Neodiplopeltula gen. nov. from the west coast of Sweden and reappraisal of the genus Diplopeltula Gerlach, 1950 (Nematoda, Diplopeltidae)

The new genus Neodiplopeltula gen. nov. is proposed to accommodate those species from the genus Diplopeltula Gerlach, 1950 that possess the following morphological characters: amphids in the shape of an elongated loop, a well-developed subcylindrical stoma and outstretched ovaries. The genus Diplopeltula is considered genus inquirendum et incertae sedis. Four species placed in Neodiplopeltula gen. nov. are redescribed. The following taxonomic changes are proposed: Neodiplopeltula asymmetrica (Allgén, 1935) gen. et comb. nov.; Neodiplopeltula barentsi (Steiner, 1916) gen. et comb. nov.; Neodiplopeltula bathmanni (Jensen, 1991) gen. et comb. nov.; Neodiplopeltula cuspidiboja (Leduc, 2017) gen. et comb. nov.; Neodiplopeltula indica (Gerlach, 1962) gen. et comb. nov.; Neodiplopeltula intermedia (Gerlach, 1954) gen. et comb. nov.; Neodiplopeltula obesa (Nguyen Vu Thahn, Nguyen Thahn Hien & Gagarin, 2012) gen. et comb. nov.; Neodiplopeltula onusta (Wieser, 1956) gen. et comb. nov.; Neodiplopeltula ovalis (Ditlevsen, 1928) gen. et comb. nov. and Neodiplopeltula tchesunovi (Fadeeva & Mordukhovich, 2013) gen. et comb. nov. New synonyms include: Diplopeltis asymmetricus Allgén, 1935 and Diplopeltis ovalis Ditlevsen, 1928 are synonimised with Neodiplopeltula barentsi (Steiner, 1916) gen. et comb. nov.; Diplopeltula tchesunovi Fadeeva & Mordukhovich, 2013 is synonimised with Neodiplopeltula onusta (Wieser, 1956) gen. et comb. nov.; the male of Diplopeltula cuspidiboja Leduc, 2017 is synonimised with Neodiplopeltula barentsi gen. et comb. nov. and the female with N. bathmanni gen. et comb. nov. A key to the species of Neodiplopeltula gen. nov. is provided.


Introduction
The genus Diplopeltula Gerlach, 1950 was proposed for two new and one known species, but without an explicit generic diagnosis. Diagnostic characters of this genus mentioned in the identifi cation key from the original publication (amphid is loop shaped, with equally long ventral and dorsal branches; amphid not located on the shield-like cuticular plate) match a number of genera currently included in the family Diplopeltidae, such as Morlaxia Vincx & Gourbault, 1988;Mudwigglus Leduc, 2013;Pararaeolaimus Timm, 1961;Pseudaraeolaimus Chitwood, 1951 andeven Araeolaimus de Man, 1888. Taxonomic instability of the genus is further sustained by the fact that the original description of the type species Diplopeltula breviceps Gerlach, 1950 does not include clear descriptions of characters currently considered diagnostic at the genus and family level, such as the exact structure of the pharynx or of the female reproductive system (Table 1), and because the type material was not preserved by S.A. Gerlach. As a result, the genus Diplopeltula recently underwent several taxonomic revisions, with more and more species being moved to other known and newly proposed genera (Holovachov et al. 2009;Leduc 2013;Holovachov 2017;Holovachov & Boström 2017). In this manuscript we propose to move all the remaining properly described species from the genus Diplopeltula to a new genus Neodiplopeltula gen. nov., treat both Diplopeltula breviceps and D. longiceps Gerlach, 1950 as species inquirenda et incertae sedis, treat Diplopeltula ostrita Boucher &Helléouët, 1977 andD. striolata Vincx &Gourbault 1992 as species incertae sedis, and consider Diplopeltula a genus inquirendum et incertae sedis.

