Belgopeltula belgica (Vincx & Gourbault, 1992) gen. et comb. nov. and Mudwigglus micramphidium sp. nov. from the west coast of Sweden, and reappraisal of the genus Pseudaraeolaimus Chitwood, 1951 (Nematoda: Araeolaimida: Diplopeltidae)

Two species of the nematode family Diplopeltidae are described from Skagerrak. The new genus Belgopeltula gen. nov. is proposed for Diplopeltula belgica Vincx & Gourbault, 1992 and is characterised by: amphidial fovea circular in female and double-loop-shaped in male; excretory pore located at the level of cephalic setae bases; oral opening on the dorsal side of the body; pharynx subdivided into strongly muscularised fusiform corpus and weakly muscularised narrow and long postcorpus; female didelphic with antidromously refl exed ovaries; supplements absent. Mudwigglus micramphidium sp. nov. is characterised by: a body of 0.6 mm long; cephalic sensilla 1.5 μm long; amphidial fovea loop-shaped, 8 μm long and 3.5 μm wide; gymnostom without cuticularised ring; tail elongate conoid, with subcylindrical distal part; terminal setae absent; spicules 15 μm long; gubernaculum present; two midventral precloacal setae. It is distinguished from M. macramphidium Leduc, 2013 in having shorter amphidial fovea, shorter spicules and presence of two precloacal setae. Redescription of Diplopeltis cylindricauda Allgén, 1932 is provided based on type material. Diplopeltula minuta Vitiello, 1972 is transferred to the genus Mudwigglus Leduc, 2013. Diplopeltis cylindricauda Allgén, 1932, Diplopeltula laminata Vitiello, 1972 and Diplopeltula cassidaignensis Vitiello, 1972 are transferred to the genus Pseudaraeolaimus Chitwood, 1951.


Diagnosis
Cuticle transversely striated; striae without ornamentation. Lateral alae absent. Body pores and epidermal glands absent. Somatic sensilla indistinct. Labial region bluntly rounded; lips fused. Inner labial sensilla invisible if present. Outer labial sensilla papilliform, located on the anterior surface of lips. Cephalic sensilla setiform; their bases located at the base of the labial region, anterior to amphid. Subcephalic and cervical sensilla, deirid and ocelli absent. Amphidial fovea unispural (circular) in female and double-loop-shaped in male (starting from the porus amphidialis, fovea extends anteriorly for a short distance, makes a loop towards ventral side of the body and extends posteriorly for the full length of the amphidial fovea, makes another loop towards dorsal side of the body and extends anteriorly almost to the level of the porus amphidialis). Secretory-excretory system present; renette cell located opposite to the anterior part of intestine. Cuticularised secretory-excretory duct very short, opens to the exterior at the level of cephalic setae bases. Oral opening shifted towards the dorsal side of the body. Buccal cavity small and narrow, cheilostom short, remaining part of stoma undifferentiated; its lining is uniform with the lining of the pharynx. Pharyngeal tubes absent. Pharynx subdivided into anterior corpus and posterior postcorpus; corpus fusiform, muscular, with evenly distributed myofi laments; postcorpus narrow and long, weakly muscularised; pharyngeal lumen uniform in thickness along the entire pharynx length; valves absent. Pharyngeal gland orifi ces penetrate pharyngeal lumen as the base of the stoma. Postcorpus is surrounded by six large pseudocoelomocyles. Female reproductive system didelphic-amphidelphic, with equally developed branches, ovaries refl exed antidromously. Spermatheca present. Vulva equatorial. Vagina straight; pars proximalis vaginae encircled by single sphincter muscle; pars refringens vaginae and epiptygmata absent. Male reproductive system diorchic, testes opposed (anterior testis outstretched, posterior testis refl exed). Spicules symmetrical, arcuate; gubernaculum present. Supplements, precloacal and postcloacal sensilla absent. Three caudal glands present, their cells and nuclei are incaudal. Spinneret present.''

