A rimstone pool in Sai Cave, Khao Daeng Subdistrict, Kui Buri District, Prachuap Khiri Khan Province, western Thailand; 12°10'46"N; 100°00'26"E, altitude: 107 m above sea level.

Holotype: one adult male, NHMUK 2018.1043, dissected and mounted on one slide; allotype: one adult female, NHMUK 2018.1044, dissected and mounted in one slide; paratypes: one ovigerous female, two adult females and three adult males, NHMUK 2018.1045-1050, preserved in 70% ethanol in 1.5 ml microtube and three adult females and three adult males, NPU 2018–003, preserved in 70% ethanol in 1.5 ml microtube; all specimens collected on 1 December 2016 by author.

Anal operculum ovate and serrate. P1–P4 with acute projections on distal margin of intercoxal sclerite; no inner coxal seta. Basis of P1 with inner spine. P1–P4 with two-segmented Exp and Enp. Setal and spine formula of P1–P4Exp-2 as follows: 5.5.5.4; 3.3.3.3; with no blunt-tipped setae; P1–P4Enp-2 as 3.4.5.3 and 1.1.1.1. Male P3Enp-2 with six elements, including one transformed spine and seta: transformed spine with acute tip; slightly swollen in medians part, armed with strong spinules; transformed seta bare, strong.

Preserved specimens colourless. Nauplius eye and refractile points on integument absent. Body length (Figs 2A, 3A) (n = 5) measured from anterior margin of rostrum to posterior margin of caudal rami 398–415 μm (mean 406 μm, n = 5); greatest width at distal part of cephalothorax 156–169 μm (mean 164 μm, n = 5); body length/width ratio 2.5. Cephalothorax completely fused to pediger I, without dorsal scar. Prosome length 245–248 μm (mean 246 μm, n = 5), urosome length 165–169 μm (mean 167 μm, n = 5); prosome/urosome length ratio 1.5. Prosomites with posterior dorsal margins smooth; urosomites serrated; pediger V finely serrated, and genital double-somite and two later urosomites coarsely serrated, both dorsally and ventrally (Fig. 3B, D). Genital double-somite (Figs 2A–C, 3F) symmetrical; length 73–76 μm (mean 75 μm, n = 5), width 98–100 μm (mean 99 μm, n = 5); 1.3 times wider than broad. Pair of sclerotized structures dorsolaterally, remnant of ancestral segment both dorsally and ventrolaterally; copulatory pore situated midventrally. Anal somite (Figs 2A–C, 3D–E) with strong spinules along entire posterior margin; pair of long sensilla above base of anal operculum. Anal operculum ovate, coarsely serrated, extending over three-fourths of caudal ramus length.

Caudal rami (Figs 2A–C, 3D–E) divergent, slightly tapering on distal half, 1.5 times as long as wide. Each ramus with six setae (seta II–VII): anterolateral (II) seta bare, slightly shorter than ramus; posterolateral (III) seta pinnate, 1.3 times as long as ramus, with strong spinules near venterolateral insertion; outer terminal (IV) and inner terminal (V) setae longest, pinnate, with fracture plane; terminal accessory (VI) seta shortest, bare, 0.5 times as long as ramus; dorsal (VII) seta located on distal margin of dorsal keel, bare, 2.2 times as long as ramus, with articulate insertion.

Antennule (Figure 2D) 11-segmented do not reach posterior margin of cephalothorax. Armature formula: 1(6S), 2(2S), 3(5S), 4(2S), 5(0), 6(1S), 7(3S), 8(2S), 9(2S), 10(2S), 11(7S+1A).

Antenna (Figure 4A) uniramous, consisting of coxobasis, and three-segmented Enp. Coxobasis with one smooth seta on distal inner corner. Enp-1–3 with 1, 5, and 7 setae respectively; with row of spinules on outer margin.

Mandible (Figure 4B) with six strongly chitinized teeth; dorsal seta on gnathobase. Palp reduced to single seta.

Maxillule (Figure 4C) with four strongly chitinized teeth on precoxal arthrite; inner margin with four bare setae and one plumose seta. Palp two-segmented: proximal segment with distal plumose seta; two bare setae distally; one bare seta dorsally; distal segment with three bare setae.

Maxilla (Figure 4D) with precoxal endite with two pinnate setae. Coxa with two endites: proximal endite with one bare seta; distal endite with one pinnate, and one bare setae. Basis with two strong claw-like expansions, bare seta close to its base. Two-segmented Enp, Enp-1 with unipinnate seta; Enp-2 with one unipinnate seta, two bare setae.

Maxilliped (Figure 4E) syncoxal endite with two strong spiniform setae, and row of spinules on anterior surface. Basis with spiniform seta, two rows of spinules on anterior surface. Two-segmented Enp; Enp-1 with one pinnate seta; Enp-2 with two bare setae.

