Siamcyclops isanus, Watiroyram & Koompoot, 2024

Watiroyram, Santi & Koompoot, Kamonwan, 2024, Siamcyclops isanus sp. nov. (Copepoda, Cyclopoida, Cyclopidae), a new cave-dwelling species from northeastern Thailand, Subterranean Biology 49, pp. 185-200 : 185-200

publication ID	https://doi.org/ 10.3897/subtbiol.49.135303
publication LSID	lsid:zoobank.org:pub:F3B1718A-795E-4946-ACC1-9585C414053F
DOI	https://doi.org/10.5281/zenodo.14008990
persistent identifier	https://treatment.plazi.org/id/7B285B72-0665-4AE7-8F0F-0F40F5452BDB
taxon LSID	lsid:zoobank.org:act:7B285B72-0665-4AE7-8F0F-0F40F5452BDB
treatment provided by	Subterranean Biology by Pensoft
scientific name	Siamcyclops isanus
status	sp. nov.

Treatment

Siamcyclops isanus sp. nov.

Figs 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 View Figure 6

Type locality.

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

Description of adult female

(holotype). Habitus (Fig. 2 A View Figure 2 ) 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 View Figure 2 ) 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 View Figure 2 ) with about 1.6 times as long as wide; posterior margin with irregularly serrated free hyaline fringe. Anal somite (Fig. 2 A – D View Figure 2 ) 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 View Figure 2 ) reaching slightly beyond distal end of anal somite; semicircular; free margin smooth. Caudal rami slightly divergent (Fig. 2 A – C View Figure 2 ). 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 View Figure 3 ). 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 View Figure 3 ). 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 View Figure 3 ). 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 View Figure 3 ). 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 View Figure 3 ). 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 View Figure 3 ). 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 View Figure 4 ). 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 View Figure 4 ). 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 View Figure 4 ). 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 View Figure 4 ). 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 View Figure 4 ). 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 View Figure 4 ). 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 View Figure 2 ). 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 View Figure 3 ). Each sac with two large eggs; each egg about 70 µm in diameter.

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

Description of adult male

(allotype). Body length (Fig. 5 A View Figure 5 ), excluding caudal rami, 540 µm; smaller than female. General segmentation and ornamentation (Fig. 5 A – C View Figure 5 ) similar to female except genital somite and antennules. Genital somite (Fig. 5 B, C View Figure 5 ) 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 View Figure 5 ) with about 1.5 times as long as wide; posterior margin with irregularly serrated free hyaline fringe.

Antennule (Fig. 5 D View Figure 5 ). 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 View Figure 6 ). 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 View Figure 6 ). 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 View Figure 5 ). 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 View 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 View Figure 7 ), while it is more reduced in Bryocyclops , where it is represented by a small prominent knob (Fig. 7 A, B View Figure 7 ). In addition, the setae tend to be shorter and stronger, resembling spiniform setae in some species (Fig. 7 B View Figure 7 ), which is advantageous for species living in the confined spaces of the epikarst.