Relationships
The new genus is proposed to accommodate the single species Diplopeltula belgica and is particularly characterised by following unique characters: amphidial fovea circular in female and double-loop-shaped in male; secretory-excretory pore located at the level of cephalic setae bases; oral opening subterminal, located on the dorsal side of the body; pharynx subdivided into strongly muscularised fusiform corpus and weakly muscularised narrow and long postcorpus; female didelphic with antidromously refl exed ovaries; male diorchic with opposed testes; pre-and postcloacal supplements absent.
The morphology of the pharynx in Belgopeltula gen. nov. needs to be clarifi ed. In the original publication, the pharynx is described as: "muscular with an elongated terminal bulb". The drawing shows rather cylindrical anterior part of the pharynx that rapidly widens short distance posterior to the amphideal fovea; the nerve ring is drawn immediately in front of this pharyngeal expansion, at 54% of the pharynx length. The only other known record of the same species published by Fadeeva & Mordukhovich (2013) describes the pharynx as follows: "pharynx terminates in a small bulb-like expansion". Nerve ring is not depicted in the fi gure but is said to encircle pharynx at 77% of its length.
Videos of the pharyngeal region of the holotype of Diplopeltula belgica were examined. Morphology of the pharynx and surrounding tissues in the holotype confi rms well with observations of pharyngeal region in recent specimens: fusiform corpus and narrow postcorpus; large pseudocoelomocyles surrounding anterior part of postcorpus and glandular cells surrounding posterior part of it. Photographs European Journal of Taxonomy 383: 1-21 (2017) 4 of the specimen from the Sea of Japan also show most of the features observed in specimens from the coast of Sweden, despite it being not in the best quality: distinct fusiform corpus and narrower postcorpus; large pseudocoelomocyles surrounding postcorpus along most of its length; basal part of the pharynx surrounded by small glandular cells.
The unique shape of the male amphid (starting from the porus amphidialis, fovea extends anteriorly for a short distance, makes a loop towards ventral side of the body and extends posteriorly for the full length of the amphid, makes another loop towards dorsal side of the body and extends anteriorly almost to the level of the porus amphidialis) separates Belgopeltula gen. nov. from all other genera classifi ed in the family Diplopeltidae (Leduc 2013;Fonseca & Bezerra 2014), which have unispiral or single loopshaped amphid.
Belgopeltula gen. nov. and Mudwigglus Leduc, 2013 are the only two genera in the family Diplopeltidae with antidromously refl exed ovaries, while the rest of the genera are characterised by outstretched ovaries. In this respect, both genera show similarities to the family Diplopeltoididae Tchesunov, 1990(Tchesunov 1990) (currently placed in the order Plectida Gadea, 1973(Gadea 1973). However, as discussed in Leduc (2013), the morphology of female ovaries (outstretched vs refl exed) may not always be consistent in all species within large taxonomic categories, and in some cases may not have diagnostic value. To make things more complicated, the morphology of female ovaries has not been described in large number of species placed in the genus Diplopeltula, including its type species D. breviceps.
In having oral opening located on the dorsal side of the body, the new genus resembles the genus Campylaimus Cobb, 1920(Cobb 1920 and some species of the genus Diplopeltula. It can be easily separated from both genera in the morphology of the amphidial fovea (circular in female and characteristic double-loop-shaped in male vs single loop in both females and males in Diplopeltula and Campylaimus), the female reproductive system (ovaries refl exed in Belgopeltula gen. nov. vs ovaries outstretched in Diplopeltula and Campylaimus), the pharynx (subdivided into fusiform corpus and narrow postcorpus in Belgopeltula gen. nov. vs cylindrical in Diplopeltula and Campylaimus) and the position of excretory pore (located at the level of cephalic setae bases in Belgopeltula gen. nov. vs at the level of posterior part of pharynx or anterior part of intestine in Diplopeltula and Campylaimus). (Vincx & Gourbault, 1992) (Vincx & Gourbault, 1992) gen. et comb. nov., female entire view. Scale bar = 20 μm.

