Sipadantonius roihani Boonyanusith, Wongkamhaeng & Azman, 2024, sp. nov.

Sipadantonius roihani sp. nov. Figs 2, 3, 4, 5, 6 (female); 6–8 (male) Material examined. Holotype • ♀ (adult), 0.95 mm long; 3 August 2023; coll. Azman, B. A. R.; light trap; UKMMZ -1631. Allotype • ♂ (adult), 0.87 mm long, collection data for holotype; UKMMZ -1632. Paratypes • 1 ♀ (adult) and 1 ♂ (adult); each was utterly dissected and mounted on a slide in glycerol and then sealed with nail varnish; the data was identical to that of the holotype; UKMMZ -1633 –1634. Additional material. • 2 ♂♂ (adult); the data was identical to that of the holotype; preserved in 70 % ethanol, subsequently retained in collection of the Sabah Parks Zoological Collection in Semporna, Sabah. Type locality. The marine cave “Turtle Tomb”, Sipadan Island, Sabah, Malaysia; the entrance is located at 4°07'04.8"N, 118°37'41.0"E. Samples were collected in the cave at a depth of 22.0 meters below the sea surface, ~ 100 meters from the entrance. Description of adult female. Body (Fig. 2 A, B) with a total length of 0.91–0.95 mm (measured from anterior margin of cephalosome to tip of furcal rami; mean: 0.93 mm; n = 3), slightly dorsoventrally flattened; integument covered with hair-like spinules. Prosome six-segmented, elliptical, ~ 70 % of body length and 2.2 × as long as urosome, ~ 2.3 × as long as wide, with greatest width at posterior margin of the first pedigerous somite (P 1 - bearing somite) (Fig. 2 A). Cephalosome and all pedigerous somites free, with smooth hyaline frill on posterior margin of cephalosome and first four pedigerous somites (Fig. 2 A, B); postero-lateral corners symmetrical and rounded (Fig. 2 B). Naupliar eye not discernible. Urosome four-segmented, comprising genital double-somite, and three free abdominal somites; all somites with finely serrated hyaline frill on posterior margin (Fig. 2 C – F). Genital double-somite barrel-shaped (Fig. 2 E), ~ 27 % of urosome length, as long as wide, with greatest width at mid-length of double-somite, with lingual-shaped genital operculum ventromedially; two gonoporal plates triangular-shaped, partially hidden under genital operculum (Fig. 2 E). Three free abdominal somites subequal in length. Anal somite with slightly developed anal operculum; posterior margin with serrated hyaline frill dorso-laterally (Fig. 2 F, G); anal operculum with smooth free margin, ornamented with row of spinules (Fig. 2 F). Furcal rami (Fig. 2 F, G) symmetrical, parallel, each ~ 3 × as long as wide, with serrated hyaline frill on distal margin and two cuticular pores laterally (Fig. 2 F, G); furcal setae I and III absent; furcal seta II spiniform, with setulae along inner margin; seta IV shorter than seta V, with breaking planes and plumose; seta V longest, with breaking plane and plumose, sub-equal to urosome length, ~ 1.2 × as long as seta IV; seta VI plumose, ~ 0.8 × as long as seta V; seta VII very short and plumose, inserted beside seta VI (Fig. 3 A). Length ratio of furcal setae II to ramus length ~ 0.5 and the ratio of setae to ramus length from seta IV to seta VII: 3.3: 3.9: 3.0: 0.2. Rostrum (Figs 2 B, 3 D) well developed, single plate and V-shaped; base broad, completely fused to anterior margin of cephalic shield and tapering to rounded tip between bases of antennules, with two sensillae, lacking rostral filaments. Antennules (Figs 2 A, 3 C, D) symmetrical, representing 26 - segmented, reaching distal margin of urosomite 2; ancestral segments II and III completely fused, representing evident segment II; ancestral segments X – XI partly fused, with remnant of ancestral articulation; ultimate segments ~ 1 / 3 of the length of pre-ultimate segment. Armature formula as follows (Roman numerals correspond to ancestral segment): 1 + ae (I), 2 + ae (II – III), 1 + ae (IV), 2 + ae (V), 2 + ae (VI), 2 + ae (VII), 2 + ae (VIII), 2 + ae (IX), 2 + ae (X), 2 + ae (XI), 2 + ae (XII), 2 + ae (XIII), 2 + ae (XIV), 2 + ae (XV), 2 + ae (XVI), 2 + ae (XVII), 2 + ae (XVIII), 2 + ae (XIX), 2 + ae (XX), 2 + ae (XXI), 1 (XXII), 1 (XXIII), 2 (XXIV), 2 + ae (XXV), 2 (XXVI), 5 + ae (XXVII – XXVIII). Antennae (Fig. 4 A) biramous. Coxa short, bearing one seta on distomedial corner. Basis with two setae on distomedial corner. Exp nine-segmented, setal formula from proximal to distal segments: 1.1. 1.1. 1.1. 1.1. 3. Endp two-segmented; proximal segment bearing two setae inserted at the same place on medial margin; distal segment bilobed, bearing nine apical setae on medial lobe and seven apical setae on distal lobe, with curved row of spinules on outer margin. Mandibles (Fig. 4 B) with sclerotised gnathobase comprising nine cuspidate teeth and one small dorsal seta on cutting edge. Mandibular palp biramous; basis with four setae on inner margin. Exp five-segmented, setal formula from proximal to distal segments: 1.1. 1.1. 