Mongolodiaptomus nakhonphanomensis sp. nov. (Copepoda, Calanoida, Diaptomidae) from north-eastern Thailand, with a note on M. pectinidactylus (Shen & Tai, 1964)

Mongolodiaptomus nakhonphanomensis sp. nov. (Copepoda, Calanoida, Diaptomidae) from north-eastern Thailand, with a note on M. pectinidactylus (Shen & Tai, 1964) Santi Watiroyram1, Piyathida Monongdern1, Kamonwan Koompoot2 1 Division of Biology, Faculty of Science, Nakhon Phanom University, Nakhon Phanom 48000, Thailand 2 DZVA Unit, Walai Rukhavej Botanical Research Institute, Mahasarakham University, Maha Sarakham 44150, Thailand https://zoobank.org/101DA9B5-8A4E-446F-95C9-E8B64A6D4FEF Corresponding author: Santi Watiroyram ([email protected]) Academic editor: Kay Van Damme ♦ Received 10 June 2025 ♦ Accepted 18 September 2025 ♦ Published 8 October 2025 Abstract A new species of diaptomid copepod, Mongolodiaptomus nakhonphanomensis sp. nov., is described from surface water bodies in Nakhon Phanom Province, north-eastern Thailand. The new species is assigned to the M. mephistopheles (Brehm, 1933) species group and is morphologically most similar to M. pectinidactylus (Shen & Tai, 1964), a rare species in Thailand. It can be distinguished from M. pectinidactylus by the following characters: I) In females, the new species has asymmetrical lateral wings on pediger 5 and the second exopod of the fifth leg, whereas M. pectinidactylus (Shen & Tai, 1964) shows symmetry in these structures; II) the female of the new species has a shorter and more robust seta on the second exopod of the fifth leg compared to that of M. pectinidactylus; III) in males, the new species lacks a spinular row on the basis of the right fifth leg, which is present in M. pectinidactylus; IV) the second exopod of the male fifth leg in the new species is distally expanded, whereas it is oblong in M. pectinidactylus; V) the lateral spine of the second exopod of the male fifth leg is bent outwards in the new species, whereas it is straight in M. pectinidactylus. A brief re-description of M. pectinidactylus and a comparison with the new species are provided. This discovery raises the number of recognised species in the genus Mongolodiaptomus to fifteen worldwide. A biogeographical map is also presented. Key Words Endemic species, Indo-malayan realm, M. pectinidactylus, Songkhram River Basin Introduction The genus Mongolodiaptomus Kiefer, 1937 is primarily distributed in inland regions of Southeast Asia (SEA) and represents the most diverse diaptomid genus in Thailand (Sanoamuang and Koompoot 2024). Six species have more extensive ranges, reaching northwards into south China (Guangdong, Guangxi and Hainan Provinces): M. birulai (Rylov, 1923), M. calcarus (Shen & Tai, 1965), M. formosanus Kiefer, 1937, M. gladiolus (Shen & Lee, 1963), M. pectinidactylus (Shen & Tai, 1964) and M. uenoi (Kikuchi, 1936). Amongst these, only M. birulai (Rylov, 1922) has been reported as far north as central China (Hunan Province), north China (Hebei and Heilongjiang Provinces) and east China (Fujian, Jiangsu and Zhejiang Provinces), as well as Taiwan (Li et al. 2018). In addition, three species have been recorded from cave habitats: M. botulifer (Kiefer, 1974) and M. malaindosinensis (Lai & Fernando, 1978) (Thailand) and M. birulai (Rylov, 1923) (China) (Li et al. 2018; Watiroyram 2021). Two species remain doubtful and are subject to differing opinions amongst taxonomists regarding whether they represent distinct species or are synonyms, namely M. birulai and M. formosanus (Li et al. 2018; Sanoamuang and Watiroyram 2018; Ranga Reddy et al. 2000; Sanoamuang and Koompoot 2024). Examination of material from both species is