Metacyclops brancelji, Athibai & Wongkamhaeng & Boonyanusith, 2022

Athibai, Sujeephon, Wongkamhaeng, Koraon & Boonyanusith, Chaichat, 2022, Two new species of Metacyclops Kiefer, 1927 (Copepoda, Cyclopoida) from Thailand and an up-to-date key to the species recorded in Asia, European Journal of Taxonomy 787 (1), pp. 146-181 : 161-174

Metacyclops brancelji sp. nov.

urn:lsid:zoobank.org:act:C420CB65-D63C-4E04-A734-3F5B62D67043

Figs 10–15 ; Tables 1–2

Diagnosis

FEMALE. Body size moderate (0.89–0.96 mm; n = 6), without integumental pits. Posterior margin of second pedigerous somite undulated, those of third and fifth pedigerous somite with serrated hyaline frill. Genital double-somite dorsally with two sensilla and transverse suture, representing the remnant of ancestral articulation of the siXth thoracic somite and the first abdominal somite. Anal operculum developed, reaching insertion of caudal ramus; free margin smooth and concave. Caudal rami ca 2.6–2.9× as long as wide, ornamented with 3–4 spinules at anterior third length on lateral surface and few spinules at base of seta II, combined with a row of strong spinules latero-ventrally at base of seta III. Seta VI slightly shorter than seta III. Setal and spine formulae of exp-2 of P1–P4 5.5.5.5 and 3.4.4.3, respectively. P4 exp-2 with single apical spine; spine slightly shorter than segment. Inner spine of free segment of P5 longer than segment, ca 1.5× as long as segment; outer seta on P5 ca 2.5× as long as inner spine.

MALE. Body length 0.71–0.76 mm (n = 3). Caudal rami ca 2.5–2.8 × as long as wide. P6 with two elements; inner (ventral) spine robust, slightly shorter than outer seta.

Etymology

The name is a masculine noun in genitive singular, raised after Professor Dr Anton Brancelj (National Institute of Biology, Ljubljana, Slovenia) in honor of his great contribution to the diversity of subterranean Copepoda in Thailand.

Type material

Holotype THAILAND • ♀ (completely dissected and mounted on a slide in glycerol and sealed with nail polish); Satun Province, Phupha Phet Cave; 7°07′28.94″ N, 99°59′52.14″ E; 92 m a.s.l.; 17 Dec. 2014; C. Boonyanusith leg.; temporary pool; hand net; ZMB 34231 slide No. 5124.

Allotype THAILAND • ♂ (completely dissected and mounted on a slide in glycerol and sealed with nail polish); same collection data as for holotype; ZMB 34231 slide No. 5125.

Paratypes THAILAND • 1 ♀, 1 ♂ (each completely dissected and mounted on a slide in glycerol and sealed with nail polish); same collection data as for holotype; ZMB 34231 slide No. 5126–5127 • 1 ♀ (stored in a miXture of glycerol and 70% ethanol (ratio ~1: 10 v /v)); same collection data as for holotype; ZMB 34231a .

Additional material examined

THAILAND – Satun Province • 1 ♀, 1 ♂ (each completely dissected and mounted on one slide in glycerol and sealed with nail polish); Rakhang Thong Cave; 7°05′42.18″ N, 99°55′05.64″ E; 55 m a.s.l.; 31 Jul. 2015; C. Boonyanusith leg; rimstone; hand net; ZMB 34231 slide No. 5128–5129 • 1 ♂ (specimen stored in a miXture of glycerol and 70% ethanol (ratio ~1: 10 v /v)); same collection data as for preceding; ZMB 34231b • 2 ♀♀, 1 ♂ (processed for taking photographs by SEM); same collection data as for the preceding; collection of the third author (CB) . – Songkhla Province • 1 ♀ (completely dissected and mounted on a slide in glycerol and sealed with nail polish); Khao Nui Cave; 6°59′32.16″ N, 100°08′28.35″ E; 78 m a.s.l.; 15 Dec. 2014; C. Boonyanusith leg;; pool filled by dripping water; hand net; ZMB 34231 slide No. 5130 • 1 ♀ (specimen stored in a miXture of glycerol and 70% ethanol (ratio ~1: 10 v /v)); same collection data as for preceding; ZMB 34231c

