Three new species of free-living marine nematodes of the Microlaimus genus (Nematoda: Microlaimidae) from the continental shelf off northeastern Brazil (Atlantic Ocean)

Three new species of the Microlaimus genus (Nematoda: Microlaimidae) are described from sample sediments collected in the South Atlantic, along the Continental Shelf break of Northeastern Brazil. Microlaimus paraundulatus sp. n. possesses four setiform cephalic sensillae, a buccal cavity with three small teeth, arched and slender spicules and a wave-shaped gubernaculum. Microlaimus modestus sp. n. is characterized by four small cephalic sensillae, a buccal cavity with three teeth (one large dorsal tooth), cephalated spicules and a strongly arched gubernaculum in the distal region. Microlaimus nordestinus sp. n. is characterized by the following set of features: relatively long body, eight rows of hypodermal glands that extend longitudinally along the body and a funnel-shaped gubernaculum surrounding the spicules at the distal end. An amendment of the diagnosis is proposed for the genus.


INTRODUCTION
Among the representatives of the superfamily Microlaimoidea Micoletzky, 1922, the family Microlaimidae Micoletzky, 1922 encompasses the largest number of genera and species (Tchesunov, Jeong & Lee, 2021).Amid the genera placed within Microlaimidae, records of species originally described from samples sediment of the South Atlantic are still scarce (WoRMS Editorial Board, 2024).In research carried out in Brazil in the 1950s, Gerlach described several new species of Nematoda for the hitherto "unexplored Brazilian coast".As part of their results, four species of the genus Microlaimus de Man, 1880 were described (M.papillatus Gerlach, 1956;M. capillaris Gerlach, 1957a;M. spinosus Gerlach, 1957b andM. formosus Gerlach, 1957b) from samples collected in mangroves and sandy beaches of the southeastern coast of Brazil.Later, Aponema papillatum Pastor de Ward, 1980 and M. decoratus Pastor de Ward, 1989 were described from the Ria Deseado (Santa Cruz, Argentina).Recently, Lima, Neres & Esteves (2022) described three species of Microlaimus (M.campiensis, M. alexandri and M. vitorius) from the continental shelf of the Campos Basin, southeastern Brazil.

Laboratory processing
In the laboratory, sediment samples were sieved using a 500 mm mesh followed by a 45 mm mesh sieve which was used to retain the meiobenthic organisms.The samples remaining in the 45 mm mesh were extracted with colloidal silica (Somerfield, Warwick & Moens, 2005).
Nematoda were counted (and removed) under a stereomicroscope using a Dolffus plate.All individuals were transferred to a small glass container containing a solution with 99% formaldehyde (4%) + 1% glycerin (Solution 1-De Grisse, 1969).The methodology for impregnating each animal's body with glycerin was then applied, followed by diaphanization, according to the method described by De Grisse (1969).The individuals were mounted permanently on glass slides, as an adaptation of the method described by Cobb (1920).The genus was identified using keys provided by Warwick, Platt & Somerfield (1998) and Decraemer & Smol (2006).Species were identified through the comparison of their characteristics with those provided in the original descriptions.Drawings were made with the aid of an Olympus CX 31 optical microscope fitted with a drawing tube.Body measurements were taken using a mechanical map meter.
The holotype and one paratype (female) of each species are deposited in the Nematoda Collection of the Museum of Oceanography Prof. Petronio Alves Coelho (MOUFPE), Brazil.Other paratypes are deposited in the Meiofauna Laboratory, Zoology Department, Federal University of Pernambuco (NM LMZOO-UFPE).
The electronic version of this article in Portable Document Format (PDF) will represent a published study according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under the Code from the electronic edition alone.This published research and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN.The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/.The LSID for this publication is: urn:lsid:zoobank.org:pub: 414C399D-A60E-494E-9E36-C4866FBC9539.The online version of this research is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Reproductive system didelphic-amphidelphic, with outstretched ovaries.Anterior ovary situated to the right side of intestine, posterior ovary to the left side of intestine.Tail conical, about 5.5 times the anal body diameter.