required in future studies to clarify their taxonomic status. Due to the lack of speciZoosyst. Evol. 101 (4) 2025, 1965–1983|DOI 10.3897/zse.101.161679 Copyright Watiroyram, S. et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. zse.pensoft.net Watiroyram, S. et al.: A new species of Mongolodiaptomus Kiefer, 1938 from north-eastern Thailand 1966 men examination and the absence of detailed published descriptions supporting their status, we follow the World Copepoda Database in recognising them as distinct species in the present study (Walter and Boxshall 2025). Thus, there are currently 14 previously known species worldwide, including ten recorded from Thailand, with the exception of M. birulai, M. formosanus, M. gladiolus and M. mephistopheles (Brehm, 1933) (Sanoamuang and Koompoot 2024; Walter and Boxshall 2025). During our examination of samples collected from tributaries of the Mekong River Basin in Thailand, eight known and one undescribed species of Mongolodiaptomus were recorded (Koompoot and Watiroyram 2025). Most of Mongolodiaptomus species previously reported from the country, except M. phutakaensis Sanoamuang & Koompoot, 2024, were found during this field trip, providing an excellent opportunity for an intensive taxonomic study. This resulted in clear morphological differentiation amongst closely-related species, along with changes in species composition and distribution (Koompoot and Watiroyram 2025). These findings led to the description of a new species, as well as the recording of M. pectinidactylus (Shen & Tai, 1964), a rare species observed in the present study. A biogeographic map of the genus Mongolodiaptomus is also presented, based on available published literature. Materials and methods Site description The Nawa District, situated in the westernmost part of the Nakhon Phanom Province in Thailand, comprises six sub-districts and spans an area of approximately 520 km2 (Fig. 1A, B). It is one of twelve districts across three provinces located in the lower floodplain of the Songkhram River Basin (Blake and Pitakthepsombut 2006). The Songkhram River is a major tributary of the Mekong River that flows through the northern part of north-eastern Thailand. It originates in the Sakhon Nakhon Province and flows eastwards through the Udon Thani, Sakhon Nakhon and Bueng Kan Provinces before joining the Mekong River at Ban Chai Buri City in the Nakhon Phanom Province (Hortle and Suntornratana 2008). Sampling and specimen preparation Forty-eight sampling sites across six sub-districts of the Nawa District were selected for copepod collection conducted between July and August 2023 (Fig. 1A, B). Samples were collected qualitatively using a hand net and a plankton net with a 60 µm mesh size from various types Figure 1. Sampling site and two habitats of Mongolodiaptomus nakhonphanomensis sp. nov.: A. Location of the Nakhon Phanom Province in the north-eastern Thailand (indicated in yellow); B. Location of the Nawa District within the Nakhon Phanom Province (indicated in green) and sampling sites (indicated in red spots); C. Irrigation canal (temporary waterbody); D. Permanent pond. Zoosyst. Evol. 101 (4) 2025, 1965–1983 zse.pensoft.net 1967 of waterbodies, including rice fields, roadside canals, irrigation canals, buffalo bogs, ponds and reservoirs (Fig. 1C, D). Samples were immediately fixed in approximately 5% formalin on spot. Water quality parameters, including pH, temperature, conductivity and total dissolved solids (TDS), were measured at each sampling site using a multiprobe device (Laqua Horiba, WQ-310-K). Adult animals were picked out and preserved with 70% ethanol in 1.5ml microtubes in the laboratory. Adult