The genera Tripyla (Bastian, 1865), Trischistoma (Cobb, 1913) and Tripylina (Brzeski, 1963) are in the family Tripylidae (De Man, 1876), which is assigned to either the order Triplonchida (de Ley & Blaxter, 2004) or Enoplida (Andrássy, 2007). Paratripyla has been synonymized with Tripyla and the family Tripylidae currently consists of the genera Tripyla, Tripylella,
In recent classifications of the Nematoda, the three subfamilies recognized
within the family Tripylidae de Man, 1876, order Enoplida Filipjev, 1929 were
Tripylinae de Man, 1876, Trischistomatinae Andrassy, 2007 and Tobriliinae
Andrassy, 2007 (Andrassy 2007)
The subfamily Tripylinae included the genera
Brzeski, 1963 and
Tripylella Brzeski &
The Trischistomatinae had only the genus,
Trischistoma Cobb, 1913, and the Tobriliinae had only one
Tobrilia and Abunema have some characters that seem to separate
them from the other genera and various authors have proposed their removal
from the family. However, since alternative families have not been suggested,
Zullini (2006) elected to retain these genera in the Tripylidae.
Tobrilia and Abunema have some characters that seem to separate them from the other genera and various authors have proposed their removal from the family. However, since alternative families have not been suggested, Zullini (2006) elected to retain these genera in the Tripylidae.
Phylogenetic analysis of small subunit (SSU) ribosomal DNA sequences distinguished the clade of Tripylina plus Trischistoma from the monophyletic group represented by species of Tripyla. Reconstruction of the evolution of ovary number on the phylogenetic tree indicates that the monovarial condition envolved in the most recent common ancestor of the Tripylina plus Trischistoma clade (Cid del Prado-Vera et al. 2010). Other molecular phylogenies separated Trischistoma from the other genera and suggested that it is closely related to the family Trefusiidae in the order Enoplida (Holterman et al. 2006; Holterman & Holovachov 2007; Meldal et al. 2007; Zhao & Buckley 2009; van Megen et al. 2009), which has affinities in spicule characteristics, intestinal tract and muscle arrangement (O. Holovachov, personal communication).
Based on the morphological and molecular evidence that Tripyla, the type genus of the Tripylidae, is not closely related to Trischistoma and Tripylina, Zhao (2011) elevated the subfamily Trischistomatinae to family rank and transferred Tripylina into that family. Consequently, the family Tripylidae includes the genera Tripyla and Tripylella while the Trischistomatidae currently includes the genera Trischistoma and Tripylina (Zhao, 2011; Zhao et al. 2012).
Morphologically and anatomically, the number of gonads, the position, shape and size of the stomatal teeth, and the proximity of whorls of labial and cephalic setae are used to distinguish genera in the Tripylidae and Trischistomatidae.
In the Tripylidae:
Tripyla is diovarial, has a striated cuticle, and the outer labial setae and cephalic setae are well separated;
Tripylella females are diovarial, amphidelphic, and the whorls of outer labial and cephalic setae are very close together so that they appear as a single whorl.
In the Trichistomatidae:
Trischistoma has a single ovary and the outer labial and cephalic setae well separated;
Tripylina has a single ovary, the outer labial and cephalic whorls of setae are close together (Tsalolikhin 1983; Zullini 2006; Cid del Prado-Vera et al. 2010; 2012).
Males of Tripylidae have the spicules surrounded by a muscular sheath, which has been used to support the suggestion that the family has greater affinity with the Triplonchida than with the Enoplida (De Ley & Blaxter 2004). However, Andrassy (2007) retained the family in the Enoplida and molecular phylogenetic analysis of SSU sequences strongly supports the relationship with the Enoplida (Zhao & Buckley 2009; Cid del Prado-Vera et al. 2010). Spicules in males in the Trischistomatidae are not surrounded by a muscular sheath (Brzeski 1965; Andrássy 1985).
The pharynx in the Tripylidae and Trischistomatidae is, in general, uniformly cylindrical throughout its length with slight enlargement in the latter portion associated with the location of the esophageal glands. There are five esophageal gland nuclei, the dorsal esophageal gland reportedly opening through the dorsal tooth, the first pair of subventral glands opening slightly posterior to that,and the second pair of subventral glands opening near the nerve ring (Chitwood & Chitwood, 1937). At the base of the pharynx is a tri-lobed structure variously termed the cardia or cardiac valve.
Nematodes of the families Tripylidae and Trischistomatidae are generalist predators of small aquatic and soil organisms. Many authors have commented on the freshwater and wet soil in which these nematodes are found, and on the nature of their prey based on observation of intestinal contents or from behavior. Among the recorded prey, as reviewed and collated by Small (1987), are nematodes, rotifers and protozoa (Cid del Prado et al., 2012).
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