Three new and five known species of Diplopeltoides Gerlach , 1962 ( Nematoda , Diplopeltoididae ) from Sweden , and a revision of the genus

Eight species of Diplopeltoides are described from the Swedish west coast. Diplopeltoides suecicus sp. nov. has the cuticle with longitudinal striation visible only under SEM; cuticular plate underlying the cephalic cuticle around the amphid present; cephalic sensilla 4–6 μm long; amphid an inverted U-shape; wide space between amphidial branches areolated; spicules 27–31 μm long; gubernaculum with caudal apophysis. Diplopeltoides longicaudatus sp. nov. is characterized by a cuticle without longitudinal striation; cuticular plate underlying cephalic cuticle around amphid present; cephalic sensilla 13 μm long; amphid an inverted U-shape; narrow space between amphidial branches not ornamented; spicules unequal in size, 27–31 μm long; gubernaculum absent; midventral precloacal cuticular ridge present. D. grandis sp. nov. is characterized by a cuticle with longitudinal striation; cuticular plate underlying cephalic cuticle around amphid present; cephalic sensilla 18.5 μm long; amphid an inverted U-shape; wide space between amphidial branches punctate. The following taxonomic changes are proposed: Diplopeltoides asetosus (Juario, 1974) comb. nov., Diplopeltoides botulus (Wieser, 1959) comb. nov., Diplopeltoides bulbosus (Vitiello, 1972) comb. nov., Diplopeltoides lucanicus (Boucher & Helléouët, 1977) comb. nov., Diplopeltoides pumilus (Vincx & Gourbault, 1992) comb. nov. and Diplopeltoides striatus (Gerlach, 1956) comb. nov. Diplopeltoides holovachovi Fadeeva & Mordukhovich, 2013 is synonymised with Diplopeltoides pumilus comb. nov. An updated key to the species of Diplopeltoides is provided.

Morphological data obtained from observations of these eight species provides grounds for the ongoing update of the taxonomy of the genus Diplopeltoides and a revision of the poorly defined genus Diplopeltula Gerlach, 1950, which implies the transfer of six known species from the genus Diplopeltula to the genus Diplopeltoides: D. botulus (Wieser, 1959) comb. nov., D. lucanicus (Boucher & Helléouët, 1977) comb.nov., D. striatus (Gerlach, 1956) comb.nov.and three species mentioned above (D. asetosus, D. bulbosus and D. pumilus).Exclusion of these six species from Diplopeltula does not require any modifications to the diagnosis of this genus, recently published by Leduc (2017).

Material and methods
Bottom sediment samples were collected at 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 fixation.Nematodes were extracted from samples using a decanting and sieving method (smallest mesh sizes: 45 µm or 70 µm).Fresh water was used during sieving to induce an osmotic shock in nematodes inducing their detachment from the substrate.Samples were immediately fixed in a 4% formaldehyde solution in fresh water.
For light microscopy, formaldehyde-preserved specimens were transferred to pure glycerine using the rapid method of Seinhorst (1959) as modified by De Grisse (1969).Permanent nematode mounts on glass slides were prepared using the paraffin wax ring method.After measuring and observations, some specimens were removed from their slides and rehydrated by first 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-fixed 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.Type and other specimens are deposited in the invertebrate collections of the Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden (SMNH).Abbreviations used in Tables are according to Hunt & Palomares-Ruis (2012).