specimens were dissected under an Olympus SZ51 stereomicroscope in a mixture of glycerol and 70% ethanol (ratio ~ 1:10 v/v). Dissected specimens were mounted in pure glycerol and sealed with transparent nail polish. Permanent slides with dissected animals were examined with an Olympus compound microscope (CX31) at 1000 times magnification. Pencil drawings were produced with a drawing tube (Olympus U-Da) mounted on a compound microscope. The drawings were subsequently scanned and finalised using the graphic software Procreate version 1.1. Specimens were selected for scanning electron microscopy (SEM) and photographed using a LEO 1450 VP scanning electron microscope, following the protocol of Watiroyram and Sanoamuang (2017). The morphological terminology follows Huys and Boxshall (1991). Type specimens are deposited at the Thailand Natural History Museum, Pathum Thani, Thailand (THNHM) and the Nakhon Phanom University, Faculty of Science, Thailand (NPU). Abbreviations The following abbreviations are used throughout the text and figures: a, aesthetasc; Enp, endopod; Exp, exopod; Exp/Enp-n, exopodal segment n/endopodal segment n; P1–P5, swimming legs 1–5; s, spine (s). Associated fauna In total, during the study, fourteen species of Calanoida and six species of Cyclopoida were identified (Table 1). Amongst the species recorded, Mongolodiaptomus nakhonphanomensis sp. nov. is new to science and is described herein. The most frequently encountered species were Mongolodiaptomus malaindosinensis, Eodiaptomus draconisignivomi, Mesocyclops affinis, and M. thermocyclopoides. Taxonomic section Order Calanoida Sars, 1903 Family Diaptomidae Baird, 1850 Genus Mongolodiaptomus Kiefer, 1937 Mongolodiaptomus nakhonphanomensis sp. nov. https://zoobank.org/D6BB71C2-3AF1-40F7-B965-D1493B296C0C Figs 2–7 Type locality. A pond in the Nong Hua Ngua Village (Fig. 1D), Na Khun Yai Sub-district, Na Wa District, Nakhon Phanom Province, north-eastern Thailand; coordinates: 17°30'01.47"N, 104°08'03.35"E, altitude: 153 m above sea level. Material examined. Holotype: one adult male dissected and mounted on one slide (THNHM-IV-21117); allotype: one adult female dissected and mounted on one slide (THNHM-IV-21118); paratypes: three adult males and three adult females dissected and mounted on one slide each (NPU 2025–01-04). All specimens collected from the type locality on 30 July 2023 by Piyathida Monongdern. Etymology. The specific name nakhonphanomensis refers to Nakhon Phanom Province in north-eastern Thailand, where the new species was discovered. The Latin suffix “-ensis” denotes origin or place. Description. Adult male. Body length excluding caudal setae 2.1 mm (holotype) (other males: mean = 2.0, n = 3) (Fig. 2A). Rostrum (Fig. 1B) with two spiniform processes. Pedigers 4 and 5 incompletely fused laterally (Fig. 1A). Lateral wings (Fig. 2A) asymmetrical: left wing prolonged downwards, the right wing expanded distally in lateral direction; both tipped with thin spine. Urosome (Fig. 2E) orientated obliquely posteriad towards right side. Genital somite short, asymmetrical, with small Table 1. Copepod species recorded from the study area. Calanoid species Cyclopoid species 1. Dentodiaptomus javanus (Grochmalicki, 1915) 1. Mesocyclops affinis Van de Velde, 1987 2. Eodiaptomus draconisignivomi Brehm, 1952 2. M. thermocyclopoides Harada, 1931 3. E. phuphanensis Sanoamuang, 2001 3. Microcyclops rubellus (Lilljeborg, 1901) 4. E. sanoamuangae Ranga Reddy & Dumont, 1998 4. Thermocyclops crassus (Fischer, 1853) 5. Heliodiaptomus elegans Kiefer, 1935 5. T. decipiens (Kiefer, 1929) 6. Mongolodiaptomus botulifer (Kiefer, 1974) 6. T. wolterecki Kiefer, 