Two new species of Metacyclops Kiefer, 1927 (Copepoda, Cyclopoida) from Thailand and an up-to-date key to the species recorded in Asia Author Athibai, Sujeephon 8CAD9425-CB2A-4E7E-A71C-F040194EB4EB Applied Taxonomic Research Center and Department of Biology, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. Department of Zoology, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand. School of Biology, Faculty of Science and Technology, Nakhon Ratchasima Rajabhat University, Nakhon Ratchasima 30000, Thailand. sujiat@kku.ac.th Author Wongkamhaeng, Koraon D34BFAAF-A3F5-47E4-9671-A1F02717D789 koraon.w@ku.ac.th Author Boonyanusith, Chaichat EB363F4D-60CB-4431-BF5D-1000E3E01F8B chaichat.b@nrru.ac.th text European Journal of Taxonomy 2022 2022-01-10 787 1 146 181 http://dx.doi.org/10.5852/ejt.2021.787.1621 journal article 2640 10.5852/ejt.2021.787.1621 5e8737ed-def0-43ef-bebf-34f81cba4b4a 2118-9773 5837679 DE392695-DE12-4E36-B60C-6D324A4834FF Metacyclops sakaeratensis sp. nov. urn:lsid:zoobank.org:act: 84533C2C-57F3-47AB-8EF3-0DF17C9B4C92 Figs 2–9 ; Tables 1–2 Diagnosis FEMALE. Body size moderate ( 0.73–0.75 mm ; n = 3), with shallow integumental pits. Posterior margin of second pedigerous somite undulated; those of third and fifth pedigerous somites 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 straight. Caudal rami ca 2.1–2.3 × as long as wide, with few spinules at anterior third length on lateral surface and 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 on free segment of P5 as long as segment; outer seta on P5 ca 4.5 × as long as inner spine. MALE. Body slenderer and smaller than female ( 0.62–0.67 mm ; n = 3). Caudal rami ca 2.2–2.4 × as long as wide. P6 with two elements; outer (dorsal) seta about twice as long as inner (ventral) spine. Etymology The species name is a noun. The specific epithet was raised after the ʻSakaeratʼ Subdistrict, where the new species was encountered. Type material Holotype THAILAND • ♀ (completely dissected and mounted on one slide in glycerol and sealed with nail polish); Nakhon Ratchasima Province , Sakaerat Subdistrict ; 14°30′21.60″ N , 101°55′08.40″ E ; 14 Sep. 2013 ; C. Boonyanusith leg.; plankton net ; ZMB 34230 slide No. 5120. Allotype THAILAND • ♂ (completely dissected and mounted on one slide in glycerol and sealed with nail polish); same collection data as for holotype ; ZMB 34230 slide No. 5121. Paratypes THAILAND • 1 ♀ , 1 ♂ (each completely dissected and mounted on one slide in glycerol and sealed with nail polish); same collection data as for holotype; ZMB 34230 slide No. 5122–5123 • 1 ♀ adult (stored in a miXture of glycerol and 70% ethanol (ratio ~1: 10 v /v)); same collection data as for holotype; ZMB 34230 . Additional material examined THAILAND • 1 ♀ , 1 ♂ (stored in a miXture of glycerol and 70% ethanol (ratio ~1: 10 v /v)); same collection data as for holotype; collection of the third author (CB) • 2 ♀♀ (processed for taking photographs by SEM); same collection data as for holotype; collection of the third author (CB) . Type locality The new species was collected in a headwater stream in SERS , Sakaerat Subdistrict, Nakhon Ratchasima Province , Northeast Thailand ( Fig. 1A–C ). The locality is located in a dry evergreen forest and has been known under the name of “Tham Ngu Jong Ang” (King cobra Cave) in Thai language. The name refers to a rock shelter, which is the characteristic of the stream bank in the type locality. The stream reach is about 30 m long and about 4–5 m wide, streambed with bedrock. During dry season, the water does not encompass the channel entirely. The new species was collected in pool-formed area, where the water flows slowly. The mean water temperature was 25.3°C, pH 6.98, conductivity 64.7 µS cm-1 , and dissolved oxygen 9.1 mg L- 1 . Description Adult female Total body length, measured from tip of rostrum to posterior margin of caudal