Type locality

The new species was collected from a pool in Phupha Phet Cave, Manang District, Satun Province, southern Thailand ( Fig. 1A, D ). The cave is located in a limestone hill of the Nakhon Sri Thammarat Mountain range. Beyond the entrance there is a very large cave tunnel, separated to several rooms and decorated well with stalactites, stalagmites, flowstones and rimstones, with a wooden bridge throughout the cave and installed lights. The collecting point is about 100 meters from the entrance, being a large temporary pool in the room, named “Dok Boa Khwam” (Upside-down lotus flower). Water comes primarily from the epikarst zone of the cave, and the depth varies according to season, ranging from 5 cm in dry season to 40 cm in rainy season. In collecting date, the water was about 40 cm deep, covering an area of about 30 m 2, transparent and colorless.

Description

Adult female

Total body length, measured from tip of rostrum to posterior margin of caudal rami, 0.89–0.96 mm (mean = 0.94 mm; n = 6; holotype = 0.89 mm) ( Fig. 10A ). Body without integumental pits ( Fig. 11A–B ). Naupliar eye not discernible. Rostrum V-shaped in frontal view, with rounded tip, completely fused to cephalothoraX, with two sensilla laterally. Prosome ca 65% of body length and ca 1.87 × as long as length of urosome. CephalothoraX anteriorly oval, ca 32% of body length and ca 1.08 × as long as wide, with greatest width at posterior end; posterior margin smooth. Posterior margin of second pedigerous somite undulated; third pedigerous somite with serrated hyaline frill on posterior margin; posterior margin of fourth pedigerous somite smooth ( Figs 10A , 11A ). Fifth pedigerous somite with two transversal rows of spinules located between proXimal seta and free segment of P5, with two sensilla dorsally; posterior margin with serrated hyaline frill. Genital double-somite symmetrical, ca 0.82× as long as wide, tapering posteriorly, dorsally with two sensilla and transverse suture, representing the remnant of ancestral articulation; posterior margin with serrated hyaline frill ( Figs 10A–B , 11B ). Seminal receptacle with clear distinction between anterior and posterior lobes; anterior lobe short and wide; posterior lobe globular, narrower than anterior one. Second and third abdominal somites narrower than genital doublesomite, ca 56% of double-somite width, with serrated hyaline frill on posterior margin ( Figs 10A , 11B ). Anal somite with row of minute spinules latero-ventrally on posterior margin and two sensilla dorsally at base of anal operculum ( Fig. 10C ). Anal operculum developed, trapezoidal, reaching insertion of caudal ramus; free margin smooth and concave ( Fig. 10C ).

CAUDAL RAMI ( Figs 10C–D , 11C ). Relative short, ca 2.7 × as long as wide, with siX setae; all setae pinnate. Seta I absent. Seta II inserted at ⅓ of caudal ramus length. Seta III spiniform, inserted at posterior outer corner of ramus. Seta IV and seta V with breaking planes. Seta V, the longest, ca 0.33 × as long as body length. Seta VI slender, slightly shorter than seta III. Seta VII inserted dorso-medially at ⅕ of ramus length. Length ratio of caudal setae to ramus length, from seta II to seta VII: 0.27: 0.84: 3.82: 4.98: 0.70: 0.82. Lateral surface ornamented with 4–5 minute spinules located at anterior ⅓ of ramus length, with few minute spinules at base of seta II and a row of strong spinules latero-ventrally at base of seta III.