Description
Adult Body fusiform in female and cylindrical in male, tapering anteriorly in the pharyngeal region and posteriorly in the tail region, ventrally curved upon fi xation. Cuticle fi nely striated along entire body, striae without longitudinal incisures or any other ornamentation visible under the light microscope. Somatic setae indistinct if present. Labial region rounded, lips fused. Inner and outer labial sensilla not observed. Cephalic sensilla small setiform, equal to 0.1-0.2 labial region diameters in length, their bases are located 3-7 μm from anterior end. Amphidial fovea dissimilar in shape and size between sexes (see descriptions of each sex). Oral opening shifted towards the dorsal side of the body, 1.5-3.5 μm from the anterior end. Stoma very small, cheilostom short, remaining part of stoma undifferentiated, its lining is uniform with the lining of the pharynx. Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus slightly fusiform, muscular; postcorpus narrow and long. Pharyngeal gland nuclei indistinct. Pharyngeal gland orifi ces penetrate pharyngeal lumen at the base of the stoma. Postcorpus is surrounded by six large pseudocoelomocytes, two cells arranged longitudinally in three body sectors: dorsal, right-subventral and left-subventral. Nerve ring is located at the posterior end of the pharynx, its exact position if obscured by small glandular cells (neuron bodies?). Secretory-excretory system present; renette cell located opposite to the ventral side of the anterior part of intestine; secretory-excretory pore opens to the exterior just posterior to cephalic setae bases. Tail elongate-conoid. Caudal glands and spinneret present.

Female
Amphidial fovea unispiral (circular). Reproductive system didelphic, amphidelphic; ovary branches symmetrical, antidromously refl exed. Anterior ovary situated to either right or left of intestine; posterior ovary situated to either right or left of intestine. Spermathecae present, fi lled with round spermathozoa. Vulva located at mid-body. Vagina straight, pars proximalis vaginae surrounded by a sphincter muscle; pars refringens vaginae absent. Intrauterine egg not seen. Rectum short.

Male
Amphidial fovea double-loop-shaped: starting from the porus amphidialis, fovea extends anteriorly for a short distance, makes a loop towards ventral side of the body and extends posteriorly for the full length of the amphid, makes another loop towards dorsal side of the body and extends anteriorly almost to the level of the porus amphidialis. Amphid length (longest) to width ratio is 2.7-5.2. Narrow space between amphidial branches not refractive and not ornamented. Reproductive system diorchic, anterior testis outstretched; posterior testis refl exed. Spicules paired and symmetrical, weakly curved, with weakly defi ned manubrium and subcylindrical shaft. Gubernaculum plate-like, with strong dorsocaudal apophysis. Supplements and caudal setae indistinct or absent.

Remarks
Both published descriptions of Diplopeltula belgica are each based on single male specimens (Vincx & Gourbault 1992;Fadeeva & Mordukhovich 2013); females of this species were unknown until now. Recent male specimens resemble type male from the North Sea (Vincx & Gourbault 1992) in structure of amphid, digestive (see above) and excretory systems, with the exception of the following: shorter body (358-418 μm in recent specimens vs 550 μm in type male), shorter amphid (13.5-18.5 μm in recent specimens vs 26 μm in type male), shorter cephalic setae (0.5-1.5 μm in recent specimens vs 4 μm in type male), shorter (16.5-18 μm in recent specimens vs 21 μm in type male) and less curved (weakly arcuate in recent specimens vs strongly arcuate in type male) spicules.