Diagnosis (emended after Holovachov 2014)
Cuticle annulated, annulation starts at level with the amphids; annules smooth or with fine longitudinal striation.Annules unequal in width; annule width increases gradually from first postcephalic annule to annule located somewhat posterior to the pharyngo-intestinal junction, which is followed by a much narrower one; annules increase in width posterior towards mid-body region.Posterior to two-thirds of body length, annule width gradually decreases again, with the most narrow annule located on tail; the most narrow caudal annule is followed posteriorly by one much wider annule and then by caudal annules, in which the width gradually decreases toward the tail terminus.Lateral alae absent.Body pores and epidermal glands absent.Somatic sensilla present.Labial region bluntly rounded; lip pairs fused into three small lobes.Inner labial sensilla invisible if present.Outer labial sensilla pore-like, located on the anterior surface of lips.Cephalic sensilla setiform or papilliform; their bases located at the base of the labial region, anterior to amphid.Subcephalic and cervical sensilla, deirid and ocelli absent.Amphids in some species lie on strongly cuticularised lateral subcuticular plates that are partly or completely connected together on the ventral and dorsal sides forming a "cephalic framework".Amphidial fovea loop-shaped (inverted U-shaped).Secretory-excretory system usually present; renette cell elongate, located opposite to the cardia and anterior part of intestine.Secretory-excretory ampulla present, located at the level of the posterior part of pharynx.Cuticularized secretory-excretory duct very short, opens to the exterior posterior to the nerve ring level.Buccal cavity small funnel-shaped: cheilostom can usually be seen as the cylindrical or funnel-shaped anterior-most part, not cuticularised; gymnostom short and not cuticularised; stegostom closed, linear, its slender lining continuous with that of the corpus.Pharyngeal tubes absent.Pharynx subdivided by breaks in muscular tissue into anterior corpus and posterior postcorpus; corpus cylindrical or fusiform, muscular, with evenly distributed myofilaments; postcorpus glandular, consisting of anterior narrower isthmus and basal swelling; dorsal sector of the basal bulb is often enlarged to accommodate dorsal pharyngeal gland; pharyngeal lumen uniform in thickness along the entire pharynx length; valves absent.Dorsal and two subventral gland orifices penetrate pharyngeal lumen at the base of stoma.Dorsal gland nucleus is visible in the basal swelling.Cardia conoid, glandular.Female reproductive system didelphic-amphidelphic with equally developed branches, ovaries reflexed antidromously.Spermatheca absent.Vulva equatorial, transverse or pore-like.Vagina straight; pars proximalis vaginae encircled by sphincter muscle; pars refringens vaginae and epiptygmata absent.Male reproductive system diorchic, testes outstretched or opposed in different species.Spicules symmetrical, arcuate; gubernaculum usually present.Supplements, precloacal and postcloacal sensilla absent.Setae in caudal region of males arranged in subventral and subdorsal pairs along the tail.Three caudal glands present, open via three separate openings, their nuclei are incaudal.Spinneret absent.
2) Diplopeltula botula Wieser, 1959 is here transferred to the genus Diplopeltoides based on the fact that it shows no qualitative differences from several species currently assigned to the genus Diplopeltoides.
3) Diplopeltula bulbosa Vitiello, 1972 is here transferred to the genus Diplopeltoides based on the morphology of the female reproductive system with reflexed ovaries in this species vs outstretched ovaries as defined for the genus Diplopeltula by Fonseca & Bezerra (2014) and Leduc (2017).4) Diplopeltula lucanica Boucher & Helléouët, 1977 is here transferred to the genus Diplopeltoides based on the characteristic morphology of the amphid (with a crenate interamphideal shield -a feature common to other species of Diplopeltoides, but not in Diplopeltula and its type species Diplopeltula breviceps Gerlach, 1950).Vincx & Gourbault, 1992 is here transferred to the genus Diplopeltoides based on its close morphological affinities to Diplopeltoides holovachovi Fadeeva & Mordukhovich, 2013 in general morphology, structure of cuticle, anterior end and spicules.Since the only known difference between the two species is in the length of the spicules, Diplopeltoides holovachovi is here considered a junior synonym of Diplopeltoides pumilus.Gerlach, 1956

Etymology
The specific epithet "suecicus" (masculine) -Swedish, refers to the country where the new species was first discovered.