ANTENNULE ( Fig. 12A ). Eleven-segmented, reaching ca ⅔ of cephalothoraX; armature formula: 1-[8], 2-[4], 3-[6], 4-[2], 5-[1+I], 6-[2], 7-[3], 8-[2+ae], 9-[2], 10-[2+ae], 11-[8]. Fifth segment with short spine on posterior outer corner. Aesthetasc on eighth and tenth segments slender, inserted near outer seta, as long as seta. Eleventh segment with acrothek sub-apically.

ANTENNA ( Fig. 12B ). Four-segmented, comprising coXobasis and three-segmented enp; setal formula 3.1.9.7. Coxobasis robust, with three transversal rows of spinules on caudal surface and two smooth setae on inner distal corner; seta representing eXp spinulose, inserted on outer distal corner and reaching tip of enp-3. Enp-1 ca 1.5× as long as wide, with smooth seta on medial margin. Enp-2 ca 1.5 × as long as wide, with longitudinal row of minute spinules on outer margin and nine setae; seven setae inserted along medial margin and two setae inserted apically. Enp-3 ca 2.0 × as long as wide, with two longitudinal rows of minute spinules along outer margin and seven smooth setae apically; outermost seta shortest.

MANDIBLE ( Fig. 12C ). Gnathobase with strongly chitinized teeth on cutting edge and spinulose seta dorsally; seta completely fused to segment. Palp reduced, one-segmented, with one short, slender seta and two long, bipinnate setae; two long setae subequal in length, ca 10 × as long as shorter one.

MAXILLULE ( Fig. 12D ). Three-segmented, composed of robust praecoxa and two-segmented maxillulary palp, representing coxobasis and enp.Arthrite of praecoxa with three strong claw-like extensions apically and one spinulose seta sub-apically. PraecoXa with seven elements along medial margin; proXimalmost seta minute, sub-proximal seta robust and spinulose, three middle setae slender and smooth, sub-distal seta robust and smooth, distalmost seta robust and spinulose. Basal segment of palp with three elements apically; outer apical seta robust and armed with long spinules on outer margin; inner apical and subapical ones smooth. Exp reduced, represented by spinulose seta near lateral segment of palp. Enp represented by lateral segment of palp, with two setae apically and one seta sub-apically; all setae spinulose, subequal in length.

MAXILLA ( Fig. 12E ). Five-segmented. Praecoxa and coxa partly fused frontally. Praecoxal endite prominent, inserted medially, with one smooth and one spinulose setae apically. CoXa with two endites; proXimal endite with one smooth seta apically; distal endite rectangular, movable, with two spinulose setae apically; spinules on proXimal seta of distal endite relatively long, those of distal one minute. Basis with claw-like endite and two setae at base of claw; longest seta strong, inserted ventrally to claw; shorter one slender, inserted on caudal surface above the longest seta; concave margin of claw with oblique row of spinules; spinules fused to basis and increased in size from frontal spinule to caudal one. Enp twosegmented; enp-1 with two robust setae; enp-2 with strong seta apically and two smooth, slender setae sub-apically.

MAXILLIPED ( Fig. 12F ). Four-segmented, composed of syncoxa, basis and two-segmented enp. Syncoxa with two endites and ornamented with arch row of spinules on outer margin; proXimal endite with two subequal spinulose setae apically; distal endite with one spinulose seta. Basis with one seta on caudal surface; basal endite with one spinulose seta apically, with a row of long spinules on frontal surface. Enp-1 with strong spinulose seta. Enp-2 with three setae; apical seta strong, two other ones slender and smooth. P1‒P4 ( Fig. 13 ). Two-segmented enp and exp. Intercoxal sclerite of P1 with few spinules on distal prominences, those of P2‒P4 with 3‒6 smaller spinules. CoXa with one seta on distal inner corner. Basis with one seta laterally and hairy medially. Setal and spine formulae of eXp-2 of P1‒P4: 5.5.5.5 and 3.4.4.3, respectively. Armature of swimming leg as in Table 1.