1938 7. M. dumonti Sanoamuang, 2001 8. M. malaindosinensis (Lai & Fernando, 1978) 9. M. nakhonphanomensis sp. nov. 10. M. pectinidactylus (Shen & Tai, 1964) 11. Neodiaptomus yangtsekiangensis Mashiko, 195 12. Phyllodiaptomus parachristineae Sanoamuang & Watiroyram, 2023 13. P. praedictus Dumont & Ranga Reddy, 1994 14. Vietodiaptomus blachei (Brehm, 1951) zse.pensoft.net Watiroyram, S. et al.: A new species of Mongolodiaptomus Kiefer, 1938 from north-eastern Thailand 1968 Figure 2. Mongolodiaptomus nakhonphanomensis sp. nov., adult male (holotype): A. Habitus, dorsal view; B. Rostrum; C, D. Right antennule, segments 1–13 and 14–21, respectively; E. Urosome, ventrolateral view. seta on outer distal margin on right side. Urosomites 2–3 each with long hairs on mid-ventral surface; former somite longer than latter. Urosomite 4 with right posterolateral margin dilated; dorsoposterior margin expanded, overlapping part of following somite. Anal somite symmetrical; anal operculum small, with posterior margin slightly concave. Caudal rami (Fig. 2A, E) symmetrical; each ramus expanded distally, with setules along both inner and outer margins; with six setae (setae II–VII): setae II–VI plumose, seta VII bare. Right ramus without chitinous processes or ventral prominences. Antennules asymmetrical, with the right antennule transformed and geniculated, exhibiting sexual dimorphism. Right side antennula (Fig. 2C, D) 22-segmented. Setal formula: 1+a (I), 3+a (II), 1+a (III), 1+a (IV), 1+a (V), 1 (VI), 1+a (VII), 1+s (VIII), 2+a (VIX), 1+s (X), 1+s (XI), 1+a+s (XII), 1+a+s (XIII), 2+a+s (XIV), 2+a+s (XV), 2+a+s (XVI), 2+s (XVII), 1+s (XVIII), 2+s (XIX), 3+s (XX), 2 (XXI), 4+a (XXII). Segments 13 to 20 enlarged and elongated; segments 18 and 19 geniculated; segment 20 (antepenultimate) with comb-like process (four teeth). Left side antennula (Fig. 4A) 25-segmented, reaching be- Zoosyst. Evol. 101 (4) 2025, 1965–1983 zse.pensoft.net 1969 Figure 3. Mongolodiaptomus nakhonphanomensis sp. nov., light microscope photograph of P5 male, posterior view. yond the caudal setae. Setal formula as follows: 1+a (I), 3+a (II), 1+a (III), 1 (IV), 1+a (V), 1 (VI), 1+a (VII), 1+s (VIII), 2+a (IX), 1 (X), 1 (XI), 1+a+s (XII), 1 (XIII), 1+a (XIV), 1 (XV), 1+a (XVI), 1 (XVII), 1 (XVIII), 1+a (XIX), 1 (XX), 1 (XXI), 2 (XXII), 2 (XXIII), 2 (XXIV), 4+a (XXV). Antenna (Fig. 4B): coxa with one seta on inner side; basis with two setae on inner distal margin. Exp seven-segmented, with 1, 3, 1, 1, 1, 1, 1 setae laterally, three setae apically. Enp two-segmented; Enp-1 with two setae at 1/2 length, Enp-2 with nine setae along inner margin and seven setae apically, accompanied by short row of spinules on distal outer margin. Mandible (Fig. 4C): coxa with five bi-cuspidate teeth and one blunt tooth on gnathobase; with small dorsal seta on distolateral corner. Basis with four setae along inner margin. Enp two-segmented; Enp-1 with four setae on distal inner corner. Enp-2 with nine setae apically, accompanied by row of spinules along outer margin. Exp 4-segmented, with 1, 1, 1, and 3 setae, respectively. Maxillule (Fig. 4D): praecoxal arthrite with ten robust setae apically and four fine setae on dorsal side. Coxal endite with three fine setae; coxal epipodite with eight robust setae. Basis with two endites, each armed with four setae; basal exite with one small seta. Enp reduced, represented by eight setae apically. Exp with seven apical setae. Maxilla (Fig. 4E): proximal and distal praecoxal endites with four and three setae, respectively. Two coxal endites with three setae each. Allobasis with three setae on basal endite. Enp two-segmented, each with three