Description Adult
Body cylindrical, posteriorly tapering in the tail region, straight or weakly ventrally curved upon fixation.Cuticle coarsely annulated along entire body, except for visually smooth anterior end (faint annulation is visible under the scanning electron microscope) and terminal part of the tail; annules 1.5-2.5 µm wide at mid-body region; longitudinal striation not observed under the light microscope but distinct under the scanning electron microscope, covers annules over entire body length.Somatic setae visible on tail.Labial region bluntly rounded, lips fused.Cuticularised plate underlying cephalic cuticle around amphid, extending from the level of the anteriormost edge of amphid to the posteriormost edge of amphid, 17-21.5 µm long and 12-14.5 µm wide at its base; cuticular plates are connected with each other on ventral and on dorsal sides.Inner labial sensilla not seen; outer labial sensilla small papilliform, located on the anterior surface of lips.Cephalic sensilla setiform, equal to 0.5-0.8labial region diameters in length, their bases are located 1.5-2 µm from anterior end.Amphids similar in shape and size between sexes: amphidial fovea an inverted U-shape with its dorsal branch longer than ventral branch.Wide space  between amphidial branches (amphidial shield) strongly cuticularised and areolated.Stoma very small, its cuticularised lining is uniform with the lining of the pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretory-excretory system present; secretory-excretory pore located along the ventral body line opposite to basal pharyngeal swelling; secretory-excretory duct very short, leading from pore to ampulla; renette cell small, its body adjacent and ventral to anterior part of intestine.Tail subcylindrical with bluntly rounded terminus.Caudal glands opening via three separate openings, spinneret absent.Cephalic setae length 6.0 5.0 (4.0-6.0)5.7 (5.5-6.0)Amphid from anterior end 7.5 5.8 (5.5-6.0)6.7 (5.5-7.5)European Journal of Taxonomy 369: 1-35 (2017)

Female
Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to either right or left of intestine; posterior ovary situated to either right or left of intestine.Vulva pore-like, located at or immediately posterior to mid-body.Vagina straight, 0.2-0.3 of the vulval body diameter, with developed sphincter muscle at its proximal part; pars refringens vaginae absent.Intrauterine egg not seen; sperm is often present in the uterus.Rectum short, 0.7-0.9 of the corresponding body diameter long.

Male
Reproductive system diorchic, both testes outstretched.Spicules paired and symmetrical, strongly curved, with weakly defined manubrium and subcylindrical shaft.Gubernaculum plate-like, with a pair of strong closely set caudal apophyses.Caudal setae present, arranged in subventral and subdorsal rows, but difficult to observe and count.

Description
Adult Body cylindrical, posteriorly tapering in tail region, straight or ventrally curved upon fixation, more so in the posterior part of males.Cuticle coarsely annulated along entire body, except for smooth anterior end and terminal part of the tail; annules 1.5-2.5 µm wide at mid-body region; longitudinal striation not observed under the light microscope.Somatic setae visible on tail.Labial region bluntly rounded, lips fused.Cuticularised plate underlying cephalic cuticle around amphid absent.Inner and outer labial sensilla not seen.Cephalic sensilla setiform, equal to 0.3-0.6 labial region diameters in length, their bases are located 3-3.5 µm from anterior end.Amphids similar in shape and size between sexes: amphidial fovea an inverted U-shape with dorsal branch longer than ventral branch.Wide space between amphidial branches (amphidial shield) strongly cuticularised, punctuated or crenate.Stoma very small, its cuticularised lining is uniform with lining of pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to basal pharyngeal swelling; secretory-excretory duct very short, leading from pore to ampulla; renette cell ovoid, its body adjacent and ventrosublateral to anterior part of intestine.Tail subcylindrical with rounded terminus.Caudal glands opening via three separate openings, spinneret absent.

Female
Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to right of intestine; posterior ovary situated to left of intestine.Vulva at or immediately posterior to mid-body.Vagina straight, 0.3 of the vulval body diameter, with developed sphincter muscle at its proximal part; pars refringens vaginae absent.Intrauterine egg not seen; sperm is visible in the uterus.Rectum short, 0.7-1 of the corresponding body diameter long.Juario, 1974) comb.nov., presented as mean and (range) for females and each male individual measurement separated with a semicolon; "?" indicates that the character can not be observed or measured.

Male
Reproductive system diorchic, both testes outstretched.Spicules paired and symmetrical, strongly curved, with dorsally bent manubrium and subcylindrical shaft.Gubernaculum plate-like, with a pair of strong closely set caudal apophyses.Caudal setae present, arranged in subventral and subdorsal rows but difficult to observe and count.

