Trischistomatidae Andrássy, 2007

Leduc, Daniel, Zhao, Zeng Qi & Sinniger, Frederic, 2020, Halanonchus scintillatulus sp. nov. from New Zealand and a review of the suborder Trefusiina (Nematoda: Enoplida), European Journal of Taxonomy 661 (661), pp. 1-45 : 12-13

Family Trischistomatidae Andrássy, 2007

Diagnosis (from Zhao 2011)

Cuticle smooth, thin, not annulated. Metanemes present. Labial region divided into three lips. Outer labial setae and cephalic setae either in one circle or two separate circles. Amphideal fovea pocket-shaped with slit-like amphideal aperture. Buccal cavity narrow, surrounded by pharyngeal musculature, with three teeth (often only one is visible) in one or two stomatal chambers. Pharynx muscular, cylindrical; cardia present or absent. Female monodelphic with anterior reflexed ovary, with or without post-vulval uterine sac; vulva located at> 59 % of body length from anterior extremity. Male reproductive system monorchic. Spicules narrow, may or may not be enclosed within a muscular pouch; when present, muscle pouch almost completely surrounds spicules. Papillose precloacal supplements present or absent. Spermatozoa elongated or globular, usually with visible nucleus. Tail with three glands and terminal spinneret.

Type genus

Trischistoma Cobb, 1913 .

Remarks

The classification of the family was last revised by Zhao (2011). Phylogenies based on SSU sequences consistently show that while Trischistoma and Tripylina form a monophyletic clade with the Trefusiidae , the two genera do not form a monophyletic clade, and Trischistoma is more closely related to the Trefusiidae than Tripylina ( Holterman et al. 2006; Meldal et al. 2007; Zhao & Buckley 2009; van Megen et al. 2009; Bik et al. 2010, Zhao et al. 2012; present study). Morphologically, Trischistoma may be considered more similar to Trefusiina than Tripylina due to its having the same arrangement of the anterior sensilla (outer labial and cephalic setae in separate circles), as well as having the buccal cavity with minute denticles only (no teeth in Trefusiina ); Tripylina differs more strongly in the arrangement of the anterior sensilla (outer labial and cephalic setae in one circle) and buccal cavity with larger teeth. Trischistoma also shares an unusual spermatozoa morphology (relatively large and/or elongated, elliptical or fusiform, with central rod and/or nucleus near one extremity) with several Trefusiidae species. In Trischistoma , spermatozoa have been described for T. equatoriale Andrassy, 2006 and T. tenuissimum Andrassy, 2011 , and they both exhibit this unusual morphology. In the Trefusiidae , similar spermatozoa have been observed in species of Trefusialaimus ( Riemann 1974; Leduc 2013), Rhabdocoma ( Ott 1977; Vincx & Vanreusel 1989), and Trefusia ( Bussau 1993) . This kind of spermatozoa morphology has not been observed in Tripylina .

The Trischistomatidae is characterised by having a buccal cavity with teeth, a feature not found in any other Trefusiina family except some Lauratonema . We therefore propose to retain this family for the time being, despite the morphological differences between the two genera, and SSU phylogenies indicating that Trischistoma and Tripylin a do not form a monophyletic group.