setae. Maxilliped (Fig. 4F): coxa with four endites, bearing 1, 2, 3 and 4 setae, respectively. Basis with three setae distally plus row of strong spinules along inner margin. Enp six-segmented, with 2, 3, 2, 2, 2 and 4 setae, respectively. P1–P4 (Fig. 5A–D). P1 with two-segmented Enp and three-segmented Exp; P2–P4 with three-segmented Enp and Exp; Exp longer than Enp. Coxa on P1–P4 with pinnate seta at distal inner corner. P4 basis with small seta at distal outer margin. Exp-1 P1–P4 with one robust spine on outer margin and pinnate seta on inner margin. Exp-2 as Exp-1, but P1 without spine on outer margin. Exp-3 with three pinnate setae on inner margin and three pinnate setae apically on P2–P4, but two on P1 inner margin. Enp-1 P1–P4 with one pinnate seta on inner margin. P1 Enp-2 with three setae on inner margin, two setae apically, one pinnate seta on outer margin. P2–P4 Enp-2 with two pinnate setae on inner margin. Enp-3 P2–P4 with three setae on inner margin, two setae apically and two pinnate setae on outer margin. P5 (Figs 3, 5E, F) asymmetrical, with right leg markedly enlarged. Intercoxal plate with rounded distal margin. Right side of P5: coxa with spiniform seta on posterior lobe, not reaching mid-length of basis. Basis approximately twice as long as wide, with narrow hyaline lamella along proximal inner margin and hyaline prominence at mid-distal region; with small seta at 3/4 length of outer margin. Exp three-segmented: Exp-1 shorter than wide, with unproduced distal outer corner and semicircular hyaline prominence distally. Exp-2 elliptical, with distal portion broader than proximal one, about 2.5 times as long as wide; with strong blunt spine at 1/2 length of outer margin and blunt process distally. Spine curved backwards, slightly shorter than half segment length. Exp-3 as sickle-shaped claw, about 1.5 times as long as Exp-2; with inner margin serrated. Enp 1-segmented, conical, gradually tapering distally, reaching the mid-length of Exp-2; with spinulated tip. Left side of P5: coxa with thin seta at distal inner margin, reaching beyond mid-half of basis. Basis with narrow hyaline lamella along distal inner margin; with short, thin posterolateral seta on outer margin. Exp three-segmented (Fig. 5E, F): Exp-1 longer than wide, gradually tapering in posterior end; with a hair field at distal half of inner margin. Exp-2 oval, smaller than Exp-1; with inner robust seta, accompanied by spinular field along inner margin. Exp-3 reduced to a short, bare segment with rounded tip. Enp one-segmented, conical, gradually tapering distally; reaching end of Exp-1, with spinulate tip. Adult female. Body length excluding caudal setae 2.5 mm (allotype) (other females: mean = 2.3, n = 3) (Fig. 6A). Antennules symmetrical, with setal formula identical to that of the left antennule of male. Antenna and mouth appendages as in male. Cephalosome and pediger 1, as well as pedigers 4 and 5, completely fused. Pediger 5 (Figs 6A, 7B) with asymmetrical posterolateral wings: right wing short, rounded, with strong large spine dorsally, tiny spine on posterior margin; left wing longer, subtriangular, with tiny spine dorsally, larger spine poste- zse.pensoft.net Watiroyram, S. et al.: A new species of Mongolodiaptomus Kiefer, 1938 from north-eastern Thailand 1970 Figure 4. Mongolodiaptomus nakhonphanomensis sp. nov., adult male (holotype): A. Left antennule; B. Antenna; C. Mandible; D. Maxillule; E. Maxilla; F. Maxilliped. riorly. Urosome (Fig. 7A) three-segmented; genital double-somite longer than two subsequent urosomites and caudal rami combined. Genital double-somite (Fig. 7A) asymmetrical; left side broadly expanded laterally at the proximal