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Free-living aquatic nematodes are abundant, diverse and of general environmental importance. However, knowledge of species distributions of both marine and freshwater nematodes is sparse. Species distribution data are crucial for evaluating environmental impacts from human activities and to conduct integrated nematode community assessments. Basic knowledge on taxonomy and species descriptions is lacking for many regions due to decreasing taxonomic expertise, yet it is essential for biodiversity research and for building molecular sequence libraries for the application of methods such as environmental DNA. In order to encourage and facilitate taxonomic and descriptive work on this understudied group, we present here a framework for nematode species description. We begin by providing a brief overview of nematology history, then provide suggestions of microscopic methods that should be used and provide a list of characters essential for morphometric species descriptions. Finally, we briefly discuss common molecular sequencing approaches that are commonly used in nematode taxonomic literature.

A new species of the genus Aporcella, collected in replanted coffee orchards in Central Highland of Vietnam, is described and illustrated. Aporcella daklakensis sp. n. is characterized by its 1.63-1.98 mm long body, lip region offset by weak constriction and 14-15 μm broad, odontostyle 13-15 μm long or equal to lip region diameter, neck 400-565 μm long, pharyngeal expansion 205-335 μm long and occupying 51-60% of the total neck length, female genital system diovarian with very poorly developed genital tract and transverse vulva (V = 53-59), tail conical with rounded terminus (31-39 µm, c = 43-57, c' = 1.0-1.4), visibly subdigitate and bearing cuticular irregularities at its ventral side, and male absent. The new species is compared with its closest Aporcella representatives.

Crassolabium unicum sp. n., collected from a natural habitat in Vietnam, is described and illustrated. The new species is characterized by its 0.770.94 mm long body, three-layered cuticle, lip region offset by depression and 1213.5 µm broad, odontostyle 13.515 µm long with wide aperture occupying 3646% of its length, neck 235260 µm long, pharyngeal expansion 117131 µm long or occupying 4752% of total neck length, anterior part of intestine presenting very distinct folds, female genital system didelphic-amphidelphic, uterus simple and 2843 µm or 0.70.9 body diameters, vulva transverse (V = 4552), prerectum bearing a blind postrectal sac, caudal region short and rounded (2125 µm, c = 4561, c' = 0.60.8) with two (dorsal and ventral) lacunae between outer and intermediate cuticle layers, and male unknown. It is compared with the similar representatives of the genus.

A new species of the genus Metarhabditis, M. giennensis sp. n., collected from a riverbank poplar forest in the southern Iberian Peninsula, is described and illustrated, including SEM observations. The new species is characterized by its 1.01-1.16 mm long body in females and 0.77-0.98 in males, cuticle with very fine transverse striation, lip region 9-14 µm broad and consisting of six swollen rounded lips fused in pairs, stoma 14-25 µm long with tubular gymno-promesostegostom, pharynx with slightly swollen metacorpus and slender isthmus, nerve ring, excretory pore and deirids located at isthmus level, female reproductive system didelphic-amphidelphic, vulva equatorial (V = 49-50), female rectum length 1.2-1.7 times the anal body width, female tail conical-elongate with acute tip (123-199 µm, c = 5.8-8.2, c' = 8.2-11.1), male tail conical (34-56 µm, c = 15.5-25.7, c' = 2.4-3.5), bursa leptoderan, 32-41 µm long spicules with rounded and ventrally bent manubrium, and 17-20 µm long gubernaculum. An updated list of species of the genus and a key for their identification are also provided. [Zoobank URL: urn:lsid:zoobank.org:act:7EBAC9DB-A148-4A1A-97E5-3B796D0839BF].

Two Metaporcelaimus species, one new and one known, are described from Ukraine. Metaporcelaimus petrophilus sp. n. is characterized by its 1.30-1.58 mm long body, lip region offset by deep constriction and 16-18 µm broad, odontostyle 15-17 µm long, neck 360-413 µm long, pharyngeal expansion occupying 47-54% of total neck length, uterus simple and 83-125 µm long or 1.6-2.7 times the corresponding body diameter, uterine eggs with granulate shell, V = 51-55%, tail conical (35-40 µm, c = 33-42, c' = 1.4-1.8 in females), spicules 43-50 µm long, and 9-13 spaced ventromedian supplements with hiatus. New data, including the first description of male specimens and SEM study, are provided for M. ovogranulosus, the Ukrainian populations of which are nearly identical to the type population from California.

