Halanonchus Cobb, 1920

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 : 23-24

Genus Halanonchus Cobb, 1920

Latilaimus Allgén, 1933 .

Diagnosis (modified from Pavlyuk 1984)

Halanonchinae . Cuticle smooth. Brown or golden granules often present along lateral, ventral and dorsal chords. Inner and outer labial sensilla either very close to each other or in separate circles. Inner labial sensilla papillose or setose; outer labial setae sometimes jointed; cephalic setae situated in separate circle further posteriorly but anterior to amphids. Buccal cavity large, barrel-shaped; both gymnostoma and stegostoma with cuticularized walls, often with curved cuticular discontinuity between gymnostoma amd stegostoma, which has been interpreted as “oval structures” supporting the buccal cavity.Amphideal fovea pocket-shaped, at level of buccal cavity or posterior to buccal cavity; oval or rounded amphideal aperture. Mid-ventral row of papillose pharyngeal supplements (complex supplements in H. bullatus Gerlach, 1964 ) usually present in males. Male copulatory apparatus consists of short, arcuate or straight spicules, and small gurbenaculum without apophyses; precloacal supplements present. Tail long and filiform. Seven species.

Type species

Halanonchus macrurus Cobb, 1920 .

Other valid species

H. arenarius Pavlyuk, 1984 .

H. bullatus Gerlach, 1964 .

H. cornutus Vitiello, 1971 .

H. longicaudatus ( Allgén, 1935) Gerlach, 1964 .

= Eumorpholaimus longicaudatus Allgén, 1935 .

H. papilatus Groza-Rojancovski, 1972 .

Species inquirendae

H. macramphidus Chitwood, 1936 .

H. renatus ( Timm, 1961) Gerlach, 1964 .

= Latilaimus renatus Timm, 1961 .

H. zosterae ( Allgén, 1933) Gerlach, 1964 .

= Latilaimus zosterae Allgén, 1933 .

Remarks

Latilaimus was synonymized with Halanonchus by Gerlach (1964). Pavlyuk (1984) considered H. renatus invalid due to the incomplete description, which did not include male specimens. We consider H. macramphidus and H. zosterae to be species inquirendae for the same reason. No females have yet been described for H. longicaudatus or H. papilatus .

Although the presence of pharyngeal supplements is given as a genus character by Pavlyuk (1984), this feature is absent in H. longicaudatus . Furthermore, only one inconspicuous pharyngeal supplement is present in H. cornutus and pharyngeal supplements were not observed in all male H. scintillatulus sp. nov. specimens. We also note that, within the genus, inner labial sensilla can be either papillose or setose, and can be situated either very close to the outer labial sensilla or in a separate circle.

Wieser & Hopper (1967) listed deeply incised lips as a diagnostic feature of the Halanonchinae . The latter authors included Cytolaimium , Rhabdocoma and Halanonchus in the subfamily, which they classified with the Tripyloididae . This family includes the genus Bathylaimus Cobb, 1894 , which is characterized by deeply incised lips. This feature was later included in the diagnosis of Halanonchus provided by Pavlyuk (1984). We did not observe this feature in the Halanonchus specimens from the Hauraki Gulf, however, and did not see it clearly described in any description of Halanonchus species. We postulate that the presence of deeply incised lip was a misinterpretation of buccal cavity structures. We also believe that the description of oval structures supporting the buccal cavity resulted from a similar misinterpretation, as explained below.

Oval structures supporting the buccal cavity were first illustrated in the description of H. macrurus by Cobb (1920). Unfortunately, the nature of these structures was not described or interpreted by the latter author. In his description of H. macramphidus Chitwood, 1936 , Chitwood (1936) described a buccal cavity similar to that described by Cobb (1920), but without any oval structure. Chitwood’s illustration shows instead a fine line demarcating the gymnostoma and stegostoma, which is consistent with a cuticular discontinuity in the buccal cavity wall. Gerlach (1964) included oval structures in his illustration of the buccal cavity of H. bullatus Gerlach, 1964 , the posterior edge of which coincides with a cuticular discontinuity between the gymnostoma and stegostoma as seen in cross-section. He did not describe or interpret, however, the nature of these oval structures beyond illustrating them. Oval structures were first explicitly discussed by Wieser & Hopper (1967) in their decription of H. macrurus . They stated that: “Each lip seems to be supported by a large oval structure which apparently was mistaken for the amphids by Allgén (1933) in his description of Latilaimus zosterae” ( Wieser & Hopper 1967: 249) . Their illustration shows a single oval structure, with a posterior margin coinciding with the limit between gymnostoma and stegostoma. Their illustration also shows two thin longitudinal lines spanning the length of the gymnostoma and stegostoma, which touch one side of the oval structure. These lines may have been interpreted by these authors as indicating the presence of deeply incised lips in this species. Our own observations, as well as the descriptions by Cobb (1920), Chitwood (1936), and Groza-Rojancovski (1972) in particular, indicate to us that the gymnostoma and stegostoma in Halanonchus are delimited by a curved cuticular discontinuity. In the case of H. scintillatulus sp. nov., the two buccal cavity compartments are also delimited by the more thickly cuticularized anterior edge of the stegostoma, which projects slightly into the buccal cavity ( Fig. 5 ). In addition, our own observations, as well as the illustrations of Wieser & Hopper (1967) and Pavlyuk (1984), indicate that the ventrosublateral and dorsal sectors of the gymnostoma and stegostoma are delimited by longitudinal cuticular discontinuities. We hypothesise that it is the presence of these cuticular discontinuities which led previous authors to describe the presence of three oval structures supporting the buccal cavity in Halanonchus species, as well as the presence of deeply incised lips.

Figures

Fig. 5. Schematic representation of buccal cavity wall cuticularization in Halanonchus based on observations of Halanonchus scintillatulus Leduc sp. nov. and illustrations of other Halanonchus species. Only the two ventrosublateral sectors are shown (dorsal sector omitted for clarity). Thin lines show contours of gymnostoma, and thick lines show contours of stegostoma.