P1–P4 (Fig. 5A–D) biramous, with two-segmented Exp and Enp. Intercoxal sclerites with no ornamentation, with acute projections on distal margin. P1 with group of tiny spinules at insertion of inner spine. All setae on P1–P4Exp-2 with normal tips. P1–P4Enp-1 with spiniform process on outer distal corner. Outer seta and inner proximal seta on P1Enp-2 as long as spine, shorter than inner distal seta. Outer seta and inner proximal seta on P2Enp-2 as long as spine, shorter than two inner distal setae. Proximal inner seta on P3Enp-2 as long as spine, shorter than the rest of segment; outer seta slightly longer than spine, followed by two inner medial setae and apical seta. Apical seta on P4Enp-2 longest, 2.5 times as long as spine, followed by inner and outer seta. Spine and setal formula of P1–P4 as follows (seta in Arabic numerals, spine in Roman numerals for outer-inner or outer-apical-inner seta/spine):

P5 (Figs 3G, 4F) fused to pediger, represented by three pinnate setae on small prominence: dorsal seta longer than two ventral setae; outer seta longer than inner one.

P6 (Figs 2A–B, 3F) situated laterodorsally next to heavily sclerotized structure on genital double-somite: with one short pinnate seta dorsally; two tiny spinules next to it ventrally element on small plate.

Adult females with pair of egg sacs (Figure 12A), each with two eggs, with a mean diameter of 64 μm (n = 8).

Body length (Figs 6A, 7A) measured from anterior margin of rostrum to posterior margin of caudal rami 369–373 μm (mean 371 μm, n = 5); smaller than female. General segmentation and ornamentation (Figs 6A–C, 7A–C) similar to female, with five-segmented urosome. Anal operculum (Figure 7C) more finely serrated than in female. Antennae, mouthparts, P1–P2, P3Exp-2, and P5 (Figs 7G, 8A–C) similar to those of female, except P2Enp-2 with longer inner medial seta.

Antennule (Figs 6D, 7F) 16-segmented, geniculate. Armature formula as follows: 1(7S+2A), 2(3S), 3(1S), 4(2S+1A), 5(2S), 6(1S), 7(2S), 8(1A), 9(1S), 10(2S), 11(1Sp), 12(0), 13(2S+1A+1Sp), 14(1S), 15(3S), 16(7S+1A).

P3 (Figure 8C) intercoxal sclerite with acute projections on distal margin. Enp-1 with inner pinnate seta. Enp-2 with outer pinnate seta; transformed apical spine with hook-like tip, thin pinnate seta; inner strong bare seta, two inner pinnate setae. Transformed apical spine with several spinules at 2/3 length of it; less-produced medial portion; with short, acute tip.

P4 (Figure 8D) intercoxal sclerite with acute projections on distal margin. Coxa without inner seta. Basis and Exp-1 with outer seta and spine, respectively. Exp-2 with three outer spines and two pinnate apical and inner setae. Enp-1 with inner pinnate seta and spiniform process on inner and outer distal margins. Enp-2 with apical spine and three long pinnate setae: apical and inner setae equal in length, 3.0 times as long as spine; outer seta 1.5 times longer than spine.

P6 (Figs 6B, C, 7D) reduced to simple plate, represented by three subequal pinnate setae.

Two out of five examined females have round rather than acute distal margins on intercoxal sclerite of P1.

The specific name asetus refers to the one of the primary characteristic that discriminates the new species from other species of group I sensuLindberg (1953) by the first swimming leg without an inner coxal seta.

This species is currently known only from Sai Cave, Khao Daeng Subdistrict, Kui Buri District, Prachuap Khiri Khan Province, Thailand.

The new species is most similar to B. maewaensis, found in caves in northern and western Thailand. They share the same morphological characteristics, such as the free margin of anal operculum ovate and serrate, the same setae and spines formula on P1–P4Exp-2 (setae: 5.5.5.4; spines: 3.3.3.3) and the same setae formula on P1–P4Enp-2 (3.4.5.3) in both sexes, except the male P3Enp-2 of B. asetus sp. n. which has five instead of four setae. The new species is clearly differentiated from B. maewaensis by its typical divergent caudal rami, the absence of a coxal seta on P1 and the absence of blunt-tipped setae on P2–P3Exp-2. Female P4Enp-1 of B. asetus sp. n. is larger than Enp-2 and has no spiniform process on the inner distal margin, which is present in B. maewaensis. Female P4Enp-2 of the new species has three long slender setae which are longer than the apical spine on the same segment, but B. maewaensis has robust and short setae that are approximately as long as spine. Male P3Enp-2 of the new species has a different transformed spine compared to B. maewaensis: the expansion is less expressed and is armed with strong spinules in the new species, but B. maewaensis has a well-expressed expansion and no ornamented surface.