part, with strong spine; right side with proximal part slightly expanded laterally. Both spines articulated, inserted on somite prominence. Urosomite 2 symmetrical, shorter than wide, partly covered by posterior end of genital double-somite. Anal somite and caudal rami as in male (Fig. 7A). Zoosyst. Evol. 101 (4) 2025, 1965–1983 zse.pensoft.net 1971 Figure 5. Mongolodiaptomus nakhonphanomensis sp. nov., adult male (holotype): A. P1; B. P2; C. P3; D. P4; E. P5, posterior view; F. P5, frontal view. zse.pensoft.net Watiroyram, S. et al.: A new species of Mongolodiaptomus Kiefer, 1938 from north-eastern Thailand 1972 Figure 6. Mongolodiaptomus nakhonphanomensis sp. nov., line drawing and SEM photographs of adult female: A. Habitus, dorsal view; B. P5, posterior view; C. P5, frontal view (1 = right side of Exp-2, 2 = left side of Exp-2); D. Right P5 Exp-2–3, posterior view. P5 (Figs 6B, C, 7C, D) asymmetrical. Coxa with a stout spine on a prominence at the distolateral corner in dorsal view; coxal spine reaching proximal margin of Exp-1. Basis with thin seta at 1/2 length of outer margin. Exp three-segmented, Enp two-segmented. Exp-1 rectangular, about twice as long as wide. Exp-2 triangular: right side shorter and stouter than left side (Fig. 6C1–2); inner margin with row of strong spinules; two longitudinal ridges ventrally; distal lateral seta strongly modified into short, stout spine (Fig. 6D). Exp-3 reduced to short, stout spine and long, thin seta. Enp cylindrical, extending beyond half-length of Exp-1. Enp-1 shorter than Enp-2; Enp-2 with circular row of spinules at narrowed apical end. Variations. In addition to body size differences, segment 20 of the right antennule in males have a comb-like process with two to five teeth (Fig. 2D). The female P5 Enp varies in length, extending from the middle to the distal end of Exp-1. The size of the hyaline lamella on the right male P5 basis also varies, appearing in long and narrow on the inner margin and rounded (Fig. 5E) in the proximal-distal region. Zoosyst. Evol. 101 (4) 2025, 1965–1983 zse.pensoft.net 1973 Figure 7. Mongolodiaptomus nakhonphanomensis sp. nov., adult female (allotype): A. Urosome, dorsal view; B. Pediger 5 and part of genital double-somite, dorsal view; C. P5, posterior view; D. P5, frontal view. zse.pensoft.net Watiroyram, S. et al.: A new species of Mongolodiaptomus Kiefer, 1938 from north-eastern Thailand 1980 Figure 10. Distribution of the genus Mongolodiaptomus Kiefer, 1937: A. In southeast Asia and China; B. In north-eastern Thailand. Zoosyst. Evol. 101 (4) 2025, 1965–1983 zse.pensoft.net 1981 from an unspecified locality in Thailand, but its presence has been confirmed in roadside canal of the Sakon Nakhon Province, north-eastern Thailand (Ranga Reddy et al. 1998; Koompoot and Watiroyram 2025); (10) M. nakhonphanomensis sp. nov., was found from both temporary and permanent waterbodies in the Nakhon Phanom Province, north-eastern Thailand (the present study); (11) M. mekongensis has been recorded from various types of waterbodies within the Mekong River system in north-eastern Thailand, Laos, Cambodia and southern Vietnam (Boonmak and Sanoamuang 2022; Chaicharoen and Sanoamuang 2022; Koompoot and Watiroyram 2025). In addition, it has been reported outside the Mekong River system from Hainan Island in southern China (Wei et al. 2023); (12) M. pectinidactylus was first described from the Hainan and later recorded from Guangdong, both in southern China (Shen and Tai 1964; Li et al. 2018). This species has been occasionally found in various types of waterbodies in four provinces of north-eastern Thailand (Sanoamuang 2002; Koompoot and Watiroyram 