A new species of the genus Tylencholaimus, collected in natural areas of Iran, is described and illustrated, including SEM observations. Tylencholaimus discus sp. n. is characterized by its 0.65-0.76 mm long body, lip region offset by weak constriction and 7-7.5 µm broad with a conspicuous perioral disc, odontostyle 6-7 µm long, odontophore 9-10.5 µm long and visibly flanged rather than knobbed, neck 230-259 µm long with anterior region weakly muscular and basal expansion 98-125 µm long or occupying 42-49% of total neck length, female genital system mono-prodelphic, posterior branch reduced to a uterine sac 2-20 µm long or much shorter (0.1-0.6 times) than body diameter but very occasionally consisting of uterus, sphincter and a small terminal mass all together measuring 35-40 µm long, V = 65-74, tail short and rounded (13-17 µm, c = 41-57, c' = 0.6-0.8), and male unknown.

One known (M. romanicus) and two new species of the genus Metaporcelaimus, collected in natural and semi-natural habitats of Ukraine, are described and illustrated. Metaporcelaimus concinnus sp. n. is characterized by its 3.09-4.58 mm long body, lip region 16.5-19.5 µm broad and visibly wider than the adjacent body, odontostyle 17-19 µm long, neck 745-865 µm long, pharyngeal expansion occupying 62-66% of total neck length, uterus tripartite and 3.9-5.7 times the corresponding body diameter, vulva longitudinal (V = 49-53%), tail conical (44-54 µm, c = 67-94, c' = 1.0-1.3 in females), spicules 79-86 µm long with strongly bifurcate distal end, and 10-14 irregularly spaced ventromedian supplements without hiatus. Metaporcelaimus declivicaudatus sp. n. is characterized by its 2.88-3.66 mm long body, lip region 23-28 µm broad, odontostyle 24-26 µm long, neck 634-788 µm long, pharyngeal expansion occupying 48-56% of total neck length, uterus tripartite and 3.0-3.4 times the corresponding body diameter long, vulva transverse (V = 51-55%), tail conical (45-65 µm, c = 53-63, c' = 1.0-1.2) with rounded terminus, spicules 102-113 µm long, and 12-14 spaced ventromedian supplements without hiatus. New information about M. romanicus is also provided, confirming available data about this species.

A new species of the genus Aporcelinus, A. elongicaudatus sp. n., collected from natural habitats of Costa Rica, is described and illustrated. The new species is characterized by its 0.92-1.05 mm long body, lip region offset by weak constriction and 15 µm broad, odontostyle 15-18 µm or 1.0-1.2 times the lip region diameter, neck 270-300 µm long, pharyngeal expansion 130-150 µm long or 45-55% of total neck length, uterus 33-41 µm long or 0.70-0.85 times the corresponding body diameter (but certainly longer), vulva transverse (V = 49-51), and tail conical elongated (45-71 µm, c = 14-22, c' = 2.0-3.2) and dorsad bent.

Two new species of the genus Metaxonchium are described from natural habitats of Vietnam. Metaxonchium bonkowskii sp. n. is characterized by its 1.69-2.23 mm long body, lip region offset by weak constriction and 9.5-10.5 µm broad, odontostyle 11-13 µm long, neck 673-868 µm long with the pharyngeal expansion occupying 66-77% of its length, anterior genital branch reduced to a uterine sac 75-135 µm long or 4-7% of body length, posterior uterus tetrapartite and including a Z-like differentiation, V = 51-55, and female tail short and rounded (37-51 µm, c = 66-97, c' = 0.5-0.7). Metaxonchium silvaticum sp. n is characterized by its 1.28-1.43 mm long body, lip region offset by weak depression and 7.0 µm broad, odontostyle 8-9 µm long, neck 502-630 µm long with the pharyngeal expansion occupying 65-76% of its length, anterior genital branch reduced to a uterine sac 65-80 µm long or 4-6% of body length, posterior uterus tripartite, non-echinophor, V = 52-57, tail short and rounded (18-23 µm, c = 56-74, c' = 0.6-0.8) with abundant blister-like or saccate bodies, spicules 41 µm long, and seven spaced ventromedian supplements.

