A temporary pond nearby the Prakaipetch Cave, Nadokkham Subdistrict, Na Duang District, Loei Province, northeastern Thailand; 17°54'23"N, 101°54'23"E; altitude: 420 m above sea level.

One adult male, NHMUK 2017.134, dissected and mounted in glycerol on one slide: collected on 5 August 2015 by S. Watiroyram.

One adult female, NHMUK 2017.135, dissected and mounted in glycerol on one slide: collected on same date by the same collector.

Ten adult females and males, NHMUK 2017.136–145, undissected and preserved in 70% ethanol in 1.5 ml microtube; 10 adult females and males, NPU 2017–001, undissected and preserved in 70% ethanol in 1.5 ml microtube: collected on same date by the same collector.

The right P5 Exp-2 of male in Mongolodiaptomus loeiensis sp. n. with principal spine slightly posterior to mid-outer margin and one spinous process each proximally and distally, fits the diagnostic features of the genus Mongolodiaptomus sensu Reddy et al. (2000).

The male of new species is most similar to M. calcarus by the segment 20 of right antennule with serrated spine, and the armature of P5: on right P5 (1) the presence of strong and stout coxal spine, (2) Exp-2 with similar shape and size, and (3) Exp-2 with strong and bent principal lateral spine; on left P5 (1) basis with long and narrow hyaline membrane on inner margin, and (2) Exp-2 with row of strong spinules on inner margin at distal half of segment. However, M. loeiensis sp. n. differs from M. calcarus by following morphological characters: (1) the new species with three chitinous processes on ventral surface of the right caudal ramus while M. calcarus with only two chitinous processes, (2) intercoxal plate of the new species with outgrowth process into two-spine like lobes at distal margin while M. calcarus without any outgrowth process, (3) basis of right P5 in the new species with inner hyaline membrane and with sub-globular chitinous process on caudal surface while M. calcarus without hyaline membrane, and with spur-like chitinous process, (4) the new species with long and slender end claw while it is typical short and robust in M. calcarus. Additional differences occur in female characteristics as follows: (1) the genital somite of M. loeiensis sp. n. with bulges on proximal, sub-middle, and middle region at right margin while M. calcarus is slightly convex on those region, (2) the right side of genital somite with spine located on outgrowth process in M. loeiensis sp. n. but it is directly inserted on somite in M. calcarus, (3) the left side of genital somite with slightly proximal dilated and spine inserted on small prominence while M. calcarus with obviously proximal rounded lobe and spine directly inserted on its segment.

Body length (Fig. 1A), measured from anterior margin of rostrum to posterior margin of caudal rami, 1.0–1.3 mm. Rostral spines (Figs 1B, 4B) with two teeth-like process in anterior margin. Prosome (Fig. 1A) ovoid, with cephalosome and 5 pedigers; pediger 4 and 5 fused, partly separated at lateral side. Pediger 5 (Figs 1C, 4A) with symmetrical postero-lateral wings, reaching proximal part of genital somite; each wing with one inner and one posterior spine on posterolateral margins (former spine smaller than later one). Urosome (Fig. 1A, D) consisting of 3-somites including genital somite, urosomite 2 and anal somite, approximately 1/2 as long as prosome. Genital somite (Figs 1C–F, 4A) exceeding in length urosomite 2, anal somite and caudal ramus combined. Right side: with dilated laterally in three regions on proximal half of segment length; with postero-laterally directed outgrowth (sub-conical process), extended beyond half of somite; with a short and blunt posterior spine on respective outgrowth. Left side: with regular margin, with a spine on dorso-lateral surface at proximal part, larger and sharper than the right spine. A pair of gonopores located beneath a genital operculum on mid-ventrally proximal part. Urosomite 2 symmetrical, shorter than latter somite. Anal somite (Figs 1A, D, 4C) slightly expanded at distal end, almost as long as caudal rami; anal operculum small and slightly concave on its posterior margin.

Caudal rami (Figs 1D, 4C) symmetrical, each ramus slightly expanded on distal end, about 1.6 times as long as wide; with a row of setules along inner and outer margins. Each ramus with six setae (seta II–VII): lateral (II) seta with smooth region on proximally outer margin; dorsal seta (VII) proximally jointed, bare, and longest.

Antennule (Fig. 1G) symmetrical, 25-segmented, reaching beyond the end of caudal setae. Setal formula starting from the first to the last segment (a = aesthetasc, s = spine): 1+a, 3+a, 1+a, 1, 1+a, 1, 1+a, 1+s, 2+a, 1, 1, 1+a+s, 1, 1+a, 1, 1+a, 1, 1, 1+a, 1, 1, 2, 2, 2, 4+a.