rami, 0.73–0.75 mm (mean = 0.75 mm ; n = 3; holotype = 0.74 mm ) ( Fig. 2A ). Naupliar eye not discernible. Rostrum V-shaped in frontal view, completely fused to cephalothorax, with two sensilla laterally and rounded tip ( Fig. 2B ). Prosome ca 64% of body length and ca 1.75 × as long as length of urosome. CephalothoraX anteriorly oval, ca 33% of body length and ca 1.15 × as long as wide, with greatest width at posterior margin; 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 ( Fig. 2A, C ). Fifth pedigerous somite with two transversal rows of spinules located between proximal seta and free segment of P5 ( Fig. 3B, D ), with two sensilla dorsally; posterior margin with serrated hyaline frill ( Fig. 2D ). Genital somite and first abdominal somite fused, forming genital double-somite. Genital double-somite symmetrical, ca 0.84 × as long as wide, tapering posteriorly; dorsally with two sensilla and transverse suture, representing the remnant of ancestral articulation ( Figs 2D , 4B ); posterior margin with serrated hyaline frill ( Figs 2D , 3A ). Seminal receptacle with clear distinction between anterior and posterior lobes; anterior lobe short and wide; posterior lobe globular, narrower than anterior one ( Fig. 3A ). Second and third abdominal somites narrower than genital double-somite, ca 56% of double-somite width, with serrated hyaline frill on posterior margin ( Fig. 2D ). Anal somite with a row of minute spinules latero-ventrally on posterior margin and two sensilla dorsally at base of anal operculum ( Fig. 2D ). Anal operculum developed, trapezoidal, reaching insertion of caudal ramus; free margin smooth and straight ( Fig. 2D ). Body with numerous integumental pits; pits shallow, hard to observe and less developed on anal somite ( Figs 2C , 4B, E ). CAUDAL RAMI ( Figs 2D , 3C , 4C ). Relatively short, ca 2.25× 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 0.34 × 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: 0.37: 0.91: 4.23: 5.57: 0.86: 1.21. Lateral surface ornamented with few minute spinules located at anterior ⅓ of ramus length, few minute spinules at base of seta II, and a row of strong spinules at base of seta III. ANTENNULE ( Fig. 5A ). 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 outer seta. Eleventh segment with acrothek sub-apically. Fig. 2. Metacyclops sakaeratensis sp. nov. , paratype, ♀ (A, C; ZMB 34230) and holotype, ♀ (B, D; ZMB 34230 slide No. 5120). A . Habitus, dorsal view. B . Rostrum, frontal view. C . Integumental pits and posterior margins of cephalothorax and two subsequent pedigerous somites, dorsal view. D . Urosome, dorsal view (black and open arrows indicate spinules and transverse suture, respectively). Scale bars: A, C = 100 μm; B, D = 50 μm. Fig. 3. Metacyclops sakaeratensis sp. nov. , holotype, ♀ (ZMB 34230 slide No. 5120). A −B . Fifth pedigerous somite and genital double-somite, ventral and lateral views, respectively. C . Caudal ramus, lateral view (arrows indicate spinules). D −E . P5, lateral and ventral views, respectively (arrows in D indicate rows of spinules on the lateral surface of the fifth pedigerous somite). Scale bars = 50 μm. Fig. 4. Metacyclops sakaeratensis sp. nov. , digital photographs of allotype, ♂ (A, F; ZMB 34230 slide No. 5121) and SEM photographs of additional material, ♀♀ (B−E; collection of the third author CB). A . Posterior margins of cephalothorax and two subsequent pedigerous somites, dorso-lateral view (arrows indicate posterior margin of the second, third and fourth pedigerous somites). B . Anterior part of genital double-somite, lateral view (arrow indicates posterior margin of the transverse suture). C . Caudal ramus, lateral view (arrows indicate spinules). D . P4 enp-2, frontal view. E . P5, ♀, lateral view. F . P6, ♂, lateral view. Fig. 5. Metacyclops sakaeratensis sp. nov. , holotype, ♀ (A−B; ZMB 34230 slide No. 5120) and paratype, ♀ (C−D; ZMB 34230 slide No. 5122). A . Antennule (arrows indicate aesthetascs). B . Antenna, frontal surface. C . Coxobasis of antenna, caudal surface. D . Labrum, inner side. Scale bar = 50 μm. Fig. 6. Metacyclops sakaeratensis sp. nov. , holotype, ♀ (ZMB 34230 slide No. 5120). A−B . Right and left mandibles, respectively (the mandibular palp of the right mandible was lost during dissection). C . Maxillule, caudal view. D . Maxilla, caudal view. E . MaXilliped, frontal view. Scale bar = 50 μm. ANTENNA ( Fig. 5B–C ). 