P1 ( Fig. 13A ). Frontal and caudal surfaces of intercoXal sclerite bare; distal prominence with 2‒3 spinules. Lateral seta on basis ca 4 × as long as those of P2‒P4; inner seta pinnate, reaching mid of enp-2. EXp-1 with outer spine and inner seta. EXp-2 as long as wide, with three spines and five setae; apical spine ca 0.71 × as long as segment. Enp-1 with inner seta. Enp-2 ca 1.5× as long as wide, with outer seta inserted between claw-like eXpansion, apically with robust spine and seta, inner margin with three setae; outer seta ca 1.3 × as long as length of apical spine; apical spine strong, slightly curved, as long as segment.

P2 ( Fig. 13B ). IntercoXal sclerite as in P1, with 4‒5 spinules on distal prominence. Basis with two hooklike eXpansions: outer eXpansion located between insertions of eXp and enp; inner one smaller, located at the same place where the medial seta of basis of P1 inserted. Lateral seta on basis ca 0.25 × as long as that of P1. EXp-1 with outer spine and inner seta. EXp-2 ca 1.5× as long as wide, with four spines and five setae; apical spine slightly shorter than segment. Enp-1 with inner seta. Enp-2 ca 1.7× as long as wide, with outer seta inserted between claw-like expansion, apically with robust spine and seta, inner margin with four setae; outer seta ca 1.3× as long as length of apical spine; apical spine strong and straight, as long as segment.

P3. Frontal surface of intercoXal sclerite bare; caudal surface with transversal row of minute spinules and distal prominences with 4‒6 minute spinules ( Fig. 13C ). Basis, eXp, and enp similar to those of P2.

P4 ( Fig. 13D ). IntercoXal sclerite similar to that of P3; yet distal prominences with 3‒4 spinules. Distal margin of coxa with rows of spinules on caudal surface. Basis similar to those of P2 and P3. Exp-1 with outer spine. EXp-2 ca 1.4 × as long as wide, with three spines and five setae; spines relatively smaller than those of P1‒P3, apical spine ca 0.5× as long as segment. Enp-1 with inner seta. Enp-2 ca 1.7 as long as wide, with outer seta inserted between claw-like expansion, apically with robust spine, inner margin with four setae; outer seta ca 1.3 × as long as length of apical spine; apical spine ca 0.9 × as long as segment.

P5 ( Fig. 13E‒F ). One-segmented, inserted on postero-lateral corner of fifth pedigerous somite. ProXimal segment completely fused to somite, represented by lateral seta. Distal segment free, subquadrate, ca 1.1 × as long as wide, with one slender outer seta and one inner spine apically; inner spine ca 1.5× as long as segment and outer seta ca 2.5× as long as inner spine.

P6 ( Figs 10B , 11B ). Small, forming simple cuticular plate inserted latero-dorsally on genital doublesomite, and armed with one seta dorsally and two minute spiniform setae ventrally.

Adult male

Total body length, eXcluding caudal seta, 0.71–0.76 mm (mean 0.74 mm; n = 3; allotype = 0.71 mm) ( Fig. 14A ). Habitus smaller and slenderer than in female. Naupliar eye and rostrum as in female. Prosome ca 62% of body length and ca 1.57 × as long as length of urosome. CephalothoraX anteriorly oval, representing ca 32% of body length and ca 1.12 × as long as wide. Second to fifth pedigerous somites similar to those of female. Genital somite swollen on mediolateral margin ( Fig. 14A–B ); ca 25% length of urosome and ca 0.6× as long as wide, with hyaline frill latero-dorsally. First abdominal somite and two subsequent somites narrower than genital somite, ca 60% of genital somite width, with serrated hyaline frill on posterior margin. Anal somite and operculum similar to those of female.

CAUDAL RAMI. As long as that in female, ca 2.7× as long as wide. Armament and ornamentation similar to those of female. Length ratio of caudal setae to ramus length slightly different to that of female; ratio of caudal setae to ramus length from seta II to seta VII: 0.29: 0.82: 3.94: 5.50: 0.70: 0.97.