Remarks
The recently collected population of D. asetosus is similar to the type specimens in general morphology and measurements, except for the relatively broader amphid and longer spicules (26-31 µm vs 17 µm (along chord) in the text of the original description or 21 µm as re-measured along curved median line using the original drawing).

Description
Adult Body cylindrical, posteriorly tapering in the tail region, straight or variously curved upon fixation.Cuticle coarsely annulated along entire body, except for smooth anterior end and terminal part of tail; annules 2.5-3 µm wide at mid-body region; longitudinal striation not observed under light microscope, but distinct under scanning electron microscope, covers annules over most of body length, except for anteriormost and posteriormost parts.Somatic setae present along pharyngeal region and on tail.Labial region bluntly rounded.Cuticularised plate underlying cephalic cuticle around amphid absent.Inner and outer labial sensilla not seen.Cephalic sensilla papilliform, equal to 0.1 labial region diameter in length, their bases are located 3.5-6 µm from anterior end.Amphids similar in shape and size between sexes: amphidial fovea an inverted U-shape with ventral branch longer than dorsal branch; tip of ventral branch bent to dorsal side.Interior of amphideal fovea with fine striation visible under light and scanning electron microscopes.Narrow space between amphidial branches (amphidial shield) not cuticularised.Stoma very small, its cuticularised lining uniform with lining of pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to basal pharyngeal swelling; secretory-excretory duct very short, leading from pore to ampulla; renette cell obscure.Tail cylindrical or clavate, with bluntly rounded terminus.Caudal glands opening via three separate openings, spinneret absent.

Female
Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to either right (usually) or left (rarely) of intestine; posterior ovary situated to either left (usually) or right (rarely) of intestine.Vulva pore-like, located posterior to mid-body.Vagina straight, 0.3-0.4 of vulval body diameter, with thick walls and developed sphincter muscle at proximal part; pars Fig. 6.Diplopeltoides bulbosus (Vitiello, 1972)  refringens vaginae absent.Intrauterine egg not seen; sperm is visible in uterus.Rectum short, 0.7-0.9 of corresponding body diameter long.

Remarks
The recently collected population of D. bulbosus is similar to type specimens in general morphology, measurements and shape of amphids, except for the spicule length.Vitiello (1972) stated in the original description that the spicules are 25 µm long, compared to 45-50 µm in our specimens.Measured from the drawings in the original description, the spicule length of the type specimen appears to be closer to 34 µm.

Description
Adult Body cylindrical, posteriorly tapering in the tail region, straight or curved upon fixation.Cuticle coarsely annulated along entire body, except for smooth anterior end and terminal part of tail; annules 2-2.5 µm wide at mid-body region; longitudinal striation not observed under light microscope.Somatic setae present on tail.Labial region bluntly rounded.Cuticularised plate underlying cephalic cuticle around amphid absent.Inner and outer labial sensilla not seen.Cephalic sensilla papilliform, equal to 0.1 labial region diameter in length, their bases are located 3.5-4 µm from anterior end.Amphids similar in shape and size between sexes: amphidial fovea an inverted U-shape with dorsal branch longer than ventral branch.Narrow space between amphidial branches (amphidial shield) cuticularised with crenated edge.Stoma very small, its cuticularised lining uniform with lining of the pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretoryexcretory system present; secretory-excretory pore located along ventral body line opposite to basal pharyngeal swelling; secretory-excretory duct very short, leading from pore to ampulla; renette cell obscure.Tail subcylindrical with bluntly rounded terminus.Caudal glands opening via three separate openings, spinneret absent.

Female
Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to either right or left of intestine; posterior ovary situated to left of intestine.Vulva located immediately posterior to mid-body.Vagina straight, 0.3 of vulval body diameter, with thick walls and Fig. 7. Diplopeltoides nudus (Gerlach, 1956) developed sphincter muscle at its proximal part; pars refringens vaginae absent.Intrauterine egg not seen; sperm is visible in the uterus.Rectum short, 0.6-0.9 of the corresponding body diameter long.