Material and methods
Bottom sediment samples were collected in several locations in the southern part of the Skagerrak and in the Gullmarn Fjord off the west coast of Sweden. All samples were collected with a bottom dredge or box corer and further sieved in the laboratory before fi xation. Nematodes were extracted from samples using a decanting and sieving method (smallest mesh sizes: 45 μm or 70 μm). Freshwater was used during sieving to induce an osmotic shock in nematodes inducing their detachment from the substrate. Material retained on the sieves was immediately fi xed in a 4% formaldehyde solution in freshwater.
For light microscopy, formaldehyde-preserved specimens were transferred to pure glycerine using Seinhorst's (1959) rapid method as modifi ed by De Grisse (1969). Permanent nematode mounts on glass slides were prepared using the paraffi n wax ring method. After observations, six females and fi ve males of N. incisa (Southern, 1914) gen. et comb. nov. from the sample #S053 (SWEDEN: Skagerrak, 58°17′32″ N, 11°11′24″ E, coarse sediment with algae at a deep of 45-55 m, 9 Aug. 2011, O. Holovachov leg.) were removed from slides and rehydrated by fi rst gradually adding drops of S2 (5% glycerine, 95% ethanol) to glycerine in an embryo-dish, starting with a 1:4 ratio of S2 to glycerine, until the volume tripled, then gradually adding distilled water until the volume tripled again. The specimens were then washed in distilled water before resuspension in formaldehyde. For SEM, specimens were post-fi xed in 1% osmium tetroxide (OsO 4 ) and transferred to pure acetone through an acetone/distilled water series. Specimens were critical point dried in liquid CO 2 , mounted on stubs, gold-plated under vacuum to a thickness of 200 Å in an Agar High Resolution Sputter Coater Model 20, and examined in a Hitachi S-4300 SEM at an accelerating voltage of 10 kV.
All curved structures were measured along the curved median line. Measurements in all tables are presented in μm as mean and (range) where appropriate. Terminology follows Maggenti (2005). Specimens are deposited in the invertebrate collection of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden (SMNH). Type specimens of Diplopeltis asymmetricus Allgén, 1935 deposited in the Swedish Museum of Natural History in Stockholm, Diplopeltis ovalis Ditlevsen, 1928 deposited in the Natural History Museum of Denmark in Copenhagen and Diplopeltula ostrita Boucher & Helléouët, 1977 deposited in the National Museum of Natural History in Paris were also examined.

Diagnosis
Cuticle transversely striated; striae visibly smooth under light microscope, may have fi ne longitudinal incisures visible under SEM. Lateral alae absent. Body pores and epidermal glands absent. Somatic sensilla present, most prominent along anterior part of pharyngeal region and on tail. Labial region bluntly rounded; lips fused. Six inner labial sensilla small, pore-like, located on anterior surface of lips, discernible under SEM only. Six outer labial sensilla papilliform, located on anterior surface of lips. Four cephalic sensilla setiform; their bases located at base of labial region, at level with anteriormost part of amphid. Bases of dorsosublateral setae often located more posteriorly than bases of ventrosublateral setae. Subcephalic sensilla absent. Cervical sensilla present in some species, setiform or papilliform, arranged in dorsosublateral and ventrosublateral rows at level with amphid and subcuticular periamphideal plates. Deirid and ocelli absent. Amphids in some species on strongly refractive lateral subcuticular periamphideal plates that can be connected together on ventral and dorsal sides forming a 'cephalic framework'. Amphidial fovea loop-shaped (inverted U-shaped), dorsal branch often longer than ventral branch; both branches of amphideal fovea closely adjacent. Secretory-  (Southern, 1914) gen. et comb. nov. and generic diagnosis (this publication), Diplopeltula Gerlach, 1950 based on the type species D. breviceps Gerlach, 1950, andDiplopeltoides Gerlach, 1962 based on the type species D. ornatus (Gerlach, 1950) and emended generic diagnosis in Holovachov & Boström (2017).

Etymology
The genus name is composed of a prefi x 'neo-', meaning 'new' and Diplopeltula, and defi nes a new taxon proposed to accommodate species formerly placed in the genus Diplopeltula.