ANTENNULE ( Fig. 14C–E ). 16-segmented, geniculate. Armature formula as follows: 1-[8+3ae], 2-[4], 3-[2], 4-[2+ae], 5-[1], 6-[2], 7-[2], 8-[2], 9-[1+ae+I], 10-[2], 11-[2], 12-[I], 13-[2+ae], 14-[0], 15-[1], 16-[12]. Eleventh and thirteenth segments with pinnate seta each.

ANTENNA, MANDIBLE, MAXILLULE, MAXILLA, MAXILLIPED. Similar to those of female.

P1. Frontal and caudal surfaces of intercoXal sclerite bare and distal prominences with 2‒3 minute spinules ( Fig. 15A ). Coxa, basis, enp and exp similar to those of female.

P2. IntercoXal sclerite as in P1, distal prominences with 4‒5 minute spinules ( Fig. 15B ). CoXa, basis, enp and exp similar to those of female.

P3. Frontal surface of intercoxal sclerite bare, caudal one with transversal row of minute spinules and distal prominences with 4‒5 minute spinules ( Fig. 15C ). CoXa, basis, enp and eXp similar to those of female.

P4. Frontal surface of intercoxal sclerite bare, caudal one with transverse row of minute spinules and distal prominences with 3‒4 minute spinules ( Fig. 15D ). CoXa, basis, enp and eXp similar to that of female; apical spine on enp-2 ca 0.8× as long as segment.

P5 ( Figs 11D , 14B , 15E‒F ). Similar to that of female. Free segment ca 1.1× as long as wide, with one slender outer seta and one inner spine apically; inner spine strong, ca 1.5× as long as segment bearing it; outer seta ca 2.5× as long as inner spine.

P6 ( Figs 11E , 14B , 15G ). Reduced to cuticular plate with two elements; inner spine strong, slightly shorter than outer seta.

Variability

The female specimens collected in Phupha Phet Cave are slightly smaller (0.89 mm each, n = 2) than those of Rakhang Thong Cave (0.95–0.96 mm, n = 2) and Khao Nui Cave (0.95–0.96 mm, n = 2). Similarly, the male specimens collected in Phupha Phet Cave (0.71 mm, n = 1) are smaller than those of Rakhang Thong Cave (0.75–0.76 mm, n = 2). Unfortunately, because the male has not yet been encountered from Khao Nui Cave, the length of the male is doubtful for this cave. Minor variation in the length/width ratio of the caudal rami, the length of spines of the female P5 and the male P6 are shown in Table 2 .

Distribution

Metacyclops brancelji sp. nov. was encountered in three caves: Phupha Phet Cave and Rakhang Thong Cave (Satun Province; Fig. 1D–G ), and Khao Nui Cave (Songkhla Province; Fig. 1D, H ).