A new species of the genus Myolaimus is described from agricultural areas in the southeast of the Iberian Peninsula. Myolaimus ibericus sp. n. is characterized by its 560-783 μm long body in females and 526-731 μm in males, cuticle often appearing swollen and baggy, lateral field with one longitudinal wing, lip region with six amalgamated lips having ten (6 + 4) setiform papillae, stoma consisting of a wider anterior chamber (cheilo-gymnostom) and a tubular posterior part (stegostom) separated by a well developed dorsal tooth and two small, lateral (one right and another left) teeth, glottoid-like apparatus structure appearing at metastom, pharyngeal corpus 1.7-2.6 times the isthmus length, excretory pore and deirids located at metacorpus level, deirids pore-like in females and seta-like in males, female reproductive system monodelphic-prodelphic with antidromous ovary, post-uterine sac 1.7-2.7 times the corresponding body diameter long, V = 50-56, female rectum 1.3-2.0 times the anal body diameter long, female tail conical-elongate (54-70 μm, c = 9.9-13.1, c' = 4.4-5.8) often enveloped by the baggy cuticle, male tail conoid (8-10 μm, c = 58.4-73.1, c' = 1.1-1.4) and ventrally constricted at its middle, bursal structure with seven (4+1+2) pairs of genital papillae. This is the first record of the genus from the Iberian Peninsula. Description, measurements and illustrations, including SEM photographs are provided. An illustrated compendium of the posterior ends of males in Myolaimus species is also presented.

A new species of the genus Protorhabditis is described from agricultural areas in the South East of the Iberian Peninsula. Protorhabditis hortulana sp. n. is distinguished by its very small body length (189-222 μm in females), lateral field with two longitudinal wings, lip region rounded with fused lips, stoma 10-13 μm long lacking glottoid apparatus, pharynx with distinctly swollen metacorpus, excretory pore and deirids at basal bulb level, female reproductive system outstretched and spermatheca with self-sperm, vulva slightly postequatorial (V=54-62), female tail conoid (14-19 μm, c= 11.7-17.6, c'= 1.6-2.4) with finely rounded tip, and males unknown. Description, measurements and illustrations, including SEM photographs are provided. A compendium of species of Protorhabditis is also given and illustrated.

Two new species of Aporcelinus from Vietnamese natural habitats are studied, described and illustrated, including line drawings and light microscope (LM) pictures. Aporcelinus paramamillatus sp. n. is characterized by its 1.71-2.14 mm long body, lip region 21-22 μm broad, odontostyle 22-24 μm long at its ventral side, neck 451-503 μm long, uterus tripartite and 71-94 µm long, V = 53-54, tail conical with finely rounded tip, ending in a short peg-like terminus, occasionally slightly re-curved dorsad (24-34 μm, c = 50-77, c' = 0.6-0.9) and unknown male. Aporcelinus paraseychellensis sp. n. is distinguished by its 1.20-1.46 mm long body, lip region 16-18 μm broad, odontostyle 20 μm long at its ventral side, neck 354-368 μm long, uterus tripartite and 176-241 µm long, V = 52-54, tail conical with acute tip and hardly re-curved dorsad, spicules 62 μm long, and 12 regularly spaced ventromedian supplements lacking hiatus.

Recently, specimens of Cephalobus topali, collected from an agricultural area from northern Vietnam, are described, including SEM study. They are characterized by their its 306-347 µm long body in females, lateral field with two alae or 3-4 longitudinal incisures, lip region with three pairs of asymmetrical conoid lips, three low and rounded labial probolae separate from the corresponding pair of lips by an incisure, U-shaped primary axils without guard processes, V-shaped secondary axils without guard processes, pharyngeal corpus slightly fusiform and 2.6-3.1 times the isthmus length, spermatheca and postvulval sac poorly developed, female tail conoid (18-24 µm, c = 12.7-17.0, c' = 1.8-2.7) with rounded terminus ending in an acute or wart-shaped mucro, phasmid at 29-34% of tail length, and male unknown. SEM pictures reveal that lip region pattern significantly differs from that found in members of Cephalobus and fits that observed in Acrobeloides, so that the species is transferred to this genus. Cephalobus quadrilineatus is provisionally maintained as different from C. topali.