Antenna (Fig. 2A) 11-segmented. Coxa with one seta on distal corner. Basis with two inner setae on distal corner. Exp-1–6 with 1, 3, 1, 1, 1, and 1 inner seta, respectively. Exp-7 with one inner and three apical setae. Enp-1 with two inner setae. Enp-2 with nine inner and seven apical setae.

Mandible (Fig. 2B) with four strongly chitinized teeth and a single seta on gnathobase. Basis with four inner setae. Enp 2-segmented: Enp-1 with four inner setae, Enp-2 with nine apical setae plus tiny spinules along outer margin. Exp 4-segmented, with 1, 1, 1, and 3 setae, respectively.

Maxillule (Fig. 2C) with seven spines and six setae on praecoxal arthrite. Coxal endite and coxal epipodite with three and nine setae, respectively. Proximal and distal endites each with four setae; basal exite with one seta. Enp reduced, represented by eight apical setae. Exp with six setae plus a row of setules on median margin.

Maxilla (Fig. 2D) with three setae on proximal praecoxal and coxal endites, and distal praecoxal and coxal endites. Allobasis with three setae. Enp reduced to two segmented, each with three setae.

Maxilliped (Fig. 2E) with four endites on syncoxa: 1, 2, 3, 3 setae inserted on respective endites; endite 4 with tiny spinules on distal corner. Basis with three setae, ornamented with spinules on proximal half of segment. Enp 6-segmented, with 2, 3, 2, 2, 2, and 4 setae, respectively.

P1–P4 (Figs 3A–D), biramous, Exp longer than Enp. P1 with 3-segmented Exp and 2-segmented Enp, P2–P4 with 3-segmented Exp and Enp. Exp and Enp with longitudinal setules on inner and outer margin, respectively. Armature formula of P1–P4 as follows (legend: outer-inner seta/spine; outer-apical-inner; Arabic numerals represent setae, Roman numerals represent spines):

P5 (Figs 3E–F, 5F–I) symmetrical. Coxa with a blunt, stout spine on protuberance at distal outer corner on caudal surface. Basis with a thin, bare seta on outer margin, reaching middle of Exp-1. Exp 3-segmented and Enp 2-segmented. Exp-1 sub-rectangular, more than 2.0 times as long as wide, slightly longer than Enp. Exp-2 sub-triangular, drawn out into claw-like, with a row of strong spinules along middle of both margins; with short and bare lateral spine. Exp-3 reduced into small prominence, with short lateral spine, and long apical seta. Enp subconical, Enp-1 rectangular, slightly shorter than wide. Enp-2 narrowed distally, with a circular row of spinules on distal end.

Additional ornamentation of P1–P5 as in Figs 3A–F, 5F–I.

Adult females with a single egg sac containing 8–10 eggs.

Body length (Fig. 6A) measured from anterior margin of rostrum to posterior margin of caudal rami, 0.9 –1.1 mm (mean = 1.0 mm, n = 5), smaller than female. Prosome as in female but lateral wings not well developed compared to those in female, pediger 5 without inner spine inserted on dorso-posterior margin of each wing. Urosome (Figs 4D, 6B–C) 5-segmented and asymmetrical, oriented downward to right side. Genital somite (Fig. 6B) dilated postero-laterally on right side accompanied with a small seta on distal corner. Urosomites 2–3 (Figs 4D, 6C) with a field of long hairs ventrally along middle of segment. Urosomite 4 (Figs 4D–E, 6B) with posterolateral dilated on right side; dorso-posterior margin expanded beyond anal operculum. Anal somite (Figs 4D–E, 6B–D) similar to female but asymmetrical, right side located at lower position than opposite side. Caudal rami (Figs 4D–E, 6B–D) similar to female in setation but having different shape and ornamentation on right ramus. Ramus asymmetrical and cylindrical shaped: right ramus with three ventral chitinous prominences: two teeth-like on large bulge at proximal region (outer one small and with sharped tip; inner one large and with rounded tip), and one semi-circular ridge located above insertion of caudal seta IV and V.

Antennule (Figs 4F–G, 6E) asymmetrical, reaching beyond end of caudal setae. Right antennule 22-segmented, setal formula (a = aesthetasc, s = spine): 1+a, 3+a, 1+a, 1, 1+a, 1, 1+a, 1+s, 2+a, 1+s, 1+s, 1+a+s, 1+a+s, 2+a+s, 2+a+s, 2+a, 2+s, 1+s, 2, 3+s, 2, 4+a; geniculated between segment 18 and 19; segment 20 (antepenultimate) with comb-like spine (3–5 teeth).

Left antennule, antenna, mouthparts, and P1–P4 similar to those in female.