Relationships
The new genus differs from all other taxa currently included in the family Diplopeltidae (Holovachov 2017) in having amphids in the shape of an elongated loop, a well-developed subcylindrical stoma and outstretched ovaries. It is proposed in order to accommodate a number of species previously placed in the genus Diplopeltula and to solve a long lasting taxonomic conundrum (Vincx & Gourbault 1992;Holovachov et al. 2009;Holovachov 2017;Holovachov & Boström 2017). The description of Diplopeltula breviceps, type species of the genus Diplopeltula, does not include clear descriptions of several important taxonomic characters (see Table 1) and the absence of such data prevents an unequivocal placement of the species D. breviceps and the genus Diplopeltula in the classifi cation system of Nematoda. As a result, D. breviceps is considered species inquirenda et incertae sedis and the genus Diplopeltula is accordingly considered genus inquirendum et incertae sedis. The second species, D. longiceps, is also considered species inquirenda et incertae sedis for the same reasons (incomplete description and unclear systematic position).

Nomenclatorial changes and notes
1) The description of Diplopeltis incisus sensu Gerlach, 1950 is based on specimens collected in several localities in the Kiel Bay and is likely to be a mixture of at least two different species: based on the oval shape and relatively small size of the periamphideal refractive plate, the position of cephalic setae posterior to the oral opening, and the position of the oral opening close to the anterior end, N. barentsi gen. et comb. nov. can be identifi ed on fi g. 3h, 3k & 3m in Gerlach (1950), while fi g. 3b likely depicts N. incisa sensu stricto, based on the irregular shape and relatively large size of the periamphideal refractive plate, the position of cephalic setae posterior to the oral opening, and the position of the oral opening close to the anterior end. Unfortunately, exact measurements of morphological characters cannot be attained from the fi gures in Gerlach's publication due to the absence of proper scale bars. Thus, our hypothesis on the taxonomic identity of specimens studied by Gerlach (1950) must solely rely on illustrations and cannot be verifi ed any further.
2) The description of Diplopeltula incisa sensu Voronov, 1982 is based on specimens collected in six different locations from the White Sea, within an area of more than 100 × 200 km in size and ranging in depth from 50 to 250 meters. Many measurements given by Voronov vary within 2×-4× range. Morphological features of the anterior end also show a considerable variability (noted by Voronov himself) suggesting that his description is based on more than one morphospecies. Here we suggest that there are four different species of Neodiplopeltula gen. nov. illustrated in Voronov's publication: fi g. 1a likely depicts N. bathmanni gen. et comb. nov. with very narrow periamphideal plates and an oral opening located at level with the cephalic setae bases; both specimens depicted on fi g. 1б and 1в match our concept of N. barentsi gen. et comb. nov. with relatively broad oval periamphideal plates and an oral opening located close to the anterior terminus, anterior to the cephalic setae bases; the presence of irregularly shaped periamphideal plates and an oral opening located close to the anterior terminus, anterior to cephalic setae bases in specimen depicted on 1г suggest it to belong to N. incisa gen. et comb. nov.; while the anterior end shown on fi g. 1д completely matches with N. onusta gen. et comb. nov. (and confi rmed by Voronov in his fi gure caption) in having periamphideal plates merging on the dorsal and ventral body sides and an oral opening located at level with the cephalic setae bases.
3) The holotype of Diplopeltis ovalis described by Ditlevsen (1928) from Greenland is available in the collection of the Natural History Museum of Denmark in Copenhagen and is in rather acceptable shape. Some morphological features, such as cephalic setae and the spinneret cannot be seen due to deterioration, while the vagina and rectum are difficult to observe since the specimen is not in perfect lateral position on the slide. Morphological  4) The lectotype of Diplopeltis asymmetricus described by Allgén (1935) from Öresund was found in the collection and is in poor shape. The specimen is partly dried, thus making it impossible to observe and measure smaller features such as cephalic setae, spinneret and rectum. It is also slightly fl attened, which affects a-and c'-ratios. Morphological features that are visible, show no differences from N. barentsi gen. et comb. nov. -in particular, the position of the oral opening and the morphology of refractive plates underlying the amphids are identical (Figs 14B, 15C). Consequently, Diplopeltis asymmetricus is being transferred to the genus Neodiplopeltula gen. nov. and synonimised with N. barentsi gen. et comb. nov. Sergeeva, 1977 is similar to N. barentsi gen. et comb. nov. in the position of the oral opening on the dorsal side of the body but close to the anterior end, length and arrangement of the cephalic setae (7.5 μm vs 3.5-7.5 μm in present specimens), and the shape of refractive plates (elongated ovoid, not connected).