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; 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. LABRUM ( Fig. 5D ). Trapezoidal in frontal view, with two rows of hairs; cutting edge with 12 teeth medially between two obtuse lateral teeth. MANDIBLE ( Fig. 6A‒B ). 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. 6C ). 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. 6D ). Five-segmented. Praecoxa and coxa partly fused frontally. Praecoxal endite prominent, inserted medially, with one smooth and one spinulose seta 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 on 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 two-segmented; enp-1 with two robust setae; enp-2 with strong seta apically and two smooth, slender setae sub-apically. MAXILLIPED ( Fig. 6E ). Four-segmented, composed of syncoxa, basis and two-segmented enp. Syncoxa with two endites and ornamented with a 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. Enp-1 with strong spinulose seta. Enp-2 with three setae; apical seta strong, two other ones slender and smooth. P1‒P4 ( Fig. 7 ). Two-segmented enp and exp. Intercoxal sclerite with minute spinules on distal prominences. 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. 7A ). Frontal and caudal surfaces of intercoXal sclerite bare, with 2‒3 minute spinules on distal prominences. Lateral seta on basis ca 4× as long as those of P2‒P4; medial 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 0.7 × 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 1.2× as long as length of apical spine; apical spine strong, slightly curved, as long as segment. Table 1. Armature formula of P1–P4 in Metacyclops sakaeratensis sp. nov. and M. brancelji sp. nov. Roman numerals represent number of spines and Arabic numerals represent number of setae.

Swimming leg	Coxa	Basis	Exopod 1 2		Endopod 1 2
P1	0-1	1-1	I-1	II-I+1-4	0-1	1-I+1-3
P2	0-1	1-0	I-1	III-I+1-4	0-1	1-I+1-4
P3	0-1	1-0	I-1	III-I+1-4	0-1	1-I+1-4
P4	0-1	1-0	I-0	II-I+1-4	0-1	1-I-3

P2 ( Fig. 7B ). IntercoXal sclerite as in P1, yet distal prominences with 3‒4 minute spinules. Basis with two hook-like 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 1.3× as long as wide, with four spines and five setae; apical spine slightly shorter than segment. Enp-1 with inner seta. Enp-2 ca 1.5× 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 inserted between claw-like eXtensions, ca 1.3× as long as length of apical spine; apical spine strong and straight, as long as segment. P3 ( Fig. 7C‒D ). Frontal surface of intercoXal sclerite bare; caudal surface with transversal row of minute spinules and distal prominences with 3‒4 minute spinules. Basis, eXp, and enp similar to those of P2. P4 ( Figs 4D , 7E ). Intercoxal sclerite similar to that of P3. Coxa with rows of spinules on caudal surface. Basis similar to those of P2 and P3. EXp-1 with outer spine. EXp-2 ca 2.0 × as long as wide, with three spines and five setae; spines smaller than those of P1‒P3, apical spine ca 0.5× as long as segment. Enp- 1 with inner seta. Enp-2 ca 2.0× 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 ca 0.8× as long as segment. P5 ( Figs 3A‒B, D‒E , 4E ). 