Microlaimidae Micoletzky, 1922

Leduc, Daniel, 2016, One new genus and three new species of deep-sea nematodes (Nematoda: Microlaimidae) from the Southwest Pacific Ocean and Ross Sea, Zootaxa 4079 (2), pp. 255-271 : 256-257

Family Microlaimidae Micoletzky, 1922

Diagnosis. (modified from Tchesunov (2014)) Body colourless or yellow-brownish. Cuticle usually annulated (except Ixonema Lorenzen, 1971 ), rarely with lateral differentiation in the form of lateral alae ( Microlaimus falciferus Leduc & Wharton, 2008 ); finely punctated, spiny, or with fine longitudinal bars in some species. Head cuticle smooth but not thickened or modified into a cephalic capsule; head usually slightly set off from the body. Inner labial sensilla as minute papillae, outer labial sensilla as papillae or setae, cephalic sensilla usually as longer setae. Amphideal fovea circular, cryptocircular or spiral. Cheilostoma with twelve longitudinal folds. Pharyngostoma with cuticularised walls; buccal cavity with small to medium-sized dorsal tooth and two smaller subventral teeth, sometimes without teeth. Pharynx with rounded, pyriform, or oval posterior bulb, rarely elongated ( Maragnopsia hadalis n. gen. n. sp.). Pore of secretory-excretory system usually posterior to nerve ring. Two opposed, outstretched ovaries. Two opposed testes or only one anterior testis. Tail short and conical, rarely long and conico-cylindrical ( Maragnopsia hadalis n. gen. n. sp.).

Remarks. Microlaimus is by far the most diverse genus of the family Microlaimidae . It closely resembles a number of genera, namely Bolbolaimus Cobb, 1920 , Aponema Jensen, 1978 , and Calomicrolaimus Lorenzen, 1976 . There has been disagreement about which morphological traits to use for differentiating between these genera, which has resulted in changes in the placement of some species.

Cobb (1920) first described the genus Bolbolaimus with B. pellucidus Cobb, 1920 as type species, and also described B. punctatus Cobb, 1920 in the same publication. He did not provide a diagnosis for the genus, but the type species is characterised by the following traits: head not set off from rest of body, circular to cryptocircular amphideal fovea with faint outline, strongly cuticularised buccal cavity with large dorsal tooth and smaller subventral teeth, pharynx with conspicuous anterior and posterior bulbs, spicules short and strongly cuticularised, small pre-cloacal supplements present, two opposed and outstretched ovaries, and conical tail. The illustration shows the presence of six cephalic setae and six outer labial papillae, but the inner labial sensillae are not shown. The presence of six cephalic setae, instead of the usual four, seems unlikely and it is possible that the outer labial setae were mistaken for cephalic setae, and that the cephalic setae were overlooked. B. punctatus is characterised by the same key traits as B. pellucidus , but no male was described and the illustration clearly shows that the four cephalic setae are situated at the same level as the outer labial setae. Jensen (1978) first placed Bolbolaimus within the Microlaimidae mainly based on the presence of outstretched ovaries in females, and emphasised the importance of the structure of the pharynx, the position of the excretory pore posterior to the nerve ring, and the heavy cuticularisation of the buccal cavity and copulatory apparatus as key diagnostic traits of the genus. More recently, Platt & Warwick (1988) and Tchesunov (2014) emphasised the presence of an anterior pharyngeal bulb and strongly cuticularised buccal cavity as the most important traits for differentiating Bolbolaimus from Microlaimus . Platt & Warwick (1988), however, noted that these traits are also found in several Microlaimus species, such as M. robustidens Stekhoven & De Coninck, 1965 , M. acinaces Warwick & Platt, 1971 , and M. conothelis (Lorenzen, 1973) . Conversely, Tchesunov (2014) noted that Bolbolaimus bahari Muthumbi & Vincx and B. abebei Muthumbi & Vincx, 1999 lack a well-developed pharyngeal bulb. In their review of Microlaimidae, Kovalyev & Tchesunov (2005) argue that the position of the excretory pore is not a reliable trait for differentiating among genera. More work is clearly needed to clarify the relationship between Microlaimus and Bolbolaimus ; the absence of a constriction setting off the head from the rest of the body in Bolbolaimus may be an important trait differentiating the two genera. Other traits that have not been considered previously, such as the arrangement of the head sensillae (e.g., relative positions of outer labial sensillae and cephalic setae) and whether the amphids are completely or incompletely surrounded by cuticle annulations, may also be informative, but more detailed observations of B. pellucidus type material are required.

The genus Aponema was established by Jensen (1978) with A. torosus as type species. This species is characterised by a single anterior testis and presence of dorso-caudal apophyses, the latter trait being listed by Jensen (1978) among the diagnostic features of the genus. Other authors, however, later argued that the presence of a single anterior testis is the most important feature differentiating Aponema from other microlaimid genera ( Kovalyev & Tchesunov 2005; Kovalyev & Miljutina 2009). In the latest treatment of the family, Tchesunov (2014) provided a detailed discussion on the taxonomy of Aponema and maintained that the presence of dorsocaudal apophyses is the best trait to use for distinguishing this genus.

In the original description of the genus Calomicrolaimus, Lorenzen (1976) lists the presence of cervical setae, thickening of the ventral cuticle in the pre-cloacal region, and male with protruding corpus gelatum as key diagnostic features of the genus. None of the species described subsequently possesses all three of these characteristics, which led Kovalyev & Tchesunov (2005) to regard Calomicrolaimus as junior synonym of Microlaimus . Tchesunov (2014) considered Calomicrolaimus to be a valid monotypic genus, and recommended transferring or returning all species described since the original description of the genus by Lorenzen (1976) to the genus Microlaimus .