5) Diplopeltis ovalis sensu
6) The male of Diplopeltula cuspidiboja gen. et comb. nov. is identical to N. barentsi gen. et comb. nov. in the position of the oral opening, size and shape of refractive plates and the great majority of quantitative characters (see Table 3), except for a slightly longer amphid (32 μm vs 21-25.5 μm) and gubernaculum (20 μm vs 10-11.5 μm). 9) Diplopeltula indica and Diplopeltula intermedia both match the diagnostic characters of the genus Neodiplopeltula gen. nov.: amphids in the shape of an elongated loop, a well-developed subcylindrical stoma and outstretched ovaries. Diplopeltula obesa also has amphids in the shape of an elongated loop and outstretched ovaries, however, the structure of the stoma in this species is described as "not expressed", leaving some doubts as to its exact morphology.
10) Diplopeltula tchesunovi is identical to D. onusta in all qualitative and quantitative characters (Table 4), with the exception of longer refractive plates (41-48 μm vs 32-38 μm), which can be attributed to geographic variability. 11) Diplopeltula striolata is described based only on one male, thus the structure of the female reproductive system remains unknown, questioning its systematic affi nities. It is similar to the genus Neodiplopeltula gen. nov. in the morphology of the amphid (in the shape of an elongated loop) and stoma (small but well developed, subcylindrical). The morphology of the pharynx (uniformly cylindrical) also matches that of Neodiplopeltula gen. nov. However, D. striolata strongly differs from Neodiplopeltula gen. nov. in having a strongly annulated cuticle, the position of the excretory pore at the level with the anterior part of the intestine, the presence of a spinneret, the absence of a gubernaculum. Although this species can easily be identifi ed, and its validity is unquestionable, its taxonomic position remains unresolved as it cannot be assigned to any of the existing genera with confi dence (species incertae sedis).
12) All available type specimens of Diplopeltula ostrita were examined, but not all morphological characters can be observed. That includes the female reproductive system and the structure of the ovaries. However, D. ostrita strongly differs from Neodiplopeltula gen. nov. in having a strongly annulated cuticle, a minute buccal cavity, weak spicules and gubernaculum. Similar to Diplopletula striolata, D. ostrita is here considered to be a valid species, which, however, cannot be assigned to any existing genus with confidence and must be considered species incertae sedis.
microscope, but distinct under the scanning electron microscope, covers cuticle over entire body length. Somatic sensilla visible along pharyngeal region (cervical setae, see below) and on tail. Labial region bluntly rounded, lips fused. Refractive plates underlying cephalic cuticle around amphids present (periamphideal), extending from level of anteriormost edge of amphid posteriorly some distance behind posteriormost edge of amphid; refractive plates irregular in shape (with incised edges), not connected with each other on ventral and dorsal sides. Inner labial sensilla small pore-like, located on anterior surface of lips, discernible under SEM only. Outer labial sensilla papilliform, located on anterior surface of lips, clearly visible under SEM only. Cephalic sensilla setiform, bases of dorsosublateral setae located posterior to oral opening. Cervical sensilla papilliform, arranged in four sublateral rows starting at level with middle of amphid and ending at level with posterior edge of refractive plate, two or three per row. Amphids similar in shape and size between sexes: amphidial fovea inverted U-shape with dorsal branch 0.5-3.5 μm longer than ventral branch. Oral opening shifted towards dorsal side of body. Stoma barrel-shaped: cheilostom broad; gymnostom barrel-shaped, with weakly cuticularised walls; stegostom short conoid, its lining uniform with lining of pharynx. Pharyngeal tubes absent. Pharynx subcylindrical, muscular, with evenly distributed myofi laments, gradually expanding towards posterior end; not subdivided in distinct sections; pharyngeal lumen uniform in thickness along entire pharynx length; valves absent. Cardia ovoid, almost entirely embedded into intestinal tissue. Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to posterior 3/5 th of pharynx; secretory-excretory duct very short, leading from pore to ampulla; renette cell small, its body adjacent and ventral to posterior part of pharynx. Tail cylindro-conical with bluntly rounded terminus. Caudal glands opening via three separate subterminal openings, spinneret absent. 2.9 ± 0.7 (2.0-3.5) 2.0