Habitat

Metacyclops brancelji sp. nov. was collected during the rainy season from temporary water bodies which are seasonally filled primarily by water from the epikarst zone of the cave. Specimens were collected by hand net from both a large pool (n = 53) and a rimstone (n = 10) ( Fig. 1E–F ), about 100 meters from the entrance of Phupha Phet Cave. The type locality is in the dark zone where the temperature is relatively constant. Based on four sampling occasions from December 2014 to December 2015, the temperature in the dark zone of Phupha Phet Cave varied from 24.4°C in July 2015 to 27.8°C in April 2015. In Khao Nui Cave, the new species was collected in December 2014 from a water body filled by dripping water (n = 4) at the deepest part of the twilight zone, about 20 meters from the entrance ( Fig. 1H ). In Rakhang Thong Cave, the new species was collected in July 2015 from a rimstone (n = 33) at the cave entrance. The entrance of Rakhang Thong Cave is located at the base of the hill. The area in front of the entrance is a rock shelter which has been modified for religious propose. Near the rimstone is a Buddha statue and the gutter was created to collect water from the top of the shelter above the rimstone ( Fig. 1G ). These let us hypothesize that the discovery of M. brancelji sp. nov. in the rimstone of Rakhang Thong Cave is accidental and the new species could be encountered in water bodies around the hill and in the cave, like in Phupha Phet Cave and Khao Nui Cave. Furthermore, if compared to the regional distribution of the three latest described copepod species in Satun Province, including Onychocamptus satunensis Boonyanusith, Saetang, Wongkamhaeng & Maiphae, 2018, Boholina laorsriae Boonyanusith, Wongkamhaeng & Athibai, 2020 and Rangabradya (Siamorangabradya) wongkamhaengae Boonyanusith & Athibai, 2021, the distribution of M. brancelji sp. nov. is wider since it was collected in three separate caves located along the south of the mountain range, while the three above-mentioned species were only encountered in a single cave. The characteristics mentioned above suggest a stygophylic nature of M. brancelji sp. nov. rather than stygophilic, as suggested by many authors that stygophile exhibits an adaptation to spend their whole life and reproduce in subterranean habitats and can live in epigean environments ( Galassi 2001; Sket 2008; Brancelj 2015).

Differential diagnosis and remarks

Currently, four different types of spine formula of enp-2 of P1–P4 have been recognized within the genus Metacyclops ( Mercado-Salas et al. 2013) . Most of the species (54) and two Thai new species have the type of spine formula of 3.4.4.3. Two species, including M. thailandicus from Thailand and M. cushae Reid, 1991 from Louisiana ( USA), bear a spine formula of 3.4.3.3. Metacyclops mortoni Pesce, De Laurentiis & Humphreys, 1996 is the only species that bears a spine formula of 3.4.4.2. The last type of spine formula is 3.3.3.3, represented by two species comprising M. trispinosus Dumont, 1981 from Africa and M. margaretae (Lindberg, 1938) from India (Karanovic 2004a, 2004b; Karanovic et al. 2011). Among the species with a 3.4.4.3 formula, M. sakaeratensis sp. nov. and M. brancelji sp. nov. most closely resemble the Cambodian M. woni Lee & Chang, 2015 . Based on characteristics described by Lee & Chang (2015) for M. woni , the two Thai new species and M. woni share the combination of the following characteristics, including:

(1) female antennules 11-segmented

(2) posterior margin of second pedigerous somite undulated

(3) P4 enp-2 with one apical spine

(4) seta VI of caudal ramus shorter than seta III

(5) caudal rami with spinules at anterior third length of lateral surface

(6) male P6 bearing 2 elements

However, the two new species can be easily distinguished from M. woni by having: 1) transverse suture representing the remnant of ancestral articulation on the dorsal surface of the genital double-somite (absent in M. woni ; Chang pers. com.), 2) serrated hyaline frill on the posterior margin of the third thoracic somite (absent in M. woni ), 3) different length/width ratio of the caudal ramus (ca 2.3× in M. sakaeratensis sp. nov. and ca 2.7 × in M. brancelji sp. nov. vs ca 2.5× in M. woni ), and 4) a row of spinules on the intercoxal sclerite of P3 (absent in M. woni ) ( Table 2 ).

The armature of P5 and P6 are additional characters, separating the species ( Table 2 ). In M. sakaeratensis sp. nov. and M. woni , the inner spine of P5 is minute, as long as the free segment, whereas it is ca 1.5× as long as the segment in M. brancelji sp. nov. Conversely, the inner spine of male P6 is subequal in length with the outer seta in M. brancelji sp. nov. and M. woni , while it is much shorter in M. sakaeratensis sp. nov. Additionally, in M. sakaeratensis sp. nov. there are 2–3 spinules in the row of spinules on the anterior third of the lateral surface of the caudal ramus, while in M. brancelji sp. nov. and M. woni the row of spinules is composed of 4–5 spinules. Other differences between the two new species and M. woni are listed in Table 2 .