Male
Reproductive system diorchic, both testes outstretched.Spicules paired and symmetrical, strongly curved, with dorsally bent manubrium and subcylindrical, gradually narrowing shaft.Gubernaculum plate-like, with a pair of strong closely set caudal apophyses.Caudal setae present, arranged in subventral and subdorsal rows but difficult to observe and count.(Gerlach, 1956), presented as mean and (range) for males and each female individual measurement separated with a semicolon.

Remarks
The recently collected population of D. nudus is similar to the type specimens in general morphology and measurements, including the characteristic spicule shape, except for the somewhat longer spicules (35-38 µm vs 29 µm in type specimens) and the presence of gubernaculum apophyses (vs absent in type specimens).As noted in the redescription of D. bulbosus, gubernaculum apophyses may be indistinct or absent in some specimens and should not be used as the sole diagnostic character.

Description
Adult Body cylindrical, posteriorly tapering in tail region, straight or curved upon fixation.Cuticle coarsely annulated along entire body, except for smooth anterior end (annulation starts at level of anterior ¼ of amphid) and terminal part of tail; annules 2.5 µm wide at mid-body region; longitudinal striation is distinct under light microscope, covers annules over entire body length.Somatic setae present on tail.Labial region bluntly rounded.Cuticularised plate underlying cephalic cuticle around amphid absent.Inner and outer labial sensilla not seen.Cephalic sensilla setiform, equal to 0.2-0.3labial region diameters in length, their bases are located 2-5 µm from anterior end.Amphids similar in shape and size between sexes: amphidial fovea an inverted U-shape with branches equal in length.Narrow space between amphidial branches (amphidial shield) not cuticularised and not ornamented.Stoma very small, its cuticularised lining is uniform with lining of pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretory-excretory system indistinct.Tail conoid.Caudal glands opening via three separate openings, spinneret absent.

Female
Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to either right or left of intestine; posterior ovary situated to either left or right of intestine.Vulva located immediately posterior to mid-body.Vagina straight, 0.2 of the vulval body diameter, with thick walls; pars refringens vaginae absent.Intrauterine egg not seen; sperm is sometimes visible in the uterus.Rectum short, 0.7-0.8 of corresponding body diameter long.

Male
Reproductive system diorchic, testes opposed: anterior testis outstretched and posterior testis reflexed.Spicules paired and symmetrical, strongly curved, with weakly defined manubrium and conoid shaft.Gubernaculum plate-like, without apophyses.Caudal setae present, arranged in subventral and subdorsal rows but difficult to observe and count.

Remarks
The recently collected population of D. pumilus is similar to the type specimen in general morphology and measurements, structure of cuticle and anterior end, differing only in the larger body size (0.50-0.58 mm vs 0.29 mm in the type specimen), the longer amphid (16)(17)(18)(19) (Vincx & Gourbault, 1992) comb.nov., presented as mean and (range) for females and and each male individual measurement separated with a semicolon; "?" indicates that the character can not be observed or measured.
the longer tail (c' = 4.0-4.5 vs c' = 3.5 in the type specimen) and the longer spicules (24-26 µm vs 19 µm in the type specimen).It is also similar to D. holovachovi from the Sea of Japan in general morphology, structure of cuticle and anterior end, except for the longer body (0.50-0.58 mm vs 0.43-0.50mm in D. holovachovi) and less pronounced sexual dimorphism in the shape of the amphid (more elongated in females vs more rounded in females from the type population).

Description
Adult male Body cylindrical, posteriorly tapering in tail region, weakly curved upon fixation.Cuticle coarsely annulated along entire body, except for smooth anterior end (annulation starts at level of posterior ¼ of amphid) and terminal part of tail; annules 3-4 µm wide at mid-body region; longitudinal striation is distinct under the light microscope, covers annules over entire body length.Somatic setae not observed.Labial region bluntly rounded.Cuticularised plate underlying cephalic cuticle around amphid strongly developed, extending from level of anteriormost edge of amphid to level of posteriormost edge, 18.5-19 µm long and 13-14.5 µm wide at base; cuticular plates connected with each other on ventral and on dorsal sides.Inner and outer labial sensilla not seen.Cephalic sensilla setiform, equal to 0.3 labial region diameters in length, their bases are located 3.5-4 µm from anterior end.Amphidial fovea an inverted U-shape with ventral branch longer than dorsal branch.Narrow space between amphidial branches (amphidial shield) not cuticularised and not ornamented.Stoma very small, its cuticularised lining is uniform with lining of the pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus cylindrical or slightly fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei large, orifices of pharyngeal glands indistinct.Nerve ring surrounding isthmus.Secretory-excretory system present; secretory-excretory pore located along ventral body line opposite to isthmus; secretory-excretory duct very short; renette cell not observed.Tail conoid.Caudal glands opening via three separate openings, spinneret absent.Reproductive system diorchic, testes opposed: anterior testis outstretched and posterior testis reflexed.Spicules paired and symmetrical, strongly curved, with round manubrium and subcylindrical shaft.Gubernaculum plate-like, without apophyses.Caudal setae: two subventral pairs and single subdorsal.