Female
Reproductive system didelphic, amphidelphic, ovary branches outstretched and symmetrical, on opposite sides of intestine. Anterior genital tube 248-280 μm long, situated to either right (n = 1) or left (n = 4) of intestine; posterior genital tube 224-259 μm long, situated to either left (n = 1) or right (n = 4) of intestine. Vulva a transverse slit, located posterior to midbody. Vagina straight, with developed sphincter muscle surrounding proximal part and distinct epiptygmata in distal part; pars refringens vaginae absent. Sack-like spermatheca present, fi lled with oval spermatozoa in fertilized specimens. Rectum short.

Remarks
Despite the fact that the original description by Southern (1914) is rather brief, the present population closely resembles the type specimens in most morphological and morphometric features, including the shape of the refractive plates underlying the amphids (described as "shield shaped" with "front and lateral walls are notched"). Spicules are slightly shorter in the type specimens (66 μm vs 75-79 μm in recent specimens).  (Steiner, 1916) gen. et comb. nov.

Neodiplopeltula barentsi
Figs 5-7, 14-15; Table 3 Diagnosis (based on combined data) Neodiplopeltula barentsi gen. et comb. nov. is characterised by a 0.94-1.67 mm long body; refractive plates underlying cephalic cuticle around amphids present, elongated ovoid in shape, not joined, 23.5-32 μm long and 12-16 μm wide; cephalic setae 3.5-7.5 μm long; amphidial fovea 21-32 μm long and 4-7 μm wide; oral opening 1-2 from anterior end, posterior to cephalic setae bases; secretory-excretory  part of tail (striation can be observed under LM but very fi ne and shallow and cannot be measured with confi dence); longitudinal striation not observed. Somatic sensilla visible along pharyngeal region (cervical sensilla, see below) and on tail. Labial region bluntly rounded, lips fused. Refractive plates underlying cephalic cuticle around amphids present (periamphideal), extending from level of anteriormost edge of amphid posteriorly some distance behind posteriormost edge of amphid; refractive plates elongated ovoid in shape (plate edges not incised), not connected with each other on ventral and on dorsal sides. Inner labial sensilla indistinct. Outer labial sensilla papilliform, located on anterior surface of lips. Cephalic sensilla setiform, bases of dorsosublateral setae located posterior to oral opening. Cervical sensilla papilliform, arranged in four sublateral rows starting at level with middle of amphid and ending at level with posterior edge of refractive plate, one or two per row. Amphids similar in shape and size between sexes: amphidial fovea inverted U-shape with dorsal branch usually 0.5-2.0 μm longer than ventral branch (in two specimens ventral branch 0.5-1.0 μm longer than dorsal branch). Oral opening shifted towards dorsal side of body. Stoma barrel-shaped: cheilostom broad; gymnostom subcylindrical, with weakly cuticularised walls; stegostom short conoid, its lining uniform with lining of pharynx. Pharyngeal tubes absent. Pharynx subcylindrical, muscular, with evenly distributed myofi laments, gradually expanding towards posterior end; not subdivided in distinct sections; pharyngeal lumen uniform in thickness along entire pharynx length; valves absent. Cardia ovoid, entirely embedded into intestinal tissue. Secretoryexcretory system present; secretory-excretory pore along ventral body line opposite to 3/5 th of pharynx; secretory-excretory duct very short, leading from pore to ampulla; renette cell small, its body adjacent and ventral to posterior part of pharynx. Tail conoid with bluntly rounded terminus. Caudal glands opening via three separate subterminal openings, spinneret absent.