As suggested by Lee & Chang (2015), the combination of the following characteristics, including 11-segmented antennule, spine formula of enp-2 of P1–P4 3.4.4.3, and P4 enp-2 bearing one single apical spine, are present in twelve other species, including M. communis (Lindberg, 1938) , M. curtispinosus Dussart, 1984 , M. denticulatus Dussart & Frutos, 1985 , M. deserticus Mercado-Salas & Suárez-Morales, 2013 , M. grandispinifer ( Lindberg, 1940) , M. hannensis Defaye, 1992 , M. lusitanus Lindberg, 1961 , M. malayicus ( Kiefer, 1930) , M. minutus (Claus, 1863) , M. minutus prolatus (Kiefer, 1935) , M. pectiniatus Shen & Tai, 1964 , and M. subdolus Kiefer, 1938 ( Lee & Chang 2015). However, in the two new species the length/width ratio of the caudal ramus is shorter than 3, while it is longer than 3 in M. communis (3–4; Lindberg 1940), M. deserticus (3.5–3.8; Mercado-Salas et al. 2013), M. grandispinifer (3.7–5.7; Lindberg 1940), M. hannensis (3.2–3.4; Defaye 1992; Mercado-Salas et al. 2013), M. lusitanus (ca 3.75; Lindberg 1961; Herbst 1988), M. minutus (4–5; Tai & Chen 1979), M. prolatus (4.5–5.5; Dussart 1982; Lindberg 1961), and M. pectiniatus (3.4–3.5; Herbst 1988). According to Kiefer (1930), the caudal ramus’s length/width ratio is ca 2 in M. malayicus . It is less than that in M. sakaeratensis sp. nov. (ca 2.3) and M. brancelji sp. nov. (ca 2.7). Based on the key of Herbst (1988), the length/width ratio of the caudal ramus is more or less similar to both new species in M. subdolus (2.7–3.4), M. denticulatus (2.7–2.8), and M. curtispinosus (2.4–2.8) ( Mercado-Salas et al. 2013; Herbst 1988; Defaye & Por 2010; Pesce 1978). Nevertheless, the seta III of the caudal ramus is shorter than seta VI in M. curtispinosus and M. subdolus . In comparison, the seta III is longer than the seta VI in the two Thai new species. In M. denticulatus , the seta VI approXimately reaches the middle of the length of the seta III ( Dussart & Frutos 1985), while they are subequal in length in the two Thai new species.

Previously, Lee & Chang (2015) argued that the ornamentation of the spinule on the anterior third of the lateral surface of the caudal ramus is very characteristic for M. woni , but we realized that it has previously been described or illustrated in many Brazilian species of Metacyclops , such as M. campestris Reid, 1987 , M. hirsutus Rocha, 1994 , M. oraemaris Rocha, 1994 , M. paludicola (Herbst, 1959) , and in M. ryukyuensis Ishida, 1995 from Japan ( Reid 1987; Rocha 1994; Ishida 1995). However, the two new species and M. woni differ from M. campestris , M. hirsutus , M. oraemaris , and M. paludicola by having one apical spine on P4 enp-2 instead of two apical ones. Both new species and M. woni share several characters with M. ryukyuensis but the Japanese species could be easily distinguished from the new species by having 12-segmented antennule (vs 11-segmented antennule in both new species) and seta III shorter than seta VI (vs seta III longer than seta VI in the two new species).