Diagnosis.Microlaimus paraundulatus sp.n. characterized by its body length (439-465 mm).Cuticle finely annulated.Head slightly set off.Four cephalic sensillae in the shape of thin setae (3 mm long), corresponding to 43% of head diameter.Amphidial fovea occupying 44% of the corresponding body diameter in the males and 56% in the female, located at about 1.6-1.9times the head diameter.Buccal cavity with three small teeth, one dorsal and two ventrosublateral, the dorsal is slightly larger.Spicule arched and slender (1.8-2.3 times the cloacal body diameter) with a wave-shaped gubernaculum.Tail conical which corresponds to 4-6 cloacal or anal body diameter.
Differential diagnosis (Table 3).Firstly, it is important to establish that only males of each species mentioned in this section were used in the comparisons with males of the new species.More detailed information, such as some measurements and proportions, about females is absent in the original descriptions of some of the species in question.The measurements and proportions missing from the descriptions were obtained from available images.Microlaimus paraundulatus sp.n. resembles M. undulatus Gerlach, 1953 mainly due to the peculiar shape of the gubernaculum (wave-shaped).Additionally, in both species the amphidial fovea occupies a similar proportion of the corresponding body diameter (44% in M. paraundulatus sp.n. and 42% in M. undulatus) and the spicules are very similar in length (23-27 mm in M. paraundulatus sp.n. and 27 mm in M. undulatus).However, the species differ in the length of the cephalic setae (3 mm in the new species vs 7 mm in M. undulatus), the position of the amphidial fovea from the anterior end (1.6 times the diameter of the head in M. paraundulatus sp.n. vs 0.7 times the diameter head diameter in M. undulatus); the shape of the spicules (thin in the new species vs robust in M. undulatus) and the ratio between the length of the spicules and the diameter of the cloaca (1.8-2.3 in M. paraundulatus sp.n. vs 1.1 in M. undulatus).
The other four species of the genus (M.copulatus Jensen, 1988; M. limnophilus Turpeenniemi, 1997;M. porus Bussau, 1993 andM. zosterae Allgén, 1930) that morphologically resemble M. paraundulatus sp.n. are included in Table 3. Males of these species share the combination of three features with the new species: length of cephalic setae less than 50% of the head diameter; amphidial fovea with anterior edge positioned between 1.6 and 1.9 times the head diameter in relation to the anterior end; amphidial fovea occupies less than 50% of the corresponding body diameter.Only the percentage of the corresponding body diameter occupied by the amphidial fovea for M. porus was obtained from the drawing of the female paratype (proportion not reported and anterior end not drawn in the original description of the male holotype; sexual dimorphism absent).Microlaimus undulatus, like the other species of the genus, does not present these characteristics simultaneously and therefore the species was not included in Table 3.
The new species differs from M. copulatus with regard to index "a" (27-30.5 in M. paraundulatus sp.n. vs 18-20 in M. copulatus), the ratio (%) of gubernaculum length in relation to spicule length along arc (48-56% in the new species vs 30% in M. copulatus) and the precloacal supplement (absent in M. paraundulatus vs a precloacal papilla in M. copulatus).Additionally, the gubernaculum has dorsal apophyses and an irregular shape in M. copulatus and is wave-shaped without apophyses in M. paraundulatus sp.n.Microlaimus paraundulatus sp.n. differs from M. limnophilus in terms of the length (%) of the cephalic setae in relation to head diameter (43% in M. paraundulatus sp.n. vs 23-25% in M. limnophilus), the shape of the gubernaculum (wave-shaped in M. paraundulatus sp.n. vs rod-like in M. limnophilus) and the ratio of the spicules in relation to the cloacal body diameter (2.3 in M. paraundulatus sp.n. vs 1.3 in M. limnophilus).Furthermore, M. limnophilus possesses two pore-like precloacal supplements, while in the new species the precloacal supplements are absent.
