Siamcyclops isanus sp. nov.

Figs 2, 3, 4, 5, 6

Type locality.

The Maholarn cave, Nonghin Subdistrict, Nonghin District, Loei Province, northeastern Thailand: 17°06'23.04"N, 101°52'48.54"E, 315 m altitude. Sample collected from a rimstone pool in the innermost zone, filled with dripping water.

Etymology.

Siamcyclops isanus sp. nov. is named after ‘ Isan’, the local name for northeastern Thailand. The species epithet is a masculine, singular adjective.

Type specimens.

Holotype • one adult female dissected and mounted on one slide, THNHM-IV-20465 . Allotype • one adult male dissected and mounted on one slide, THNHM-IV-20466 . Paratypes • one adult female and male dissected and mounted on two separate slides, (NPU 2024-01 – 02) • four adult females and nine adult males stored in a 1.5 ml microtube with 70 % ethanol, (NPU 2024-01 – 02) • one adult female and male dissected and mounted on two separate slides, NPU 2020-003 • one adult male dissected and mounted on one slide, THNHM-IV-20467 . All samples were collected by the first author on 5 July 2013.

Description of adult female

(holotype). Habitus (Fig. 2 A) cyclopiform. Body length, excluding caudal setae, 550 μm, with prosome / urosome ratio of 1.5. Body surface ornamented with refractile points (not figured). Nauplius eye indiscernible. Posterior margin of prosomites with smooth free margin of hyaline fringe. Cephalosome completely fused with first thoracic somite, forming cephalothorax. Urosome length 212 μm; length / width ratio of 1.1. Pseudosegment present between pediger 5 and genital double-somite. Genital double-somite (Fig. 2 B – D) enlarged; anterior part slightly wider than posterior part; 1.2 times as wide as long; with pair of sclerotised semi-rounded structures laterally and P 6 dorsolaterally; posterior margin with irregularly serrated free hyaline fringe. Urosomites 3–4 (Fig. 2 B – D) with about 1.6 times as long as wide; posterior margin with irregularly serrated free hyaline fringe. Anal somite (Fig. 2 A – D) short, 1.6 times as long as wide; with a pair of dorsal sensilla at base of anal operculum. Anal operculum (Fig. 2 B, D) reaching slightly beyond distal end of anal somite; semicircular; free margin smooth. Caudal rami slightly divergent (Fig. 2 A – C). Caudal ramus rectangular, about 2.5 times as long as wide, without dorsal longitudinal keel. Anterolateral accessory seta (I) reduced. Lateral seta (II) pinnate, slightly shorter than 1 / 2 length of caudal ramus, inserted at 1 / 3 length of caudal ramus. Posterolateral seta (III) pinnate, strong, spine like, shorter than caudal ramus, with spinules at insertion point on ventrolateral side. Outer terminal seta (IV) pinnate, about 2.5 times as long as caudal ramus, without fracture plane. Inner terminal seta (V) pinnate, about 4.0 times as long as caudal ramus, without fracture plane. Innermost terminal seta (VI) bare; shortest. Dorsal seta (VII) pinnate, articulated, longer than caudal ramus.

Antennule (Fig. 3 A). 11 - segmented, not reaching posterior margin of cephalothorax. Armature formula as follows: 7.2. 5.2. 0 + S. 2.3. 2 + A. 2.2 + A. 7 + A. Aesthetascs cylindrical.

Antenna (Fig. 3 B). 4 - segmented; coxobasis smooth, with seta on distal inner corner. Enp 1–3 with small spinular rows on outer margin, Enp - 1 with seta on distomedial margin; Enp - 2 with medial seta and four distal outer setae; Enp - 3 with seven apical setae, two of them geniculated. All setae smooth.

Mandible (Fig. 3 C). Gnathobase with eight strongly chitinized teeth, and a small seta dorsally. Basis very reduced, with two long bipinnate setae and a short smooth seta, representing Exp and Enp.

Maxillule (Fig. 3 D). Precoxal arthrite with strongly chitinized teeth, one tricuspidate and two bicuspidate; four strong smooth setae and one spinulate seta on inner margin. Coxobasis with spinulate seta and two smooth setae distally. Exp completely reduced, represented by a smooth seta. Enp with three smooth setae.

Maxilla (Fig. 3 E). Precoxal endite with two pinnate setae. Coxa with two endites: proximal endite with smooth seta; distal endite with smooth seta and pinnate seta. Basis drawn out into claw-like expansions, with slender seta on proximal inner margin. Two-segmented Enp: Enp - 1 with two smooth setae; Enp - 2 with three smooth setae.

Maxilliped (Fig. 3 F). Syncoxa with two spinulate setae on distal inner margin. Basis with two spinulate setae on distal inner margin, accompanied by two spinule rows on its surface. Enp - 1 with inner seta; Enp - 2 with three apical setae; all setae smooth.