Female
Not found.

Remarks
The recently collected population of D. linkei is similar to the type specimens from the Norwegian Sea in general morphology, structure of cuticle and anterior end, except for the somewhat longer body (0.62-0.63 mm vs 0.48-0.50mm in type specimens) and longer spicules (32-36 µm vs 29 µm).

Etymology
The specific epithet refers to the characteristic long tail in this species.

Adult male
Body cylindrical, posteriorly tapering in tail region, strongly ventrally curved upon fixation.Cuticle finely annulated along entire body, except for smooth anterior end and terminal part of tail; annules 1.5 µm wide at mid-body region; longitudinal striation not observed under light microscope.Somatic setae indistinct.Labial region truncate conoid; lips fused.Cuticularised plate underlying cephalic cuticle around amphid weakly developed, extending from level of cephalic setae bases to posteriormost edge of amphid, 9 µm long and 7.5 µm wide at base; cuticular plates connected with each other on ventral and on dorsal sides.Inner and outer labial sensilla not observed.Cephalic sensilla setiform, equal to 2.7 labial region diameters in length, their bases are located 1.5 µm from anterior end.Amphidial fovea an inverted U-shape with dorsal branch longer than ventral branch.Narrow space between amphidial branches (amphidial shield) not cuticularised and not ornamented.Stoma very small, its cuticularised lining uniform with lining of the pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and weakly developed basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring not observed.Secretory-excretory system not observed.Tail elongate conoid.Caudal glands opening via three separate openings, spinneret absent.Reproductive system obscure.Spicules paired and somewhat unequal in size, arcuate, with small ovoid manubrium and subcylindrical shaft.Gubernaculum absent.Midventral cuticular ridge extending anteriorly from about level of middle of spicules for 150 µm towards mid-body.Caudal setae not observed.

Female
Not found.

Remarks
Diplopeltoides longicaudatus sp.nov. is unique among known species of Diplopeltoides in having a precloacal cuticular ridge in males -a feature found in several species of the related genus Tarvaia Allgén, 1934 (Holovachov et al. 2012).In having a cuticularised plate underlying the cuticle of the anterior end, the new species is similar to D. ornatus, D. linkei, D. axayacatli, D. grandis sp. nov. and D. suecicus sp.nov.The new species further differs from D. ornatus and D. linkei in the structure of cuticle (smooth vs striated under the light microscope); a body length of 1.03 mm vs 0.43-0.63mm, the length of the cephalic setae (13 µm vs 2-4 µm) and tail (c' = 6.4 vs 3.2-4.2);it differs from D. axayacatli, D. grandis sp. nov. and D. suecicus sp. nov. in

Etymology
The specific epithet refers to the large body size in this species.