Fig. 2. Belgopeltula belgica
HOLOVACHOV O., Genera Belgopeltula, Mudwigglus and Pseudaraeolaimus 9 Specimen from the Sea of Japan (Fadeeva & Mordukhovich 2013) is closer in size to specimens from Skagerrak (Table 1) -the only substantial differences are the length of the amphidial fovea (13.5-18.5 μm in recent specimens vs 37 μm in the male from the Sea of Japan) and less curved (weakly arcuate in recent specimens vs strongly arcuate in the male from the Sea of Japan) spicules.
Greatest discrepancies between all three populations are in the assumed position of the nerve ring and the morphology of male gonad. In the original description, the nerve ring is drawn short distance behind the amphid, at 54% of the pharynx length (Vincx & Gourbault 1992). Male specimen from the Sea of Japan is described to have the nerve ring at 77% of the pharynx length (Fadeeva & Mordukhovich 2013), but it is not drawn on the fi gure. In recent specimens, the position of the nerve ring can not be identifi ed with confi dence, and further observations are required to clarify this. Regarding the morphology of the male gonad, type specimen is said to have both testes outstretched (Vincx & Gourbault 1992), while our observations clearly show that the posterior testis in this species is refl exed posteriad.
Genus Mudwigglus Leduc, 2013 Type species Leduc, 2013 Diagnosis (emended after Leduc 2013) Cuticle transversely striated; striae without ornamentation. Lateral alae absent. Body pores and epidermal glands present. Somatic sensilla present. Labial region bluntly rounded; lips fused. Inner labial sensilla invisible if present. Outer labial sensilla papilliform, located on the anterior surface of lips. Cephalic sensilla setiform; their bases located at the base of the labial region, anterior to amphid. Subcephalic and cervical sensilla, deirid and ocelli absent. Amphidial aperture loop-shaped (inverted U-shaped). Secretory-excretory system present; renette cell located opposite to the posterior part of the pharynx and anterior part of intestine. Cuticularised secretory-excretory duct very short, opens to the exterior at the level of the posterior part of pharynx. Oral opening apical. Buccal cavity small but distinctly subdivided into three sections: cheilostom narrow tubular; gymnostom barrel-shaped or truncate conoid, with weakly cuticularised walls; stegostom short conoid, its lining is uniform with the lining of the pharynx. Pharyngeal tubes absent. Pharynx subdivided into anterior corpus and posterior bulbus; corpus cylindrical, muscular, with evenly distributed myofi laments; bulbus muscular; pharyngeal lumen uniform in thickness along the entire pharynx length; valves absent. Dorsal gland orifi ce penetrates pharyngeal lumen at the base of the stoma, ventrosublateral gland orifi ces indistinct. Dorsal gland nucleus is visible in the basal bulbus. Female reproductive system didelphicamphidelphic with equally developed branches, ovaries antidromously refl exed. Spermatheca absent. Vulva equatorial. Vagina straight or oblique; pars proximalis vaginae encircled by single sphincter muscle; pars refringens vaginae and epiptygmata absent. Male reproductive system diorchic; anterior testis outstretched, posterior testis outstretched, with posteriorly refl exed terminal section of the germinal zone. Spicules symmetrical, arcuate; gubernaculum present. Supplements and precloacal sensilla present. Setae in caudal region of males arranged in subventral and subdorsal pairs along the tail. Three caudal glands present, their nuclei are incaudal. Caudal glands open separately via three pores (this structure is obscure in some species).

Etymology
The specifi c epithet refers to the overall similarity between the new species and M. macramphidium, as well as one of the major differences between these two species (amphid size).

Adult male
Body cylindrical, tapering posteriorly in the tail region, strongly ventrally curved upon fi xation. Cuticle fi nely striated along the entire body, annules without longitudinal striation or any other ornamentation visible under the light microscope. Somatic setae present, sparsely distributed over entire body, with one ventrosublateral pair in the pharyngeal region just posterior to nerve ring, and three pairs along the tail: one dorsosublateral pair short distance posterior to cloacal opening, one ventrosublateral pair at about the middle of tail length, and one sublateral pair close to tail terminus. Labial region truncate conoid; lips fused. Inner and outer labial sensilla not observed. Cephalic sensilla small setiform, equal to 0.2 labial region diameters in length, their bases are located 5 μm from anterior end. Amphidial fovea an inverted U-shape with its dorsal branch slightly longer (8 μm) than ventral branch (6.5 μm). Amphidial fovea length (longest) to width ratio is 2.2. Narrow space between amphidial branches not refractive and not ornamented. Distinct cell with large nucleus is located behind the amphid, and is connected to amphidial fovea. Stoma very small, but distinctly subdivided into three sections: cheilostom narrow tubular, 3 μm long; gymnostom truncate conoid, narrowing towards anterior end, with weakly cuticularised walls, 3 μm long and 2 μm wide, without cuticularised ring; stegostom very short conoid, its lining is uniform with the lining of the pharynx. Pharynx distinctly subdivided into anterior corpus and posterior basal bulbus; corpus subcylindrical, muscular; basal bulbus ovoid, muscular. Dorsal pharyngeal gland nucleus located in the basal bulbus; dorsal pharyngeal glad orifi ce penetrates pharyngeal lumen at the base of the stoma. Ventrosublateral pharyngeal gland nuclei and orifi ces indistinct. Nerve ring encircling pharynx at posterior third of its length. Secretory-excretory system present; renette cell located opposite to the ventral side of the basal bulbus and anterior part of the intestine; secretory-excretory pore opens to the exterior at level of the basal bulbus. Reproductive system diorchic; anterior testis outstretched, posterior testis outstretched, with posteriorly refl exed terminal section of the germinal zone. Large spermatogonia are visible. Spicules paired and symmetrical, arcuate, with weakly developed manubrium Fig. 3. Mudwigglus micramphidium sp. nov., holotype male (SMNH Type-8889) entire view. Scale bar = 20 μm. and subcylindrical shaft. Gubernaculum present, platelike. Male accessory apparatus composed of two midventral precloacal setiform sensilla. Tail elongate conoid, subcylindrical distally. Terminal setae absent. Caudal glands and their outlets obscure.