2. Endp two-segmented; proximal segment with four setae on distomedial corner; distal segment short, with ten apical setae. Maxillulae (Fig. 4 C) with praecoxal arthrite bearing nine spinulose and spiniform marginal setae and one smooth marginal seta, with one seta on anterior surface and four setae on posterior surface; proximalmost one on posterior surface longest, unipinnate. Coxal epipodite with nine apical setae; two proximal ones shortest; coxal endite with six apical setae. Basis fused to Endp, proximal and distal endites armed with four and five apical setae, respectively; basal exite knob-like appearance, unarmed. Exp with eleven setae along apical and outer margin. Endp two-segmented, proximal and middle segments completely fused, setal formula: 4.3. 7. Maxillae (Fig. 4 D) seven-segmented, comprising praecoxa, coxa, basis and four-segmented Endp. Proximal and distal praecoxal endites with five and three apical setae, respectively. Coxa with two endites, each armed with three apical setae, with long spinules near distal margin of endite. Basis with large basal endite, armed with four apical setae; one of which strong and spiniform. Endp with setal formula: 3.2.2.3. Maxillipeds (Fig. 4 E) eight-segmented, comprising syncoxa, basis, and six-segmented Endp. Syncoxa with four syncoxal endites, setal formula: 1.2.4.4; one seta on second and third endite ~ 2 × as long as seta arising nearby. Basis with three medial setae, with row of spinules on anterior surface. Endp with setal formula: 2.4. 4.3. 3 + 1.4; seta 4 on segment II of Endp spiniform, with feather-like tip (Fig. 4 G). P 1 – P 4 (Figs 5 A – H, 6 A – D) biramous, comprising coxa, basis, and three-segmented rami. Integument ornamented with numerous spinules and short hairs. Intercoxal sclerite sub-rectangular. Coxa with medial seta on distomedial corner. Basis of P 2 – P 4 with cuticular window representing remnant of armament on outer margin (Figs 5 G, 6 C, D) but the remnant not discernible in P 1 (Fig. 6 B). Outer spine of P 1 Exp setiform; those of Exp of P 2 – P 4 stronger and oar-shaped. Distolateral corner of P 1 Exp - 2 drawn out into mint leaf-like process; those of Exp- 1 and Exp - 2 of P 2 – P 4 slightly extended, with two acutely protrusions beside spine; inner protrusion larger and longer than outer one. Anterior surfaces of Exp- 1 and Exp - 2 with cuticular pores near insertion of outer spine; Exp - 2 with acutely minute process near distomedial corner in P 2 – P 4 but not discernible in P 1; Exp - 3 with cuticular pores near insertion of both proximalmost outer spine and outer apical one. Endp of all swimming legs with cuticular pores near distal margin of Endp- 1 and at distal ~ 1 / 4 of Endp - 3. Outer and outer apical spines of all swimming legs relatively short, oar-shaped; inner apical spine with serrated cuticular expansion on outer margin. Armature of swimming legs as presented in Table 1. Some other characteristics of P 1 – P 4 as following described. P 1 (Figs 5 A – C, 6 A, B). Coxa trapezoidal; anterior surface with triangular expansion on proximomedial corner and oval integumental window near distomedial corner; integumental window surrounded by curved spinules. Basis bearing medial seta on distomedial corner but lacking either lateral seta or remnant of armament on outer margin; posterior surface with short curved hyaline process near insertion of Exp (Fig. 6 B). Anterior surface of Exp- 1 with row of long spinules on distolateral corner. Endp- 1 with bifid indenture on distolateral corner and long spinules along posterior margin; Endp - 2 with an acute indenture on distolateral corner. P 2 (Fig. 5 D, E). Coxa rectangular. Posterior surface of Exp- 1 with circular integumental window near insertion of inner seta; Exp - 3 with two outer spines. Endp- 1 with an acute indenture on distolateral corner; that of Exp - 2 bifid. P 3 (Figs 5 F, G, 6 C) as that of P 2 but Exp- 1 lacking integumental window on posterior surfaces and Exp - 3 with three outer spines. P 4 (Figs 5 H, 6 D) Coxa sub-quadrate. Basis and rami as those of P 3. P 5 (Figs 6 E, 7 A) biramous, with both rami three-segmented; armament as in Table 1. Integument and intercoxal sclerite as described for P 2 – P 4. Coxa rectangular, longer than wide; medial seta shorter than those of P 1 – P 4. Basis, Exp- 1 and Exp - 2 as in P 3 and P 4, but inner seta on Exp- 1 shorter than those of P 1 – P 4. Exp - 3 ~ 2.5 × as long as wide, offset on inner side at distal ~ 1 / 3 of segment. Endp- 1 and Endp - 2 with an acute indenture on distolateral corner each; Endp - 3 longer than length of Endp- 1 and Endp - 2 combined. Ornamentation as that of P 3 and P 4. Description of adult male. Body (Fig. 7 B) with a total length of 0.87 and 0.89 mm (measured from anterior margin of cephalosome to tip of furcal rami; mean: 0.89 mm; n = 2). Habitus slightly smaller and slenderer than in female (Fig. 7 B, C). Prosome six-segmented, elliptical, ~ 70 % of body length and 2.2 × as long as urosome, ~ 2.6 × as long as wide, with greatest width at posterior margin of the first pedigerous somite. Cephalosome and first three pedigerous somites similar to those in female. Naupliar eye not