P5 (Figs 5A–E, 7) asymmetrical, right leg reaching beyond caudal setae. Intercoxal plate with two tooth-like lobes on distal margin, its tip bent forward to left leg. Right P5: coxa with strong and stout spine inserted on well-developed posterior lobe on caudal surface. Basis with long and narrow hyaline lamella at inner margin; small chitinous prominence (sub-globular in shape) located approximately mid-distal of segment on caudal surface; distal outer margin with short, thin seta on frontal surface. Exp 2-segmented: Exp-1 small and shorter than wide, approximately 0.6 times as long as wide; with two prominent knobs on caudal surface; distal outer corner produced. Exp-2 enlarged, approximately 2.0 times as long as wide; proximal and distal part enlarged in similar size, inner margin slightly convex and outer margin concave; with two minute knobs and one principal (lateral) spine at outer margin (knob-like projection located on proximal and distal region; lateral spine inserted slightly anterior to mid-outer margin of segment). Lateral spine strong, cylindrical, approximately 1/2 of segment length; its tip bent upward to posterolateral direction on caudal view. End claw sickle-shaped, strong and pointed tip; approximately 1.5 times as long as Exp-2, inner and outer margins smooth. Enp 1-segmented, conical, reaching to proximal expansion of Exp-2, with cluster of spinules at rounded tip. Left P5: coxa with thin seta inserted on posterior lobe at distal inner corner, exceeding basis, and posterolateral margin with semi-circular concave on caudal surface. Basis with long, narrow hyaline lamella at inner margin (small size than right P5); with long, thin seta at posterolateral margin on caudal surface. Exp 2-segmented: Exp-1 longer than wide, gradually tapering posteriorly, inner margin concave and outer one convex; with field of setules on inner margin at distal end. Exp-2 smaller than Exp-1, conical; with seta on inner margin at distal end on frontal surface, as long as segment; with a cluster of strong spinules along inner margin; apical process stout, bare, and short. Enp 1-segmented, longer than Exp-1, with a cluster of spinules at its tip.

The specific name loeiensis refers to the place “Loei” where the new species was first recognized. The name with the Latin suffix “-ensis” is an adjective for place.

Mongolodiaptomus loeiensis sp. n. was found in a temporary pond with a mean water temperature of 25.5 °C, conductivity 259 μS/cm, and pH 7.6. The new species was a single calanoid copepod occurring in type locality. However, it was found co-occurring with other microcrustaceans, i.e. Diaphanosoma excisum Sars, 1885, Thermocyclops decipiens (Kiefer, 1929), and Mesocyclops thermocyclopoides Harada, 1931. In over 3,000 samples collected from Thailand (59 samples from Loei Province provided by the first author and about 3,000 samples from other provinces by the second author and her colleagues), the new species can by considered as a rare species because it was present only in the type locality from Loei Province (Sanoamuang 2002).

The occurrence of the new species is similar to that of Phyllodiaptomus thailandicus Sanoamuang & Teeramaethee, 2006 and Tropodiaptomus cf. ruttneri (Brehm, 1923), which have so far been known as rare species in Thailand and here each species was found in a single location or in about 2% of collected samples. P. thailandicus has hitherto been found in six provinces; Prachinburi, Chanthaburi, Sa Kaeo, Suphanburi, Kanchanaburi, and Chumphon (Sanoamuang and Teeramaethee 2006, Koompoot and Sanoamuang 2012) whereas Tropodiaptomus cf. ruttneri was only known from Phayao and Nan provinces (Sanoamuang 2002). The findings of P. thailandicus and Tropodiaptomus cf. ruttneri in this study are a new record to northeastern Thailand and provide more understanding on its distribution range in the country. Vietodiaptomus blachei (Brehm, 1951) was found in a few samples or approximately 9% of collected samples whereas P. praedictus, M. botulifer, and M. calcarus were frequently found in about 41, 39 and 6% of collected samples, respectively.

The ranges of water variables for four other diaptomids species collected from Loei Province areas are as follows: Tropodiaptomus cf. ruttneri in water with temperature 26.9–30.1 °C, conductivity 81–100 μS/cm, and pH 7.1–7.3; P. thailandicus = 25.0–30.1 °C, 310–360 μS/cm, and pH 7.3–7.5; M. calcarus = 23.6–31.2 °C, 197–287 μS/cm, and pH 7.6–8.0; M. botulifer = 26.0–30.7 °C, 57–343 μS/cm, and pH 7.2–7.8; P. praedictus = 24.6–31.8 °C, 57–386 μS/cm, and pH 7.6–8.2; V. blachei = 26.0–31.4 °C, 92–527 μS/cm, and pH 7.1–8.1.