The new species differs from M. porus Bussau, 1993 with regard to the number of teeth (three in M. paraundulatus vs two in M. porus), the shape of the gubernaculum (waveshaped in the new species vs simple in M. porus) and the values of the indices "a" (30.5 in the new species vs 21.1 in M. porus) and "c" (8 in the new species vs 5.4 in M. porus).Additionally, M. porus has rows of pores distributed throughout the body.This feature is absent in the new species.
Based on the redescription of the species, M. zosterae, provided by Kovalyev & Tchesunov (2005), whose original description was based on females alone, M. Diagnosis.Microlaimus modestus sp.n. is characterized by its small body length (331.5-359mm).Cuticle finely annulated.Head slightly set off.Inner and outer labial setae in the shape of papillae.Four short cephalic setae (2 mm long) that correspond to 22-25% of head diameter.Amphidial fovea accounts for 50-60% of the corresponding body diameter, located at about 1.1-1.2times the head diameter in males and 1.1-1.5 in females.Differential diagnosis (Table 5).Firstly, it is important to establish that only males of each species mentioned in this section were used for comparisons with males of the new species.This is because only males of M. acanthus (Jayasree & Warwick, 1977;Kovalyev & Tchesunov, 2005) have been described.Measurements and proportions missing from the descriptions were obtained from available images.Two species of the genus (M.acanthus and M. microseta Gerlach, 1953) that morphologically resemble M. modestus sp.n. are included in Table 5. Males of these species share this combination of three features with the new species: cephalic setae with a length less than or equal to 50% of the head diameter; amphidial fovea occupying more than 50% of the corresponding body diameter; amphidial fovea with anterior edge positioned between 1.1 and 1.5 times the head diameter in relation to the anterior end.M. acanthus showed a greater variation in the relative position of the amphidial fovea (1.1-1.8 times the head diameter).Nevertheless, due to other similarities with the new species, it was included in the table for comparison purposes.
M. modestus sp.n. shares the ratio between gubernaculum length and spicule length (between 50-60% in the three species), as well as the de Man's ratio c′ (between 3-4.25) with M. acanthus and M. microseta.However, the values of the other de Man's ratio (a, b and c) for M. modestus sp.n. are relatively low compared to M. acanthus and M. microseta (see Table 4).Furthermore, the ratio between the length of the spicule along the arc and cloacal body diameter is higher in M. modestus sp.n. (2-2.3) when compared to the ratios observed in M. acanthus and M. microseta (between 1.1-1.4).Additionally, the new species differs from M. acanthus and M. microseta with regard to the shape of the gubernaculum (simple with strongly arched distal region in M. modestus sp n. vs pointed and narrow proximally and expanded distally in M. acanthus vs narrow and simple in M. microseta).M. acanthus presents four to six prominent precloacal supplements in the form of robust setae, while in the new species precloacal supplements are absent.Microlaimus nordestinus sp.n. (Table 6; Figs.6-8)    Etymology.Nordestinus is the Latinized form of the term "nordestino".In Brazil, "nordestino" refers to something or someone originating from the northeastern region of the country.
Holotype male.Body cylindrical, 1,450 mm long.Maximum body diameter corresponding to 2.8 times head diameter.Cuticle striated posteriorly to cephalic setae insertion.Four sublateral, two subventral and two subdorsal rows of hypodermal glands that begin after the amphidial fovea and extend longitudinally along the body.Hypodermal glands visible up to about 57% of the total length of the tail from the cloaca.Cuticular pores and somatic setae not observed.Anterior sensilla arranged in the 6 + 6 + 4 pattern: six inner labial papilliform sensilla, six outer labial papilliform sensilla and four cephalic sensilla (6 mm long), corresponding to 67% of head diameter.Head slightly set off.Amphidial fovea unispiral, located 17 mm from anterior end (1.9 times head diameter) and occupying 46% of corresponding body diameter.Buccal cavity weakly cuticularized.Cheilostoma rugae indiscernible under a light microscope.Three small teeth, difficult to see (a slightly larger dorsal tooth and two smaller ventrosublateral).Pharynx (108 mm long) with terminal oval bulb.Bulb occupying 67% of corresponding body diameter.Cardia embedded in intestine.