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, apically with one slender outer seta and one inner spine; inner spine as long as segment and outer seta ca 4.5× as long as inner spine. P6 ( Figs 3B , 4B ). Small, forming simple cuticular plate inserted latero-dorsally on genital double-somite, and armed with one seta dorsally and two minute spiniform setae ventrally. Adult male Total body length, eXcluding caudal seta, 0.62‒0.67 mm (mean = 0.65 mm ; n = 3; allotype = 0.65 mm ). Habitus smaller and slenderer than in female ( Fig. 8A ). Naupliar eye and rostrum as in female. Prosome ca 62% of body length and ca 1.62 × as long as length of urosome. CephalothoraX anteriorly oval, representing ca 33% of body length and ca 1.16 × as long as wide. Posterior margins of cephalothoraX and two subsequent pedigerous somites ( Fig. 4A ) similar to those of female; that of fourth pedigerous somite smooth. Fifth pedigerous somite as that of female. Genital somite swollen on mediolateral margin ( Fig. 8A‒B ), ca 25% length of urosome and ca 0.62 × as long as wide, with hyaline frill latero-dorsally. First to third abdominal somites narrower than genital somite, representing ca 60% of genital somite width, with serrated hyaline frill on posterior margin. Anal somite and operculum similar to those of female. Fig. 7. Metacyclops sakaeratensis sp. nov. , holotype, ♀ (ZMB 34230 slide No. 5120). A . P1, frontal view. B . P2, frontal view. C . Frontal view of intercoxal sclerite of P3. D . P3 exp-1. E . P4, frontal view. Arrows indicate hook-like expansion. In C and E, the spinule row is on the caudal surface of intercoxal sclerite and coXa. Scale bar = 50 μm. Fig. 8. Metacyclops sakaeratensis sp. nov. , paratype, ♂ (A; ZMB 34230 slide No. 5123) and allotype, ♂ (B−C; ZMB 34230 slide No. 5121). A . Habitus, dorsal view. B . Genital and first abdominal somites, ventral view. C . Antennule (Roman numerals indicate segment of antennule). Scale bars: A = 100 μm; B−C = 50 μm. CAUDAL RAMI. Similar to that of female, ca 2.3 × as long as wide. Armament and ornamentation similar to those of female. Length ratio of caudal setae to ramus length, from seta II to seta VII: 0.42: 0.88: 4.47: 5.83: 0.82: 1.27. ANTENNULE ( Fig. 8C ). 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, AND MAXILLIPED. Similar to those of female. P1. Frontal and caudal surfaces of intercoXal sclerite bare and distal prominences with 2‒3 minute spinules ( Fig. 9A ). Coxa, basis, enp and exp similar to those of female. P2. IntercoXal sclerite as in P1, yet distal prominences with 3‒5 minute spinules ( Fig. 9B ). 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. distal prominences with 3‒5 minute spinules ( Fig. 9C ). CoXa, basis, enp and eXp similar to those of female. P4. Frontal surface of intercoxal sclerite bare, caudal one with transversal row of minute spinules and distal prominences with 3‒5 minute spinules ( Fig. 9D ). CoXa, basis, enp and eXp similar to that of female; apical spine on enp-2 ca 0.8× as long as segment. Fig. 9. Metacyclops sakaeratensis sp. nov. , paratype, ♂ (A−D; ZMB 34230 slide No. 5123) and allotype, ♂ (E−G; ZMB 34230 slide No. 5121). A−D . Intercoxal sclerites of P1, P2, P3 and P4, respectively, frontal view. E−F . Right and left P5, respectively, ventral view. G . P6, lateral view. In C − D, the spinule row is on the caudal surface of intercoXal sclerite. Scale bar = 50 μm. P5 ( Fig. 9E‒F ). Similar to that of female, yet outer seta on free segment short, ca 2.3× as long as inner spine. P6 ( Fig. 9G ). Reduced to cuticular plate with two elements, inner (ventral) one spine and outer (dorsal) one seta; inner spine ca 0.5× as long as outer seta. Variability The body length of the female specimens varies in a range of 0.73–0.75 mm (n = 3) and that of the male specimens is 0.62–0.67 mm (n = 3). The variations in the length/width ratio of caudal ramus in the female, along with the length of spine on the female P5 and the male P6 are shown in Table 2 . Distribution Metacyclops sakaeratensis sp. nov. has been known only from the type-locality.