Adult female
Body cylindrical, posteriorly tapering in tail region, curved upon fixation.Cuticle coarsely annulated along entire body, except for smooth anterior end and terminal part of the tail; annules 3 µm wide at mid-body region; longitudinal striation is distinct under light microscope, covers annules over entire body length.Somatic setae indistinct.Labial region truncate conoid, lips fused.Cuticularised plate underlying cephalic cuticle around amphid strongly developed, extending from the level of cephalic setae bases to the posteriormost edge of amphid, 38 µm long and 23 µm wide at base; cuticular plates connected with each other on ventral and on dorsal sides.Inner labial sensilla not seen, outer labial sensilla small papilliform, located on the anterior surface of lips.Cephalic sensilla setiform, equal to 1.4 labial region diameters in length, their bases located 4 µm from anterior end.Amphidial fovea an inverted U-shape with dorsal branch longer than ventral branch.Wide space between amphidial branches (amphidial shield) heavily cuticularised and punctate.Stoma very small, its cuticularised lining uniform with lining of the pharynx.Pharynx distinctly subdivided into anterior corpus and posterior postcorpus; corpus fusiform, muscular; postcorpus consists of anterior narrow non-muscular isthmus and pear-shaped glandular basal swelling.Pharyngeal gland nuclei and orifices indistinct.Nerve ring surrounding isthmus.Secretory-excretory system not observed.Tail conoid.Caudal glands opening via three separate openings, spinneret absent.Reproductive system didelphic, amphidelphic, reflexed; ovary branches symmetrical.Anterior ovary situated to right of intestine; posterior ovary situated to left of intestine.Vulva a transverse slit, located immediately posterior to mid-body.Vagina straight, 0.2 of the vulval body diameter, with thick walls; pars refringens vaginae absent.Intrauterine egg or sperm not observed.Rectum short, 0.6 of the corresponding body diameter long.

Male
Not found.

Remarks
Diplopeltoides grandis sp.nov. is most similar to D. axayacatli in having a relatively large body size, a distinctly striated cuticle and a developed cuticularised plate.The new species differs from D. axayacatli in the shape of the amphid (loop-shaped, with parallel branches vs hook-shaped, with one of the branches bent inward, in D. axayacatli) and its size (30 × 14 µm vs 11-15 × 8-9 µm in D. axayacatli).

Key to species of Diplopeltoides
See Table 7     diagnosis is questionable.In the original description, the cuticle of D. ornatus is described as having a fine longitudinal striation (Gerlach 1950).The same character is typical of Diplopeltula striata (Gerlach 1956) and maintained in the description of the genus Diplopeltula (Gerlach 1962).None of the original distinguishing characters were mentioned by Lorenzen (1981), who removed the genus Diplopeltoides from the family Diplopeltidae based on the morphology of the female reproductive system (antidromously reflexed ovaries) as one of the main diagnostic characters.The other diagnostic character was the structure of the pharynx, with "thin walls and no muscle in the middle section" and a basal swelling.Tchesunov (1990) used these characters further to support the creation of the monotypic family Diplopeltoididae.He also suspected that many species of the genus Diplopeltula would be transferred to Diplopeltoides when new data on the morphology of the digestive and reproductive structures came to light.Subsequent publications (Tchesunov 2006;Holovachov et al. 2009) proved him to be correct.
The genus Diplopeltula was retained in the family Diplopeltidae by both Lorenzen (1981) and Tchesunov (1990); one of the diagnostic characters of this family being the presence of outstretched ovaries.The problem, however, is that the original description of the type species Diplopeltula breviceps Gerlach, 1950 does not include clear descriptions of the pharynx or of the female reproductive system -features that are used not only to distinguish Diplopeltoides from Diplopeltula, but also from the recently proposed Mudwigglus Leduc, 2013(Leduc 2013).Thus, even though the majority of species currently placed in the genus Diplopeltula by Leduc (2017) are characterized by a cylindrical pharynx (with or without posterior bulb) and outstretched ovaries, both characters are unknown for the type species of the genus (Gerlach 1950) and cannot be used as diagnostic for the genus.By ignoring the female morphology in his descriptions, and not preserving any type material of his new species, S. Gerlach left marine nematology with a number of unresolved taxonomic issues, such as the taxonomic status of the genus Diplopeltula.Until new specimens of Diplopeltula breviceps are collected from the type locality and re-described in detail, the status of the genus Diplopeltula and its species will remain uncertain.
The most logical approach would be to consider Diplopeltula breviceps a species inquirenda, consider Diplopeltula a genus inquirendum and transfer all other species currently placed in it into other well defined existing or newly proposed genera.

Table 7
(continued on next page).Selected diagnostic characters of species of the genus Diplopeltoides based on literature data supplemented with recent observations (NA = data not available).