Nerve ring situated at 66% of the pharynx length, from anterior end.Ventral gland and secretory-excretory pore not observed.Reproductive system with single anterior outstretched testis on right side of intestine.Spicules arched, with proximal portion cephalized.Gubernaculum funnel-shaped surrounding the spicules at the distal end.
Two precloacal papilla present.The closest is about 16 mm from the cloaca and the second at 24 mm.Three caudal glands.Tail conical with cylindrical terminal portion, 4.8 times the cloacal body diameter.
Paratype female.Similar to male.Body measuring 1,404 mm in length, with a maximum diameter of 31 mm.Rows of hypodermal glands similar to the male.Hypodermal glands visible up to about 60% of the total length of the tail from the anus.Cephalic sensilla equivalent to 75% of head diameter.Amphidial fovea occupies 36% of corresponding body width and located 17 mm from anterior end.Buccal cavity, teeth and pharynx similar to that of males.Basal bulb occupies 64% of the corresponding body diameter.Nerve ring situated at 58% of the pharynx length, from anterior end.Secretory-excretory located after the nerve ring and 74 mm from the anterior end.Ventral gland located immediately posterior to pharynx.Vulva located 756 mm from anterior end, at 54% of body length.
Reproductive system didelphic-amphidelphic, with outstretched ovaries.In this paratype, the posterior ovary is apparently damaged.However, in Female paratype 1 it was possible to visualize the described pattern.Anterior ovary situated to the right side of intestine, posterior ovary to the left side of intestine.Three caudal glands.Tail conical, about 6.5 times the anal body diameter.
Diagnosis.Microlaimus nordestinus sp.n. is characterized by its long body length (1,080-1,450.5mm).Cuticle finely annulated.Head slightly set off.Cephalic setae 5-6 mm long and corresponding to 56-75% of head diameter.Amphidial fovea occupies 36-50% of the corresponding body diameter, located at about 1.6-2.1 times the head diameter.Buccal cavity with three small teeth, one dorsal and two ventrosublateral.Four sublateral, two subventral and two subdorsal rows of hypodermal glands that begin after the amphidial fovea and extend longitudinally along the body.Hypodermal glands visible up to about a half of the total length of the tail from the cloaca.Two precloacal papilla.Gubernaculum funnel-shaped surrounding the spicules at the distal end.Tail conical with cylindrical terminal portion (4.2-6.5 times the cloacal body diameter).
Differential diagnosis.The new species shares the following features with Microlaimus cyatholaimoides de Man, 1922: anterior sensilla arrangement, where the first two are circles of papilliform setae and the third is setiform; de Man's ratio c (9-12 in M. cyatholaimoides and 10-15 in M. nordestinus sp.n.); the presence of precloacal supplements and spicule length (33-34 mm in M. cyatholaimoides and 28-31 mm in M. nordestinus sp.n.).However, M. cyatholaimoides has a shorter total body length compared to the species described here (684-960 mm vs 1,080-1,450.5 mm in M. nordestinus sp.n.).Furthermore, the new specie differs from M. cyatholaimoides in terms of the shape of the gubernaculum (funnel-shaped surrounding the spicules at the distal end in the new specie vs lamellar in M. cyatholaimoides) and the presence of a conical tail with a cylindrical terminal portion vs conical tail in M. cyatholaimoides.Based on the illustrations provided by Man, 1922 the amphidial fovea of female M. cyatholaimoides is located 1 times the head diameter in relation to the anterior end, while in M. nordestinus sp.n. females this structure is 2.1 times the head diameter from the anterior end.Although both species have rows of hypodermic glands along the body, in M. cyatholaimoides these glands are longitudinally predominantly distributed along four sublateral rows (according to Hopper & Meyers, 1967).In M. nordestinus sp.n., the glands are distributed longitudinally along eight rows: four sublateral, two subventral and two subdorsal.