A new species of the genus Sectonema, collected from natural forests in the Spanish provinces of León and Palencia, is described and illustrated with line drawings and LM pictures. Sectonema septentrionale sp. n. is characterized by its 5.59-6.90 mm long body, lip region 25-29 μm broad and offset by deep constriction, mural tooth 18-19 μm long at its ventral side and occupying most of the stomatal lumen, pharyngeal expansion 618-926 μm long or 60-73% of total neck length, uterus tripartite and 370-493 μm long or 2.9-4.3 times the corresponding body diameter, V = 48-52, tail short and rounded (40-62 μm, c = 108-146, c' = 0.6-0.8), spicules 100-145 μm long, and 5-11 irregularly spaced ventromedian supplements with hiatus. It is very close to S. demani, but differs both in the nature of the mural tooth and of the uterus.

Paravulvus moroccanus sp. n., collected in the Moroccan eastern High Atlas, is described and illustrated.  The new species is characterized by its 1.06-1.36 mm long body, lip region offset by marked depression and 11-13 µm broad, mural tooth solididentoid and 5.5-6.5 µm long, neck 229-270 µm long, pharyngeal expansion 108-126 µm long or occupying less than one-half (42-48%) of total neck length, uterus 27-47 μm long or 0.7-1.1 times the corresponding body diameter, V = 46-56, paravulvae absent, female tail rounded conoid (21-27 µm, c = 42-64, c' = 1.1-1.5), and male unknown. The taxonomy of Paravulvus is updated, with comments on its definition, a discussion of its relationships, and the provision of a list of species along with a key to their identification and a compendium of their morphometrics.

Three new and one known species of the genus Aporcelaimellus with simple uterus and typical caudal region, mainly col-lected in natural areas of southeastern Iberian Peninsula, are studied, including descriptions, measurements, line illustrations and LM pictures. Aporcelaimellus baeticus sp. n. is characterized by a body length of 1.96-2.95 mm, lip region offset by constriction and 17-20 μm broad, odontostyle 17-22 μm long, neck 510-635 μm long, pharyngeal expansion 250-335 μm long, a dorsal cell mass at level of anterior end of intestine, uterus 60-150 μm long or 0.9-1.7 times the corresponding body diameter, vulva transverse (V = 48-54), tail convex conoid with broadly rounded terminus (30-46 μm, c = 47-80, c' = 0.7-1.1), spicules 99-103 μm long, and 18-21 irregularly spaced ventromedian supplements which lack hiatus. Aporcelaimellus brevicaudatus sp. n. is characterized by a body length of 1.54-2.14 mm long, lip region offset by a more or less distinct constriction and 14-16 μm broad, odontostyle 13-15 μm long, neck 512 μm long, pharyngeal expansion 255 μm long, uterus 39-57 μm long or 0.9-1.0 times the corresponding body diameter, V = 50-55, tail short and rounded to hemispherical (24-27 μm, c = 66-82, c' = 0.6-0.7) with inner core hardly reaching the middle of tail length, and male unknown. Aporcelaimellus rotundus sp. n. is characterized by its body 2.01-2.58 mm long, lip region offset by constriction and 19-20 μm broad, odontostyle 22-23 μm long, neck 455-579 μm long, pharyngeal expansion 226-316 μm long, a dis-tinct dorsal cell mass present at cardia level, uterus 28-74 μm long or 0.5-0.9 times the corresponding body diameter, V = 47-52, female tail short and conoid to rounded (27-33 μm, c = 64-90, c' = 0.7-0.9), and males unknown. New data, including SEM pictures, sequences and taxonomic comments, are provided for A. waenga.