2025) and one province of northern Vietnam (Tran et al. 2016). Species in the Sundaland (Sundaric) subregion (13) M. mephistopheles was previously misidentified in Thailand and Malaysia, but it is actually endemic to Java (Indonesia) (Brehm 1933; Lai and Fernando 1978, 1980; Ranga Reddy et al. 1998). Species in the Indo-Chinese and Wallacean subregions (14) M. birulai was originally described from China and has been reported from several localities under the recorded name of M. birulai and M. formosanus, including northern (Hebei, Heilongjiang), eastern (Fujian, Jiangsu, Zhejiang), central and southern China (Guangdong, Guangxi, Hainan, Hunan) (Li et al. 2018). It was later reported from Taiwan (Young et al. 2013), Vietnam (Tran et al. 2016) and the Philippines (Lopez et al. 2017); (15) M. uenoi was first described from Taiwan (Kikuchi 1936); and subsequently reported from West Java and West Kalimantan, Indonesia (Lai and Fernando 1980; Ranga Reddy et al. 2000; Alekseev et al. 2013, 2016), Kon Tum and Dak Lak Provinces in central Vietnam (Tran et al. 2016) and Hainan Island in southern China (Li et al. 2018). However, due to incomplete morphological descriptions, the occurrence of this species in Indonesia remains uncertain (Ranga Reddy et al. 2000). In north-eastern Thailand, it was found in both temporary and permanent waterbodies within the Chi River Basin, specifically in Chaiyaphum, Kalasin and Nong Bua Lam Phu Provinces (Ranga Reddy et al. 2000). However, M. uenoi was not observed in the Songkhram, Mun and Mekong River Basins during the recent field surveys, where M. mekongensis was found to be dominant (Koompoot and Watiroyram 2025). Conclusion A total of 48 samples collected from Nawa District, Nakhon Phanom Province, yielded five of the eleven Mongolodiaptomus species known from Thailand. One of them, M. nakhonphanomensis sp. nov., is described herein. The new species is most similar to M. pectinidactylus, a member of the M. mephistopheles (Brehm, 1933) species group, as they share unique characteristics, such as the morphology of the right male P5 Exp-2 and the absence of a chitinous prominence on the right caudal ramus in males. However, they can be easily distinguished by the presence or absence of a row of spinules on the left P5 basis, the shape of the principal lateral spine on the right P5 Exp-2 in males, as well as the morphology of the lateral wings and P5 Exp-2 in females. At present, although both species were collected from the study area, they have never been found together at the same locality and the new species has been found more frequently than M. pectinidactylus. The distribution of species in the study area, along with the biogeographic map generated from published literature, suggests that the genus Mongolodiaptomus has a hotspot in the Indo-Chinese subregion, particularly in the Southeast Asia. Acknowledgements This study was supported by the National Science, Research and Innovation Fund (NSRF) (Fundamental Fund: Grant No. SRI-27/2023; Proposal No. 66A134000008). We also thank the Editor of the World Copepoda Database for providing valuable literature. References Alekseev VR, Haffner DG, Vaillant JJ, Yusoff FM (2013) Cyclopoid and calanoid copepod biodiversity in Indonesia. Journal of Limnology 72(s2): 245–274. https://doi.org/10.4081/jlimnol.2013.s2.e12 Alekseev VR, Yusoff FM, Fefilova EB (2016) Continental copepod biodiversity in North-Eastern Borneo, Malaysia. Arthropoda Selecta 25(2): 183–197. https://doi.org/10.15298/arthsel.25.2.05 Blake DJH, Pitakthepsombut R (2006) Situational analysis: lower Songkhram River Basin, Thailand. 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