The occurrence of rows of hypodermic glands has also been reported for the species M. discolensis Bussau, 1993, M. porus Bussau, 1993, M. parviporosus Miljutin & Miljutina, 2009and M. vitorius Lima, Neres & Esteves, 2022.For all the previously mentioned species, the occurrence of cuticular pores was also recorded.M. sergeevae Revkova, 2020 has rows of pores along the body, however the presence of rows of hypodermic glands was not mentioned.The occurrence of cuticular pores cannot be observed in any of the specimens of the new species.
Microlaimus nordestinus sp.n. differs from M. discolensis in terms of total body length (1,080-1,450.5mm vs 425-565 mm in the latter species); external labial setae papilliform and cephalic setae setiform, while these structures are setiform and are about the same length in M. discolensis and differ with regard to de Man's ratio c' (4.2-6.5 in the new specie vs 2.3-3.3 in M. discolensis).Moreover, the tail is conical-shaped with a cylindrical terminal portion in the new species vs conical in M. discolensis.
The new species differs from M. porus in terms of total body length (2.4-3.8 times longer), the presence of two precloacal supplements vs absent in M. porus and the shape of the gubernaculum (funnel-shaped surrounding the spicules at the distal end in the new specie vs lamellar in M. porus).Moreover, M. nordestinus sp.n. has three small teeth vs two visible teeth in M. porus.
Microlaimus nordestinus sp.n. differs from M. parviporosus with regard to its cephalic setae which are much longer than the outer labial ones, whereas the outer labial setae and the cephalic setae are about the same length in M. parviporosus.Additionally, it differs from M. parviporosus in terms of spicule length (28-31 mm vs 16-18 mm), the shape of the gubernaculum (funnel-shaped surrounding the spicules at the distal end in the new specie vs rod-like, slightly bent anteriorly in M. parviporosus), the presence of precloacal supplements (vs absent in M. parviporosus) and body length, which is 2.6-4 times greater compared to that of M. parviporosus.
Microlaimus nordestinus sp.n. resembles M. sergeevae and M. vitorius in terms of the shape of the gubernaculum.In these species, this structure surrounds the spicule in its distal portion.Nevertheless, M. nordestinus sp.n. differs from M. sergeevae in terms of the absence of cervical setae (vs present in M. sergeevae), the tail (conical with cylindrical terminal portion without rows of setae in the new species vs conical with a slightly swollen final portion and a row of subventral setae in M. sergeevae) and the precloacal supplement (two papilla in M. nordestinus sp.n. vs eight thin channels in M. sergeevae).The new species differs from M. vitorius with regard to tail shape (conical with cylindrical terminal portion in the new species vs conical in M. vitorius), shorter spicules and gubernaculum (spicules: 28-31 mm vs 45-55 mm; gubernaculum: 13-17 mm vs 20-27 mm), the presence of three small teeth in M. nordestinus sp.n. vs. three large teeth in M. vitorius, the position of the amphidial fovea (relatively further from the anterior end in the new species compared to M. vitorius: ratio between the distance from the amphidial fovea to the anterior end and the head diameter = 1.6-2.1 in M. nordestinus sp.n. vs 0.5-0.9 in M. vitorius) and the precloacal supplement (two papilla in M. nordestinus sp.n. vs three small pores in M. vitorius).

