Mongolodiaptomus mekongensis, Sanoamuang & Watiroyram, 2018

Sanoamuang, La-orsri & Watiroyram, Santi, 2018, Mongolodiaptomus mekongensis, a new species of copepod (Copepoda, Calanoida, Diaptomidae) from temporary waters in the floodplain of the lower Mekong River Basin, Raffles Bulletin of Zoology 66, pp. 782-796 : 783-792

Mongolodiaptomus mekongensis , new species

( Figs. 1 – 7 )

Material examined. Holotype, one adult male ( NHMUK 2018 .1464), dissected and mounted in glycerol on one slide : Allotype, one adult female ( NHMUK 2018 .1465), dissected and mounted in glycerol on one slide : Paratypes, three adult females and males ( NHMUK 2018 .1466–1475) , three adult females and males ( NPU 2018–005 ) , three adult females and males (KKU-COP-2018S-01), undissected and preserved in 70% ethanol in 1.5 mL microtubes. All specimens collected on June 9, 2002 by L. Sanoamuang.

Type locality. A roadside canal near Kilometer Post 28, National Highway no. 2134, Na Kham Subdistrict , Sri Muang Mai District , northeastern Thailand, Ubon Ratchathani Province ; 15°22′42″N; 105°12′42″E.

Common features of both sexes. Left antennule ( Fig. 1G ) 25-segmented, reaching beyond the caudal setae. Setal formula [number of setae + aesthetasc (a) + spine (s)] of each segment as follows: 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. 3A ) coxa small, with 1 inner seta. Basis with 2 smaller medial setae on distal corner. Exp 7-segmented, with 1, 3, 1, 1, 1, 1, and 4 (one inner and 3 apical) inner setae respectively. Enp 2-segmented, Enp-1 with 2 medial setae. Enp-2 with 9 inner and 7 apical setae.

Mandible ( Fig. 3B ) coxa with 2 cuspidate and 5 blunt teeth on gnathobase plus one small dorsal seta on distolateral corner. Basis with 4 inner setae. Enp 2-segmented, Enp-1 with 4 inner setae and Enp-2 with 9 apical setae. Exp 4-segmented, with 1, 1, 1, and 3 setae, respectively.

Maxillule ( Fig. 3C ) praecoxal arthrite with 7 strong and 4 fine setae. Coxal endite with 3 fine setae and coxal epipodite with 9 strong setae. Basis with 2 endites, each with 4 fine setae; basal exite with 1 small seta. Enp reduced, represented by 8 apical setae. Exp with 6 setae.

Maxilla ( Fig. 3D ) proximal and distal praecoxal endite, each with 3 setae. Two coxal endites with 3 setae each. Allobasis with 3 setae on basal endite. Enp reduced to 2 segmented, each with 3 setae.

Maxilliped ( Fig. 3E ) praecoxa completely fused to coxa, represented by 4 endites with 1, 2, 3, 4 setae each. Basis with 3 setae distally plus row of strong spinules along inner margin. Enp 6-segmented, with 2, 3, 2, 2, 2, and 4 setae, respectively.

P1–P4 (Figs. 4A–D) biramous. Coxa with pinnate seta at distal inner corner. Basis of P4 with small seta at outer distal margin. Exp longer than Enp. P1 with 3-segmented Exp and 2-segmented Enp, P2–P4 with 3-segmented Exp and Enp. Exp-1 with one outer spine and inner pinnate seta. Exp-2 as same as Exp-1 except P1 lacking outer spine. Exp-3 with 3 inner and 3 apical pinnate setae, and one outer spine except P1 with 2 inner setae. Enp-1 with one inner pinnate seta. P1 Enp-2 with 3 inner, 2 apical, and one outer pinnate setae; P2–P4 Enp-2 with 2 inner pinnate setae. P2–P4 Enp-3 with 3 inner, 2 apical, and 2 outer pinnate setae.