P 1 – P 4 (Fig. 4 A – D). P 1 – P 4 with 2 - segmented Exp and Enp. Exp and Enp ornamented with setules on its inner and outer margin, respectively. No blunt seta on Exp - 2, all setae being equal. Armature formula (setae in Arabic numerals and spines in Roman numerals from outer-inner or outer-apical-inner margins) as follows:

P 1 (Fig. 4 A). Basis with small spinules near its insertion. Exp slightly larger than Enp. Exp - 1 much smaller than Exp - 2. Exp - 2 about twice as long as wide. Enp - 2 larger than Enp - 1, about 2.0 times. Enp - 2 with robust apical seta, as long as Enp - 1 and Enp - 2 combined, its tip bent outward; distal inner seta as long as apical spine, the two remaining setae shorter.

P 2 (Fig. 4 B). Similar to P 1 but lateral seta of basis shorter. Enp - 2 with strong spinulated apical spine, with strengthened tip; two setae as long as apical spine but two remaining setae shorter.

P 3 (Fig. 4 C). Similar to P 2 but lateral seta on basis longer, Enp - 2 with slightly less robust apical spine. Enp - 2 with two setae longer than apical spine, three remaining setae shorter.

P 4 (Fig. 4 D). Exp - 1 and Exp - 2 with weakly spinulated outer spines. Exp - 2 twice as long as wide; three inner setae longer than Exp - 1 and 2 combined, outermost seta shortest. Enp - 1 slightly shorter than wide, Enp - 2 about 1.3 times as long as wide. Enp - 2 with slender apical spine, short as long as segment bearing it; outer and innermost seta shorter than apical spine, two remaining setae longer than apical spine.

P 5 (Fig. 4 E). Completely fused to pediger 5, proximal segment represented by lateral seta, and distal segment retains two pinnate apical setae on a small lobe. Lateral seta slightly longer than apical inner seta; apical inner seta about 1.3 times as long as apical outer one.

P 6 (Fig. 2 D). Reduced to a semi-circular plate, with three short elements: dorsal seta thin and short, two ventrally positioned elements spiniform.

Egg sac (Fig. 3 A, B). Each sac with two large eggs; each egg about 70 µm in diameter.

Spermatophore (Fig. 3 C, D). Three-dimensional C-shaped.

Description of adult male

(allotype). Body length (Fig. 5 A), excluding caudal rami, 540 µm; smaller than female. General segmentation and ornamentation (Fig. 5 A – C) similar to female except genital somite and antennules. Genital somite (Fig. 5 B, C) not fused with urosomite 3 like in female, enlarged laterally; 1.3 times as wide as long; with pair of sclerotised rounded structures dorso-laterally; posterior margin with irregularly serrated free hyaline fringe. Urosomites 3–5 (Fig. 5 A – C) with about 1.5 times as long as wide; posterior margin with irregularly serrated free hyaline fringe.

Antennule (Fig. 5 D). 15 - segmented, geniculate. Armature formula as follows: 6 + 3 A. 4.2. 2 + A. 1.2.2.2 + A. 0.1. 1 + S. 1 + A. 0.1. 8 + 2 A.

Antenna, mouthparts, P 1, P 2 and P 5 similar to female.

P 3 (Fig. 6 A). Exp similar to female. Enp - 2 shorter than wide, Enp - 2 about 1.5 times as long as wide. Enp - 2 with transformed spine and smooth seta apically: transformed spine bent inward, with round tip bent outward, serrated by two rows of spinules on distal half of it. Inner subterminal seta transformed to spine-like seta, strong, bare, with blunt tip.

P 4 (Fig. 6 B). Exp and Enp similar to female but Enp with slightly longer, about 1.5 times as long as wide.

P 6 (Fig. 5 B, C). Reduced to simple plate, represented by three subequal pinnate setae.

Variability.

One of female (out of five) has P 6 with two elements.

Distribution.

Known only from the type locality.

Differential diagnosis.

The new species is similar to S. cavernicolus Boonyanusith, Sanoamuang & Brnacelj, 2018 in the following characteristics: 1) mandibular palp with three setae; 2) Exp and Enp of P 1 – P 4 are subequal; 3) spine formula of P 1 – P 4 Exp - 2 is 3.3.3.2; 4) P 4 lacks coxal seta; 5) P 4 intercoxal sclerite with round distal margin; 6) P 5 fused to pediger 5, remaining small unsegmented lobe; 7) P 6 with three elements (see Table 1). However, the new species differs from S. cavernicolus in: 1) anal operculum smooth and unproduced over segment bearing it (serrated free margin and well-developed in S. cavernicolus); 2) P 1 Enp - 2 with four elements (five elements in S. cavernicolus); 3) P 3 Enp - 2 with six elements in male (five elements in S. cavernicolus); 4) P 4 Exp - 2 with four setae (five setae in S. cavernicolus); 5) spermatophore is three-dimensional C-shaped (L-shaped in S. cavernicolus).

The SEM photos of P 5 show the degree of reduction in Siamcyclops and Bryocyclops species. The distal segment of P 5 is reduced to a small lobe on pediger 5 in Siamcyclops (Fig. 7 C, D), while it is more reduced in Bryocyclops, where it is represented by a small prominent knob (Fig. 7 A, B). In addition, the setae tend to be shorter and stronger, resembling spiniform setae in some species (Fig. 7 B), which is advantageous for species living in the confined spaces of the epikarst.