Female
Not found.

Diagnosis (emended after Fonseca & Bezerra 2014)
Cuticle visually smooth or fi nely striated; striae without ornamentation. Lateral alae absent. Somatic sensilla present. Labial region rounded; lips fused. Inner and outer labial sensilla indistinct. Cephalic sensilla setiform; their bases located at the base of the labial region, anterior to amphidial fovea. Cervical (paramphidial) sensilla present, located at the level of the amphid and posterior to it. Deirid absent.
Ocelli present or absent. Amphidial fovea loop-shaped (inverted U-shaped). Secretory-excretory system present; renette cell located opposite to the anterior part of intestine. Cuticularised secretory-excretory duct very short, opens to the exterior just posterior to cephalic setae bases. Oral opening apical. Buccal cavity small and undifferentiated, its lining is uniform with the lining of the pharynx. Pharyngeal tubes absent. Pharynx subdivided into anterior corpus and posterior postcorpus; corpus cylindrical, muscular, with evenly distributed myofi laments; postcorpus glandular; pharyngeal lumen uniform in thickness along the entire pharynx length; valves absent. Female reproductive system didelphic-amphidelphic with equally developed branches, ovaries outstretched. Vulva equatorial. Male reproductive system diorchic, anterior testis outstretched, posterior testis outstretched, with posteriorly refl exed terminal section of the germinal zone. Spicules symmetrical, arcuate; gubernaculum present. Precloacal sensillum present. Setae in caudal region of males arranged in subventral and subdorsal pairs along the tail. Three caudal glands present, their nuclei are incaudal. Spinneret present.

Remarks
According to Tchesunov & Miljutina (2008), main diagnostic characters of the genus Pseudaraeolaimus include: terminal (apical) oral opening, amphidial fovea loop-shaped, stoma small undifferentiated, pharynx glandular in its posterior part, excretory pore close to anterior end, ovaries paired, gubernaculum without apophyses. In addition, both Pseudaraeolaimus perplexus and P. ocellatus have several pairs of strongly developed cervical setae. This last feature is also known in other genera of Diplopeltidae, such as Araeolaimus de Man, 1888, Diplopeltis Cobb in Stiles & Hassall, 1905, Metaraeolaimoides De Coninck, 1936 and Southerniella Allgén, 1932, but not in type species of the genus Diplopeltula, D. breviceps.
2) Diplopeltis cylindricauda Allgén, 1932 is transferred to the genus Pseudaraeolaimus based on presence of terminal oral opening, undifferentiated stoma, loop-shaped amphidial fovea, strongly developed cervical setae, excretory pore close to anterior end (Fig. 4). Vitiello, 1972 is transferred to the genus Pseudaraeolaimus based on presence of terminal oral opening, undifferentiated stoma, loop-shaped amphidial fovea, strongly developed cervical setae.

