amplified and used to reconstruct the phylogeny of the Mononchina. Phylogenetic analyses using Neighbour Joining (NJ) and Maximum Likelihood (ML) were employed with five outgroup taxa and 65 mononch sequences including 14 new sequences from Iran. Both analyses indicated that the Anatonchus is monophyletic. Actus was placed as the sister group of Mylonchulus with weak and strong support, respectively, from the ML and NJ analyses. In both phylogenetic analyses, trees obtained from SSU rDNA alignments were subdivided into five highly- or moderately-supported clades, designated Clade I: Mylonchulus spp., Clade II: Actus salvadoricus, Clade III: Anatonchus spp., a group comprising the genera Clarkus, Coomansus, Miconchus and Prionchulus, Clade IV: Mononchus spp., and Clade V: Granonchulus sp. The 18S rDNA analysis demonstrated that this region of the nuclear genome can be used to resolve the relationships of members of this suborder.

During a survey of soil nematodes in Iran, some known species belonging to the order Mononchida were recovered: Anatonchus tridentatus, Clarkus papillatus, Miconchus studeri, Mononchus aquaticus, Mononchus pulcher, Mononchus truncatus, Mylonchulus brachyuris, Mylonchulus lacustris and Mylonchulus sigmaturus. Mylonchulus lacustris is a new geographical record from Iran. Measurements and illustrations are provided for all species. Canonical discriminant functions analysis revealed that 96.8% of the original grouped cases (Mylonchulus species) are classified correctly. A molecular study of the 18S rDNA region of A. tridentatus places this population beside other A. tridentatus populations. BlastN search results revealed that the Iranian population of A. tridentatus differs from populations from the Netherlands (AY284768; 93% identity) and Belgium (AJ966474; 96% identity) in 46 and 29 nucleotides, respectively. Phylogenetic analysis indicates that Anatonchus is a monophyletic taxon. Phylogenetic position and morphometrics of this species are presented. A compendium and a new proposed key for the species of the genus Anatonchus are given as well. Materials and methods Studies on morphologyNematodes were extracted from soil samples using Baermann's (1917) funnel technique. They were fixed with hot 4% formaldehyde solution and processed to anhydrous glycerin by the method of De Grisse (1969). Measurements were taken directly using an ocular micrometer and/or a curvimeter upon drawing the corresponding organ or structure. Drawings were made using a drawing tube attached to an Olympus CH-2 microscope. Measurements of the buccal cavity follow Andrássy (2011). Discriminant analysisDiscriminant analysis was used to identify the combination of variables that best separate the species Mylonchulus. We analysed 72 specimens belonging to three species including Mylonchulus brachyuris (Bütschli, 1873) Cobb, 1917, Mylonchulus lacustris (Cobb in Cobb, 1915) Cobb, 1917 and Mylonchulus sigmaturus Cobb, 1917. Eighteen morphometric traits within this group of nematodes including de Man's indices (a, b, c, c 0 and V), body length, neck length, tail length, buccal cavity diameter and length, dorsal tooth length, tooth apex position and length, amphid aperture width, amphid distance from the anterior end, vulva from anterior end, and anterior and posterior genital branch lengths as percentages of the total body length were used for discriminant analysis. These characters were obtained from fixed specimens. Data on the morphometric measurements of the three species were analysed using univariate ANOVA. The standardized discriminant function procedure was used to separate the species. Function coefficients were selected using Fisher's method. Fisher's method is the classical method of discriminant classification calculated on one set of discriminant function coefficients for each dependent category, using these to make the classifications. The ability of this function to identify Mylonchulus species was indicated as the percentage of indivi...