Description. Adult female. Total body length excluding caudal setae 1.32–1.41 mm (mean ± SD = 0 1.38 ± 0.01, N = 20) ( Figs. 1A , 2A ). Antennules ( Fig. 1G ) symmetrical. Rostrum ( Figs. 1B , 2B ) with pair of teeth-like process on anterior margin. Cephalosome and Pdg 1 completely fused, Pdg 4 and 5 incompletely fused. Pdg 5 ( Figs. 1C , 2C ) produced into slightly asymmetrical posterolateral wings (right wing somewhat larger than left), each ending with short, posterior spine; smaller dorsal spine located on inner margin of each wing. Urosome 3-segmented, genital doublesomite more longer than two later urosomites and caudal rami combined. Genital double-somite ( Figs. 1C–F , 2C ) asymmetrical, right posterolateral process well-developed, extended beyond haft of segment; with strong, blunt spine on respective process. Left side with strong dorsolateral spine on small prominence proximally, larger than other genital spines. Posterior margin slightly produced in dorsal and right lateral view. Urosomite 2 symmetrical, shorter than wide, partly covered by posterior end of genital double-somite. Anal somite ( Figs. 1C , 2C ) symmetrical, divergent distally, slightly shorter than later somite; anal operculum small, with slightly posterior margin concave or nearly straight.

Caudal rami ( Figs. 1C , 2A, C, D ) symmetrical, each ramus expanded on distal end, with setules along inner and outer margins. Each ramus with 6 setae (seta II–VII): seta II–VI plumose, anterolateral (II) seta with smooth region on outer margin proximally; terminal setae (seta IV and V) without fracture plane; dorsal seta (VII) articulated, bare, and longest.

P5 ( Figs. 2E–I , 4E, F) symmetrical. Coxa with stout spine on protuberance at distolateral corner in posterior view; coxal spine prolonged to Exp-1. Basis with a thin seta on distolateral margin, reaching beyond middle of Exp-1. Exp 3-segmented and Enp 2-segmented. Exp-1 enlarged rectangular-like, more than 2.0 times as long as wide. Exp- 2 sub-triangular, with a row of strong spinules along both margins; with small outer spine proximally. Exp-3 reduced into small prominence on Exp-2, with short spine and long seta apically. Enp subconical, about half as long as Exp-1. Enp-1 cylindrical, shorter than Enp-2. Enp-2 with a circular row of spinules on narrowed end apically.

Adult male. Body length excluding caudal setae 1.19–1.30 mm (mean ± SD = 0 1.24 ± 0.02, N = 20) ( Figs. 5A , 6A ), smaller than female. Pdg 4 and 5 incompletely fused. Lateral wing ( Figs. 5C , 6A ) less developed, slightly asymmetrical, left wing prolonged into downward direction and tipped with thinner spine than those on opposite wing. Urosome ( Figs. 5A , 6A–C ) 5-segmented and asymmetrical, oriented obliquely posteriad to right side. Genital somite ( Figs. 5B, C , 6A–C ) shortest, asymmetrical, right side dilated posterolaterally and with small seta on outer distal margin. Urosomites 2–3 ( Figs. 6A–C ) symmetrical, former somite longer than later one, both with long hairs ventrally. Urosomite 4 ( Figs. 6A, C ) with right posterolateral dilated; dorsoposterior margin expanded reaching to anal operculum. Anal somite ( Figs. 5A , 6A, C ) similar to female but asymmetrical posteriorly end. Caudal rami ( Figs. 5D , 6B, C ) asymmetrical and cylindrical, about 2.5 times as long as wide, with hairy inner margin. Right ramus with two ventral chitinous processes: strong spiniform process on prominence proximally and two semi-circular ridges located at base of seta IV and V. Their setation as in female.

Right antennule ( Fig. 6D ) transformed and geniculated, with 22 segments. 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+s, 1+s, 1+s, 2+s, 3+s, 2, 4+a. Segments 13 to 20 enlarged and elongated; segments 18 and 19 geniculated; segment 20 (antepenultimate) with comb-like process (4 teeth).