Male
Reproductive system diorchic, testes opposed; anterior testis 145-314 μm long, outstretched and posterior testis 117-207 μm long, refl exed. Spicules paired and symmetrical, strongly curved, with weakly defi ned elongated manubrium and shaft, cylindrical along most of its length. Gubernaculum plate-like, with pair of strong, closely set caudal apophyses variable in shape (Fig. 7C-E). Caudal setae present, several ventrosublateral and dorsosublateral pairs arranged in rows along entire tail length (often diffi cult to observe).

Remarks
The original description of this species by Steiner (1916) is rather short, with a few measurements given. The present population has a shorter pharynx and tail compared to the type specimen, but in all other respects both match very well, including the position of the oral opening and the shape of the underlying refractive plates. (Steiner, 1916) gen. et comb. nov. (SMNH-169266 and SMNH-169268). A. Female anterior end, lateral view. B. Male anterior end, lateral view. C. Spicules and gubernaculum. D-E. Variability in the shape of thegubernaculum apophyses in the same specimen (arrows). Scale bars = 10 μm. (Wieser, 1956) gen. et comb. nov. Figs 8-10; Table 4 Diagnosis (based on combined data) Neodiplopeltula onusta gen. et comb. nov. is characterised by a 0.99-1.48 mm long body; refractive plates underlying cephalic cuticle around amphids present and joined, 32.5-48 μm long and 18.5-24 μm wide; cephalic setae 3-6.5 μm long; amphidial fovea 32-44 μm long and 5-8 μm wide; oral opening  12.0 ± 0.8 (11.0-12.5) Cephalic setae length 6.5 3.5-5.

Description
Adult Body cylindrical, posteriorly tapering in tail region, straight or weakly ventrally curved upon fi xation. Cuticle fi nely transversely striated along entire body, except for visually smooth labial region and terminal part of tail (striation can be observed under LM but very fi ne and shallow and cannot be measured with confi dence); longitudinal striation not observed. Somatic sensilla visible along pharyngeal region (cervical setae, see below) and on tail. Labial region bluntly rounded, lips fused. Refractive plates underlying cephalic cuticle around amphids present (periamphideal), extending from level of anteriormost edge of amphid posteriorly some distance behind posteriormost edge of amphid; refractive plates connected with each other on ventral and on dorsal sides. Inner labial sensilla indistinct. Outer labial sensilla papilliform, located on anterior surface of lips. Cephalic sensilla setiform, bases of dorsosublateral setae located at level with oral opening. Cervical sensilla papilliform, arranged in four sublateral rows at level with amphid, one or two per row. Amphids similar in shape and size between sexes: amphidial fovea inverted U-shape with dorsal branch 1.0-2.0 μm longer than ventral branch. Oral opening shifted towards dorsal side of body. Stoma subcylindrical: cheilostom broad; gymnostom barrel-shaped, with weakly cuticularised walls; stegostom short conoid, its lining uniform with lining of pharynx. Pharyngeal tubes absent. Pharynx subcylindrical, muscular, with evenly distributed myofi laments, gradually expanding towards posterior end; not subdivided in distinct sections; pharyngeal lumen uniform in thickness along entire pharynx length; valves absent. Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to 3/5 th of pharynx; secretory-excretory duct very short, leading from pore to ampulla; renette cell small, its body adjacent and ventral to posterior part of pharynx. Tail conoid with bluntly rounded terminus. Caudal glands opening via three separate subterminal openings, spinneret absent.

Male
Reproductive system diorchic, testes opposed; anterior testis outstretched and posterior testis refl exed (poorly discernible in most specimens due to highly granular and dark content of overlapping intestine and cannot be measured). Spicules paired and symmetrical, strongly curved, with elongated ovoid manubrium and subcylindrical shaft. Gubernaculum plate-like, with pair of strong closely set caudal apophyses variable in shape (Fig. 10E-G). Caudal setae present, several ventrosublateral and dorsosublateral pairs arranged in rows along entire tail length.

