Panagrobelus stammeri Rühm, 1956 and Plectonchus hunti n. sp. Implications of new morphological observations for characterisation of these genera (Nematoda: Panagrolaimoidea)

A detailed morphological description (light and scanning electron microscope observations) is given of two isolates currently used in molecular and developmental studies: PDL0024, Panagrobelus stammeri Rühm, 1956 and PDL0025, a new species herein described as Plectonchus hunti n. sp. We redescribe the lip region, interpret the orientation of the lips in P. stammeri and provide additional morphological and morphometric diagnostic features. Plectonchus hunti n. sp. is characterised by the bifurcating spicule tip and by the placement of the male genital papillae in a 3/3 + p + 2 arrangement. Additionally, females of this new species can be differentiated from other species of Plectonchus by the more anterior position of the vulva, the more anterior location of the excretory pore and the tail morphology. An emended diagnosis for this genus is provided. In this study we demonstrate that both studied taxa share morphological similarities, such as the presence of a 'cephaloboid-type' spermatheca. Our observations also indicate slight but consistent differences in cell composition of the female reproductive system between these two taxa.