P5 ( Figs. 5E–M , 7 ) asymmetrical, right leg highly enlarged. Intercoxal plate with strong finger- like projection on distal margin, its tip bent inward. Right P5: coxa with strong spine inserted on posterior lobe (present spinous process on distal inner corner of posterior lobe, see Fig. 5F ), almost reaching distal end of basis. Basis quadrangular, about twice as long as wide; with small hyaline lamella at inner margin ( Fig. 5G ); with small chitinous prominence on posterior view mid-distally; with small seta located at distal outer margin on anterior view. Exp 3-segmented: Exp-1 quadrangular, distal outer corner produced ( Fig. 5I ); with 2 chitinous prominences on posterior view. Exp-2 slightly incurved, proximal and distal parts enlarged, about 2.5 times as long as wide; with two small processes proximally and distally, and principal spine located slightly posterior to mid-outer margin. Lateral spine strong, cylindrical, slightly longer than 1/2 of segment, about one third of spine length bent or twist to posterolateral direction distally on posterior view; with pointed tip. Exp-3 modified into end claw with sickle-shaped, thickness, long beyond 1.5 times as long as Exp-2; with inner serrate margin. Enp 1-segmented, conical, gradually tapering to distal end, reaching to quarter of Exp-2; with spinulated tip.

Left P5: coxa with thin seta inserted on posterior lobe at distal inner margin, reaching beyond mid-half of basis. Basis with long inner hyaline lamella; with short, thin posterolateral seta on posterior surface; with obviously longitudinal chitinous ridge, extending almost entire segment ( Fig. 5J ). Exp 3-segmented ( Fig. 5K, M ): Exp-1 longer than wide, gradually tapering in posterior end; with inner strongly serrate margin at distal half. Exp-2 oval, smaller than Exp-1; with inner robust seta, longer than Exp-2; with inner strongly serrate margin margin. Exp-3 reduced to bare apical process, stretched into somewhat spearhead-like. Enp 1-segmented, slightly longer than Exp-1, with spinulated tip.

Etymology. The specific name “ mekongensis ” is named after the place “the Great Mekong sub-region” where the new species was found.

Remarks. Mongolodiaptomus mekongensis , new species, fits well with the generic diagnosis of the genus Mongolodiaptomus as described in the amended delimitation by Ranga Reddy et al. (2000) in that the male right P5 Exp-2 with a principal spine on approximately mid-outer margin and two small spinous processes proximally and distally. According to the identification key given by Watiroyram & Sanoamuang (2017), the new species is similar to M. loeiensis Watiroyram & Sanoamuang, 2017 by shared morphological characteristics in both sexes. In the males, the segment 20 of right antennule with a comb-like process, the intercoxal plate of P5 with outgrowth on distal margin, and the uniqueness of P5 by shared similarities that is the right leg with (1) a strong coxal spine on prominent lobe, (2) an hyaline membrane on inner margin of basis, (3) Exp-1 distolateral corner produced, (4) a strong and bent principal lateral spine; on left leg with (1) an hyaline membrane on inner margin of basis, and (2) strong spinules along inner margin of Exp-2. The female shares with M. loeiensis the following characters: (1) the spine on Pdg 5 and genital double-somite with similar shape and size, (2) the genital double-somite with the right posterolateral outgrowth, and (3) the structure of fifth leg. The new species can be differentiated from M. loeiensis by the characteristics of the male caudal rami, and fifth leg: the right caudal ramus with two ventral chitinous processes (proximal spine-like and distal semi-circular process) while M. loeiensis with two spine-like processes proximally and semi-circular one distally; intercoxal plate of the new species with one spine-like outgrowth on its distal margin but present two spine-like outgrowths in M. loeiensis ; the new species with acute projection at the base of the right coxal spine ( Fig. 5F ) but this apparatus is absent and having the shorter and stronger coxal spine in M. loeiensis . The right leg: the principal spine of Exp-2 with acute tip in M. mekongensis , new species, but blunt in M. loeiensis ; the Exp-2 end claw of the new species is somewhat shorter and thicker than that in M. loeiensis : the left leg; the basis with obviously longitudinal chitinous ridge on posterior view, and Exp-3 with long apical process in the new species but with short and blunt process in M. loeiensis . The female Pdg 5 of the new species with asymmetrical posterolateral wings but it is symmetrical in M. loeiensis . The genital double-somite of Fig. 4. Mongolodiaptomus mekongensis , new species, female. A, P1; B, P2; C, P3; D, P4; E, P5 (A–E in posterior view); F, P 5 in anterior view. Scale bar = 100 μm.

the new species with only proximal bulge on the right outer margin but that in M. loeiensis with three bulges beginning from proximal to middle region (see Table 1).

