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Abstract The female reproductive system of the free-living nematode family Cephalobidae is examined by means of differential interference contrast, scanning electron and fluorescent microscopy. The model nematode Caenorhabditis elegans and the predatory nematode Prionchulus punctatus are also included in this study; the former mainly to test our results with the very detailed knowledge of this model organism, the latter to provide a representative of the more distantly related Enoplea. In this comparative approach, the analysed gonad structures are discussed with respect to their functional and phylogenetic significance. The general cellular composition of the cephalobid gonad – namely an oviduct comprising two rows of four cells, a distinctly offset spermatheca consisting of 8-16 cells, and a uterus composed of distinct cell rows – differs from all known Nematoda except that of the plant-parasitic Tylenchomorpha. Despite the striking evolutionary conservation of the cellular architecture of the cephalobid gonad there is a complex subcellular specialisation, namely a significant and functionally relevant variation in myofilament organisation, both among Cephalobidae and between major groups of nematodes. We demonstrate the presence of microfilaments that vary in pattern among species and that may play an important role in egg propulsion. The phenomenon of endotokia matricida, in which eggs do not leave the female body, is found to be associated with a massive rupture of the cytoskeleton in the uterus wall. The complexity of the myofibril structure and the associated potential to propagate oocytes actively cannot be solely explained by differences in phylogenetic history, but is also linked to body diameter. In the larger Acrobeloides maximus, the proximal end of the ovary sheath is adorned with 12 distinct longitudinal bands, antibody binding positively for paramyosin, while in the smaller Cephalobus cubaensis myofilament organisation is at random.

The diversity and phylogenetic relationships of the Desmodoridae, a widespread tropical family of free-living marine nematodes, is hitherto poorly known both from molecular and taxonomic points of view. We performed a molecular phylogenetic analysis of marine nematodes to: i) disentangle relationships among tropical desmodorid species; and ii) compare the performance of the nuclear SSU rDNA and mitochondrial COI nucleotide sequences in 42 and 45 nominal species, respectively, to identify species. We generated 27 new sequences of SSU rDNA belonging to five genera not previously sequenced, and 34 new sequences of COI belonging to six genera and four families not previously sequenced. The SSU rDNA tree confirmed the Enoplida to be a monophyletic sister group to the Chromadorida. The family Comesomatidae is a sister group of the Xyalidae within the Monhysterida. Both DNA markers confirmed the congruence between the morphology- and molecular-based phylogenetic inferences for most of the families. Desmodoridae was a monophyletic group, but the relationships within the family could not be recovered; the subfamilies Desmodorinae and Spiriniinae were not monophyletic meanwhile the monophyly of Stilbonematinae was not fully supported due to a few specimens of questionable identity. COI performed better than SSU rDNA to disentangle relationships among closely related species and suggested the presence of cryptic diversity within Desmodoridae. COI is effective to explore cryptic diversity and barcode species within Nematoda, with a possible threshold of genetic distance of 5% between conspecific and interspecific sequences, but DNA barcoding is limited by the poor knowledge of the diversity and taxonomy of the group and the lack of a good reference database of vouchered COI sequences.