Remarks
The single female used by Wieser (1956) to describe this species has a slightly broader labial region and longer cephalic setae than the population from Sweden, but the specimens described under the name Diplopeltula tchesunovi fi ll this gap. The position of the oral opening is at level with the cephalic setae bases and the shape of the refractive plates underlying the cephalic cuticle around the amphids (joined along the dorsal and ventral body sides) in both type female and recent specimens are identical and confi rm the conspecifi city. (Jensen, 1991) gen. et comb. nov. Figs 11-13; Table 5 Diagnosis (based on combined data)

Description
Adult Body cylindrical, posteriorly tapering in tail region, straight or weakly ventrally curved upon fi xation. Cuticle fi nely transversely striated along entire body, except for visually smooth labial region and terminal part of tail (striation can be observed under LM but very fi ne and shallow and cannot be measured with confi dence); longitudinal striation not observed. Somatic sensilla visible along pharyngeal region (cervical setae, see below) and on tail. Labial region bluntly rounded, lips fused. Refractive plates underlying cephalic cuticle around amphids present (periamphideal), extending from level of anteriormost edge of amphid posteriorly some distance behind the posteriormost edge of amphid; refractive plates are elongated ovoid in shape (plate edges not incised), not connected with each other on ventral and on dorsal sides. Inner labial sensilla indistinct. Outer labial sensilla papilliform, located on anterior surface of lips. Cephalic sensilla setiform, bases of dorsosublateral setae located at level with or anterior to oral opening. Cervical sensilla absent; small papilliform somatic sensilla visible posterior to amphid and refractive plate. Amphids similar in shape and size between sexes: amphidial fovea inverted U-shape branches unequal in length with either dorsal or ventral branch 1.0-2.0 μm longer than opposite branch. Oral opening shifted towards dorsal side of body. Stoma subcylindrical: cheilostom broad; gymnostom barrel-shaped, with weakly cuticularised walls; stegostom short conoid, its lining uniform with lining of pharynx. Pharyngeal tubes absent. Pharynx subcylindrical, muscular, with evenly distributed myofi laments, gradually expanding towards posterior end; not subdivided in distinct sections; pharyngeal lumen uniform in thickness along entire pharynx length; valves absent. Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to 4/5 th of pharynx; secretory-excretory duct very short, leading from pore to ampulla; renette cell small, its body adjacent and ventral to posterior part of pharynx. Tail conoid with bluntly rounded terminus. Caudal glands opening via three separate subterminal openings, spinneret absent.

Male
Reproductive system diorchic, testes opposed; anterior testis outstretched and posterior testis refl exed (cannot be measured). Spicules paired and symmetrical, strongly curved, with ovoid manubrium and fusiform shaft. Gubernaculum plate-like, with pair of strong closely set caudal apophyses variable in shape ( Fig. 13C-E). Caudal setae present, several ventrosublateral pairs arranged in rows along entire tail length, one dorsosublateral pair located close to tail terminus (not always visible).

Remarks
No morphological or morphometric differences can be found between the recent specimens and the original description of this species (Jensen 1991).
Data for N. barentsi gen. et comb. nov. is based on the original description, new measurements of the types of D. ovalis and D. asymmetricus, D. onustus sensu Sergeeva (1977), the male description of D. cuspidiboja (Leduc 2017) and recent data; for N. bathmanni gen. et comb. nov. on the original description (Vitiello 1972), recent data and female description of D. cuspidiboja (Leduc 2017)

Discussion
The genus Diplopeltula exhibits two pertinent problems in the systematics of marine nematodes: ignorance of the female morphology in descriptions of new taxa and the absence of type material. The 'Traditional' focus lies on male characters in species descriptions and diagnoses of genera, and the partial or complete ignorance towards the morphology of female reproductive system causes taxonomic and nomenclatorial problems in those cases where higher classifi cation of taxa relies on female-specifi c characters. The genus Diplopeltula is but one example (see detailed discussions in Holovachov et al. 2009;Leduc 2013;Holovachov 2017;Holovachov & Boström 2017). Of course, it would not be such an issue if the type material for such 'troublesome' taxa were preserved and available for restudy. Unfortunately, S. Gerlach, being one of the most prolifi c nematode taxonomists, did not preserve any type specimens and did not make any permanent slides until 1964, thus preventing any future taxonomic revisions and corrections. Regrettably, he was not alone in this practice. As it turned out, W. Wieser did not leave any trace of the enormous and taxonomically important collection of marine nematodes from Chile, consequences of which are yet to be dealt with.