Currently, only M. thailandicus has been described from Thailand ( Boonyanusith et al. 2018b). Both new species are distinguishable from M. thailandicus , according to the differences in the following characters: 1) spine formula of enp-2 of P1–P4 (3.4.4.3 in both new species vs 3.4. 3.3 in M. thailandicus ), 2) ornamentation on the intercoxal sclerite of P1–P4 (with spinule ornamentation in both new species vs without spinule ornamentation in M. thailandicus ), 3) relative length of the seta representing exp of antenna (reaching tip of enp-3 of antenna in both new species vs short, not reaching tip of enp-3 of antenna in M. thailandicus , 4) armament of the antenna (setal formula 3.1. 9.7 in both new species vs 3.1. 5.7 in M. thailandicus , and 5) length ratio of the outer seta to the apical spine on P4 enp-2 (1.3 in the new species vs 2.7 in M. thailandicus ). Furthermore, the distinct feature of the transverse suture on the dorsal surface of the genital double-somite is an additional characteristic that distinguishes the two Thai new species from M. thailandicus . In M. thailandicus , the transverse suture is discontinuous and separated into two lateral sutures, while it is continuous in the new species.

Figures

Fig. 10. Metacyclops brancelji sp. nov., paratype, ♀ (A; ZMB 34231a) and holotype, ♀ (B−D; ZMB 34231 slide No. 5124). A. Habitus, dorsal view. B. Fifth pedigerous somite and genital double-somite, lateral view. C−D. Two last abdominal somites and caudal rami, dorsal and lateral views, respectively. Scale bars: A = 100 μm; B−D = 50 μm.

Fig. 11. Metacyclops brancelji sp. nov., SEM photographs of additional material (Rakhang Thong Cave, Satun Province, collection of the third author CB), ♂ (A, C−E) and ♀ (B). A. Second and third pedigerous somites, lateral view (arrow indicates distal margin of third pedigerous somites). B. Genital double-somite and second abdominal somite, dorsal view (arrow indicates posterior margin of the transverse suture). C. Caudal ramus, lateral view (arrows indicate spinules). D. P5, lateral view. E. Male P6, lateral view.

Fig. 12. Metacyclops brancelji sp. nov., holotype, ♀ (ZMB 34231 slide No. 5124). A. Antennule. B. Antenna, caudal view. C. Mandible.D. Maxillule, frontal view. E. Maxilla, frontal view. F. Maxilliped, frontal view. Scale bar = 50 μm.

Fig. 13. Metacyclops brancelji sp. nov., holotype, ♀ (ZMB 34231 slide No. 5124). A. P1, frontal view. B. P2, frontal view. C. Frontal view of intercoxal sclerite of P3. D. P4, frontal view. E−F. Right and left P5, respectively, ventral view. In C−D, the spinule row is on the caudal surface of intercoxal sclerite and coXa. Scale bars = 50 μm.

Fig. 14. Metacyclops brancelji sp. nov., paratype, ♂ (A; ZMB 34231 slide No. 5126), allotype, ♂ (B−C, E; ZMB 34231 slide No. 5125) and additional material, ♂ (D; ZMB 34231 slide No. 5129). A. Habitus, dorsal view. B. Genital and first abdominal somites, ventral view. C. First to fourteenth segment of antennule. D. Fourteenth to sixteenth segment of antennule, E. Fifth to tenth segment of antennule (Roman numerals indicate segment of antennule). Scale bars: A = 100 μm; B−E = 50 μm.

Fig. 15. Metacyclops brancelji sp. nov., paratype, ♂ (A−D; ZMB 34231 slide No. 5127) and allotype, ♂ (E−G; ZMB 34231 slide No. 5125). A−D. Intercoxal sclerites of P1, P2, P3 and P4, respectively, frontal view. E−F. Right and left P5, respectively, ventral view. G. P6, ventral view. In C−D, the spinule row is on the caudal surface of intercoXal sclerite. Scale bar = 50 μm.

Fig. 1. Geographical location and details of the sampling sites. A. Map of Thailand and a location of Nakhon Ratchasima, Satun and Songkhla Provinces. B–C. Location (indicated by a black circle with no. 1) and sampling point of headwater stream in Nakhon Ratchasima Province. D. Locations of caves in Satun and Songkhla Provinces (indicated by black circles with nos 2, 3, 4). E–F. Sampling points in Phupha Phet Cave. G–H. Sampling points in Rakhang Thong Cave and Khao Nui Cave, respectively.