DISCUSSION
Although most species of the genus Microlaimus have three teeth in the buccal cavity, descriptive information on the species belonging to this genus varies with regard to this characteristic.Some species have an unarmed buccal cavity, as described for M. nympha Bussau, 1993; armed with two teeth, as described by Bussau (1993) and redescribed by Miljutin & Miljutina (2009) for M. porus; or with five teeth, as described for M. alexandri Lima, Neres & Esteves, 2022.We added variability in the number of teeth present in the buccal cavity to the diagnosis of the genus.
Specific characteristics, such as the relationship between the length of the cephalic setae and head diameter (%), the diameter of the amphidial fovea in the corresponding region of the body (%) and its position in relation to the anterior end of the body, helped to approximate M. paraundulatus sp.n. and M. modestus sp.n. to the most morphologically similar known species.The use of this combination of characters is frequently used in descriptions of Microlaimus species to express similarity relationships or to indicate differences between species (Kovalyev & Tchesunov, 2005;Gagarin & Tu, 2014;Revkova, 2020;Lima, Neres & Esteves, 2022).Taxonomic tools, such as de Man's ratios (a, b, c and c') and proportions between spicule length/cloacal body diameter, gubernaculum length/ spicule length (%) as well as the presence and absence of cuticular pores and precloacal supplements, helped to highlight the differences between the new species and the known species that are most morphologically similar to them.Additionally, the presence of rows of hypodermic glands, such as those visualized in M. nordestinus sp.n., which may or may not be associated with pores or/and setae, can also be used as a diagnostic feature to differentiate between Microlaimus species (Jensen, 1978;Hopper & Meyers, 1967;Muthumbi & Vincx, 1999).
Although there is variation in the number of testes in Microlaimus species (one or two), this characteristic was not used to correlate the species described here with other species.In most descriptions of Microlaimus species, especially those carried out before the 2000s, information about male gonads is missing.Tchesunov (2014) reviewed the genus Aponema Jensen, 1978 and stated that the number of testes is not reported for most microlaimid species and that other characteristics are more evident and easily observable than male gonads.
Our results recorded the first three species of the Microlaimus genus described from samples collected on the Continental Shelf of Northeast Brazil.The present study increases our knowledge on the species of this taxon present in the South Atlantic and significantly expands the available knowledge of the species richness of the genus, increasing the number of Microlaimus species originally described from sediment samples collected on the coast of Brazil from seven to 10.

Table 1
Collection stations, their respective coordinates and depth.The samples were collected at the break of the continental shelf in Northeast Brazil, South Atlantic.Manoel Cephalic region often set off.Presence or absence of association between hypodermal glands with pores or setae, small somatic setae occur in some species.Anterior sensilla arranged according to pattern 6 + 6 + 4: six inner labial setae, usually papilliform; six outer labial setae, papilliform or setiform; and four cephalic setae.Amphidial fovea cryptocircular or unispiral (=cryptospiral).Presence or absence of sexual dimorphism in amphidial fovea size.Buccal cavity small to medium-sized, armed (except in M. nympha) with two to five (three teeth in most species) small or well-developed teeth, especially the dorsal tooth.Transverse cuticularized band or ring may be present in buccal cavity.Most species have two testes extending in opposite directions; some with two anterior testes, others with only one testis, positioned anteriorly or posteriorly.Pre-cloacal supplements absent or present (papilliform, tubular, or small pores).Spicules usually short and arcuate, seldom long and slender.Gubernaculum usually present and without dorso-caudal apophysis.Female didelphic-amphidelphic, with outstretched ovaries.Tail predominantly conical.