Description
Adult male Body cylindrical, tapering posteriorly in the tail region, strongly ventrally curved upon fi xation. Cuticle fi nely striated along the entire body, striae without longitudinal incisures or any other ornamentation visible under the light microscope. Somatic setae (except cervical setae, see below) are indistinct. Labial region truncate conoid; lips fused. Inner and outer labial sensilla not observed. Cephalic sensilla setiform, equal to 1.3 labial region diameters in length, their bases are located 1.5 μm from anterior end. Amphidial fovea an inverted U-shape with its dorsal branch slightly longer (11 μm) than ventral branch (10.5 μm). Amphidial fovea length (longest) to width ratio is 3.1. Narrow space between amphidial branches not refractive and not ornamented. Stoma very small, undifferentiated, its lining is uniform with the lining of the pharynx. Pharynx poorly preserved, cylindrical along most of its length. Pharyngeal glands and nuclei indistinct. Nerve ring encircling pharynx at posterior third of its length. Secretory-excretory system present; renette cell located opposite to the ventral side of the and anterior part of the intestine; secretoryexcretory pore opens to the exterior at level with the amphid. Reproductive system diorchic; anterior testis outstretched, posterior testis outstretched, with posteriorly refl exed terminal section of the germinal zone.

Female
Not available.

Key to species of Pseudaraeolaimus Chitwood, 1951
See Table 3 for additional diagnostic characters. Cuticle visually smooth, fi nely striated or coarsely annulated. Lateral alae absent. Somatic sensilla present. Inner and outer labial sensilla papilliform. Cephalic sensilla setiform; their bases located at the base of the labial region, anterior to amphidial fovea. Cervical (paramphidial) sensilla present in some genera, located at the level of the amphid and posterior to it. Deirid absent. Ocelli present or absent. Amphidial fovea unispiral or loop-shaped. Secretory-excretory system present. Oral opening apical or shifted either to dorsal or ventral body side. Buccal cavity tubular, cylindrical or undifferentiated. Teeth-like structures absent. Pharynx variable in shape; pharyngeal lumen uniform in thickness along the entire pharynx length; valves absent. Female reproductive system didelphic-amphidelphic or monodelphic-prodelphic. Ovaries outstretched or refl exed antidromously. Male reproductive system diorchic. Posterior testis is either anteriorly oriented and refl exed in the anterior section (refl exed terminal section of the germinal zone) or is posteriorly directed. Spicules symmetrical; gubernaculum present. Precloacal sensilla present in some genera. Three caudal glands present, their nuclei are incaudal. Caudal glands open separately via three pores, or together via a common spinneret.

Discussion
The family Diplopeltidae currently includes 14 genera (including newly described Belgopeltula gen. nov.) and about 120 species (Leduc 2013;Fonseca & Bezerra 2014). Most of the genera are unequivocally defi ned and easily identifi able, while the genus Diplopeltula is a morphological and taxonomic "hodgepodge" used to hold species that did not fi t the diagnoses of other taxa of this family. As discussed in Holovachov et al. (2009), several factors are at play here. First of all, the description of the type species of the genus Diplopeltula, D. breviceps is brief and does not include many features that are now considered to be diagnostic (structure of the pharynx, excretory system, female reproductive system). Second, many species assigned to Diplopeltula are either described based on specimens of one sex, of which those based on males are prevalent, or the descriptions, even if based on specimens from both sexes, do not include data on the morphology of female reproductive system and other taxonomically important characters. The third factor mentioned in Vincx & Gourbault (1992) is "rather wide generic diagnosis", or more like an absence of proper generic diagnosis in the original description (Gerlach 1950) or subsequent revision of the group (Gerlach 1962;Vitiello 1972;Vincx & Gourbault 1992). Last, but not least, type material of D. breviceps was not kept and therefore cannot be re-examined because S. Gerlach did not make permanent slides until 1964 (F. Riemann, in littera 10 Apr. 2001). Consequently, until D. breviceps can be recollected from the type locality and properly redescribed, the taxonomy of the genus Diplopeltula will remain unclear.