The taxonomy of the genus Aporcelaimellus is revised and updated. A new definition is proposed, with several remarkable diagnostic characters: cuticle three-layered, especially distinguishable at caudal region, usually bearing a hyaline space between inner and intermediate layers at terminal portion of tail; cervical lacunae often present and well developed; oral aperture a dorso-ventral, hexagonal open slit; lip region showing bilateral symmetry and offset by a more or less distinct constriction; odontostyle short, robust and with wide aperture; uterus simple, bi- or tripartite; pars refringens vaginae present, well developed; tail similar in both sexes, short, rounded to conical with more or less rounded terminus; and ventromedian supplements 7-25 in number, separated, almost always with precloacal space (hiatus). A list of 28 valid species and their synonyms is provided as well as a key to their identification and a tabular compendium of their morphometrics. Aporcelainiellus faridpuriensis is considered to be a species inquirenda. Twenty-one species are regarded as incertae sedis: A. amplexor, A. budauniensis, A. canis, A. digicaudatus, Dorylaimus domus glauci, A. estonicus, A. gerlachi, A. goldeni, A. index, A. jairajpurii, A. monodelphus, A. nawabganjense, A. nivalis, A. paracentrocercus, A. paraobtusicaudatus, A. radicus,A. sacchari, A. seinhorsti, A. stilus, A. submissus and A. subsimilis. Two species are retained under other genera: A. bicuticulus under Labronema, and A. cocophilus under Aporcelaimus. Sixteen species are transferred to other genera: A. baqrii and A. odhneri to Crassolabium; A. dubius to Discolaimium as Discolaimium geraldi nom. n.; A. duhouxi to Labronema; A. glandus, A. heynsi, A. maitai and A. paraconicaudatus to Discolaimiun; A. insularis, A. jiaonanensis, A. kikereensis and A. propinquus to Aporcelinus; A. maximus to Aporcella; A. paracentrocercus apud Andrássy (1960) to Aporcelinus as Aporcelinus africanus sp. n.; A. saprophilus to Eudorylaimus; and A. tritici to Makatinus. Finally, A. concavus becomes a non-valid binomen.

Three new and one known species of the genus Aporcelaimellus, collected mainly in natural areas from the Iberian Peninsula and characterized by having a simple uterus and atypical caudal region are described and illustrated, including line drawings, LM pictures and/or SEM pictures. Aporcelaimellus deserticola sp. n. is distinguished by its body 1.79-2.43 mm long, lip region 14-16 pm broad and offset by expansion rather than by constriction, odontostyle 18-21 µm long, neck 464-555 µm long, pharyngeal expansion 234-283 µm long or 49-52% of total neck length, a dorsal cell mass present at level of anterior end of intestine, uterus 34-67 µm long or 0.6-1.1 times the corresponding body diameter, V= 49-55, tail conical with rounded terminus (29-43 µm, c = 47-66, c' = 0.9-1.4) and bearing a short but perceptible hyaline portion, and male unknown. Aporcelaimellus hyalinus sp. n. is characterized by its body 1.49-2.23 mm long, lip region offset by constriction and 13-16 µm broad, odontostyle 14-16 µm long, neck 389-474 µm long, pharyngeal expansion 183-242 µm long or 45-53% of total neck length, uterus 44-106 µm long or 0.7-1.8 times the corresponding body diameter, V = 48-55, tail conical with rounded terminus (27-41 µm, c = 38-68, c' = 1.0-1.4) and short inner core, spicules 56 µm long, and seven irregularly spaced ventromedian supplements. Aporcelaimellus rarus sp. n. is distinguished by its body 1.96-2.34 mm long, lip region offset by constriction and 14-16 µm broad, odontostyle 15-16 µm long, neck 411-518 µm long, pharyngeal expansion 196-270 µm long or 48-52% of total neck length, a dorsal cellular mass at a short distance behind the cardia, uterus 63-96 µm long or 0.8-1.5 times the corresponding body diameter, V= 51-56, female tail short and conical (28-40 µm, c = 50-74, c' = 0.7-0.9) with rounded terminus and bearing a dorsal concavity at its posterior half, and males unknown. New data are provided for A. salicinus. A discussion of these atypical species is also provided.

Two Iberian species of the subgenus Pseudacrobeles (Bunobus), one new and another known, collected from natural areas are described and illustrated, including morphometric and LM photographs. Pseudacrobeles (Bunobus) rotundilabiatus sp. n. is characterized by a body length of 624-805 μm long in females and 670-714 μm in males, lateral field with three incisures terminating gradually near the phasmid, lip region continuous with the adjacent body, subdorsal and subventral lips rounded, lateral lips reduced, stoma 13-17 μm long, pharyngeal corpus 3.0-4.3 times as long as isthmus, V = 63-74, spermatheca 22-62 μm long, postvulval sac 1.0-2.0 times the body diameter in length, female tail conical elongated (59-83 μm, c = 9.7-11.6, c' = 4.5-6.8) ending in a lanceolate mucro slightly tuberculate proximally, male tail conical (38-40 μm long, c = 10.1-16.8, c' = 3.9), spicules 25 m long, and gubemaculum 14 m long. Pseudacrobeles (Bunobus) pseudolatus is re-described based on type specimens. In addition, an emended diagnosis is provided for the subgenus as well as an updated list of species, a compendium of their morphometrics and a key to their identification.