Table 3
Comparison of species Microlaimus paraundulatus sp.n. with morphologically similar species (only males).M. copulatus M. limnophilus M. porus M. zosterae * M. paraundulatus differs from males of M. zosterae in terms of the shape of the gubernaculum (wave-shaped in the new species vs curved, tapering to the ends, wider in the middle part in M. zosterae) and the precloacal supplements that are absent in M. paraundulatus sp.n.vs present in M. zosterae.Bulb occupies 88% of corresponding body diameter.Cardia embedded in intestine.Nerve ring situated at 62% of the pharynx length, from anterior end.Secretory-excretory pore located 56 mm from anterior end (about 68% of the pharynx length).Ventral gland not observed.Reproductive system with single anterior outstretched testis on the right side of intestine.Spicules arched, with cephalated proximal end.Gubernaculum simple with strongly arched distal region.Precloacal supplements absent.Three caudal glands.Tail conical, about 3.2 times the cloacal body diameter.Paratype female.Similar to male.Body measuring 359 mm in length, and maximum diameter 23 mm.Cephalic sensilla equivalent to 24% of head diameter.Amphidial fovea, occupying 56% of corresponding body width and located 11 mm from anterior end.Buccal cavity, teeth and pharynx similar to that of the male.Basal bulb occupies 84% of the corresponding body diameter.Nerve ring situated at 59% of the pharynx length, from anterior end.Secretory-excretory pore located 53 mm from anterior end (about 65% of the pharynx length).Vulva located 206 mm from anterior end, at 57% of body length.
Notes:Information (measurements/proportions) of males of the species Microlaimus that concomitantly have cephalic setae <50% of head diameter; amphidial fovea between 1.6 and 1.9 times the head diameter; amphidial fovea <50% of the head body diameter.The measurements are expressed in micrometers, or if noted, as a percentage or ratio.Present (+) or absent (−); a, b, c, c′ = de Man's (1880) ratios; distance of amphidial fovea from anterior end in relation to head diameter (Amph/hd); percentage of the amphidial fovea diameter in relation to corresponding body diameter (amph%).* Based on the redescription of the species Microlaimus zosterae Allgén, 1930 provided by Kovalyev & Tchesunov (2005).** Proportion measured from the drawing of the female paratype.Missing information for the holotype male.Manoel et al. (2024), PeerJ, DOI 10.7717/peerj.173559/26 paraundulatus sp.n.Etymology.Due to its relatively small body length.Latin modestus: short in length.Holotype male.Body cylindrical 342 mm long.Maximum body diameter corresponding to 2.2 times the head diameter.Cuticle striated posteriorly to cephalic setae insertion.Cuticular pores and somatic setae not observed.Anterior sensilla arranged in the 6 + 6 + 4 pattern: six inner labial papilliform sensilla, six outer labial papilliform sensilla and four short cephalic setae (2 mm long), corresponding to 22% of head diameter.Head slightly set off.Amphidial fovea cryptocircular, located 10 mm from anterior end and occupying 57% of corresponding body diameter.Buccal cavity cuticularized.Cheilostoma rugae indiscernible under a light microscope.Three cuticularized teeth, one large dorsal tooth and two smaller ventrosublateral teeth.Pharynx (82 mm long) with terminal oval bulb.Reproductive system didelphic-amphidelphic, with outstretched ovaries.Anterior ovary situated to right side of intestine, posterior ovary to left side of intestine.Anterior and posterior ovary measuring 49 and 64 mm, respectively.Tail conical, about 3.6 times the anal body diameter.Manoel et al. (2024), PeerJ, DOI 10.7717/peerj.1735510/26

Table 5
Comparison of species Microlaimus modestus sp.n. with morphologically similar species (only males).Information (measurements/proportions) of males of the species Microlaimus that concomitantly have cephalic setae ≤50% of the head diameter; amphidial fovea between 1.1 and 1.5 times the head diameter; amphidial fovea >50% of head diameter.The measurements are expressed in micrometers, or if noted, as a percentage or ratio.Present (+) or absent (−); a, b, c, c′ = de Man's (1880) ratios; distance of amphidial fovea from anterior end in relation to head diameter (Amph/hd); percentage of the amphidial fovea diameter in relation to corresponding body diameter (amph%).