The females of the new species can be distinguished from the M. dumonti Sanoamuang, 2001 by (1) the lateral wings are asymmetrical versus symmetrical in the later species, (2) the genital double-somite with a posterolateral process on right margin but it is absent in M. dumonti . In the males, (1) the right caudal ramus on the ventral side with a large, chitinous tooth and semi-circular ridge in M. mekongensis , new species but with only two chitinous teeth in M. dumonti , (2) the P5 intercoxal plate produced into triangular process on inner distal margin in the new species but non-produced in M. dumonti , (3) the right P5 coxa with acute inner process on posterior lobe in the new species but it is absent in M. dumonti , (4) the right P5 basis with inner hyaline lamella in the new species but it is absent in M. dumonti , (5) the right P5 Exp-1 with acute process on distal outer margin in the new species but it is round in M. dumonti , and (6) the principal spine on the right P5 Exp-2 bent in the new species but it is straight in M. dumonti (see Table 1).

Figures

Fig. 1. Mongolodiaptomus mekongensis, new species, female. A, habitus, dorsal view; B, rostrum anterior view; C, pediger 5 and urosome, dorsal view; D, urosome, ventral view (without caudal rami); E, pediger 5 and urosome, right lateral view (without caudal rami); F, pediger 5 and urosome, left lateral view (without caudal rami); G, antennule. Scale bar = 200 µm.

Fig. 2. Mongolodiaptomus mekongensis, new species, SEM photographs of female (A–I). A, habitus, dorsal view; B, rostrum, frontal view; C, pediger 5 and urosome, dorsal view (white arrow pointed to spines); D, caudal rami, ventral view; E, P5 in anterior view; F, P5 in posterior view; G, P5 Exp-2–3 in anterior view; H, P5 Exp-2–3 in posterior view; I, Enp-1–2 in anterior view, white arrow indicates the segmented Enp.

Fig. 3. Mongolodiaptomus mekongensis, new species, female. A, antenna; B, mandible; C, maxillule; D, maxilla; E, maxilliped. Scale bar = 100 µm.

Fig. 5. Mongolodiaptomus mekongensis, new species, SEM photographs, male (A–M). A, habitus, dorsal view; B, lateral wings, genital somite, and urosomites 2–3, dorsolateral view (white arrow pointed to spines and black one pointed to seta); C, pediger 5, genital somite, and urosomites 2–3, dorsal view; D, right caudal ramus, ventral view (white arrow pointed spiniform process); E, P5 in posterior view; F, the right coxal spine, and basis of P5 in posterior view (white arrow pointed to spiniform process on posterior lobe); G, intercoxal plate, and basis of P5 in posterior view (white arrow pointed to hyaline membranes); H, the right P5 Exp-1–2 and Enp in posterior view; I, the right P5 Exp-1–2 in anterior view (white arrow pointed to spiniform processes); J, the left P5 basis, Exp and Enp in posterior view (white arrow pointed to longitudinal chitinous ridge); K, the left P5 Exp-1–3 and Enp in posterior view; L, P5 in anterior view; M, the left P5 Exp and Enp in anterior view.

Fig. 6. Mongolodiaptomus mekongensis, new species, male. A, habitus, dorsal view; B, urosome, right lateral view; C, urosome, ventral view (black arrows indicate chitinous spines and ridge on right caudal ramus); D, the right antennule: D1, segment 1–12; D2, segment 13–22. Scale bar = 100 µm.

Fig. 7. Mongolodiaptomus mekongensis, new species, male. A, P5 in posterior view; B, P5 in anterior view. Scale bar = 100 μm.