One dissected adult female and male mounted separately on one slide from each (seven) caves: Pha Pu Cave (NPU 2018–004-005), Na-Or Subdistrict, Muang District, Loei Province, 17°33'21"N; 101°43'39"E, altitude: 247 m, collected on 18 August 2014; Chang Phueak Cave (NPU 2018–006-007), Banna Subdistrict, Muang District, Chumphon Province, 10°26'46"N; 99°02'06"E, altitude: 104 m, collected on 10 August 2015; Mae Nang Songsi Cave (NPU 2018–008-009), Hin Tok Subdistrict, Ron Phibun District, Nakhon Si Thammarat Province, 08°14'45"N; 99°52'01"E, altitude: 45 m collected on 23 October 2015; Khao Tiphon Cave (NPU 2018–010-011), Thung Wa Subdistrict, Thung Wa District, Satun Province, 07°05'10"N; 99°47'53"E, altitude: 46 m collected on 24 October 2015; Khao Jean Cave (NPU 2018–012-013), Khlong Khut Subdistrict, Muang District, Satun Province, 06°38'32"N; 100°05'77"E, altitude: 38 m collected on 25 October 2015; Kaew Cave (NPU 2018–014-015), Laem Thong Subdistrict, Phakdi Chumphon District, Chaiyaphum Province, 15°58'27"N; 101°24'36"E, altitude: 394 m, collected on 19 September 2016; Khao Maka Cave (NPU 2018–016-017), Sala Lamduan Subdistrict, Muang District, Sa Kaeo Province, 13°47'10"N; 101°56'51"E, altitude: 121 m, collected on 5 November 2017. Four adult males and four adult females from Chang Phueak Cave mounted on one stub for SEM analysis (NPU 2018–018). All specimens collected by author.

Body length (n = 5) excluding caudal setae, 481–491 μm (mean 486 μm), width 154–156 μm (mean 155 μm), body length/width ratio approximately 3.0 (Figs 9A, 11A). Prosome length 290–293 μm (mean 292 μm), urosome length 208–213 μm (mean 210 μm); prosome/urosome length ratio 1.4. Body surface with refractile points from cephalothorax to genital double-somite. Cephalothorax with dorsally incorporated scar, pair of dorsolateral body pits. Prosomites with smooth posterior dorsal margins; pedigers I-IV with serrated lines above posterior margins; pedigers I and II with dorsolateral body pits. Urosomites with dorsally serrated posterior margins; finely serrated on pediger V; urosomites coarsely serrate both dorsally and ventrally (Fig. 9B, C, E). Genital double-somite (Fig. 9B–D) symmetrical, 1.6 times as long as wide. Pair of sclerotized structures dorsolaterally, single copulatory pore midventrally. Urosomite 3–4 (Figure 9E) with body pits on dorsolateral surface. Anal somite (Fig. 9E–F) with strong spinules along entire posterior margin; pair of long sensilla above base of anal operculum. Anal operculum ovate, coarsely serrated and reaches over three-fourths of caudal ramus.

Caudal rami (Fig. 9E–F) parallel. Ramus 1.6 times as long as wide; with well-developed dorsal keel. Each ramus with four (seta II–V) pinnate setae and two (seta VI–VII) bare setae: posterolateral (III), outer terminal (IV) and dorsal (VII) setae with spinules above its insertion; setae IV and V (inner terminal seta) with breaking plane.

P1–P4 (Fig. 11B–E) biramous with two-segmented Exp and Enp but P4 with one-segmented Enp. Intercoxal sclerites with acute projections on distal margin. P2–P3Exp-2 with two blunt-tipped setae (apical and inner seta) (Figure 11C). P1–P3Enp-1 with spiniform process on outer distal corner. P4Enp with spiniform process on mid-outer margin. Spine and setal formula of P1–P4 as follows (seta in Arabic numerals, spine in Roman numerals for outer-inner or outer-apical-inner seta/spine):

P5 (Figure 9G) completely fused to pediger V, represented by three pinnate setae: dorsal seta longest, on small prominence; two ventral setae completely fused with pediger, inner seta shortest.

P6 (Figure 9H) located on genital double-somite at 1/2 length, dorsolaterally; represented by one short pinnate seta and two tiny spinules on small plate.

Adult females with pair of egg sacs (Figure 12B), each with two eggs, with a mean diameter of 50 μm (n = 8).

Spermatophore (Figure 9C) bean-shaped, paired, visible on genital double-somite mid-posteroventrally.

Body length (Figs 10, 11F–H) smaller than female with total body lengths of 432–440 μm (mean 436 μm), prosome/urosome length ratio 1.3 (n = 5). Body ornamentation, armature of P1–P2, P3–P4Exp-2, and P5 similar to those of females. P3 (Figure 11I) with two-segmented Enp, Enp-1 with inner pinnate seta and spiniform process on outer distal margin. Enp-2 with three inner setae (distalmost seta bare, two others pinnate), transformed apical spine and outer pinnate seta. Transformed spine with medial-produced, semi-circular shape with spinules on swollen portion; with long distance between swollen portion and its tip; tip acute. P4 (Figure 11J) with two-segmented Enp; Enp-1 with inner pinnate seta and spiniform process on outer and inner distal margins; spiniform process strong and well-expressed on inner margin. Enp-2 with apical spine and three long pinnate setae, apical and inner seta subequal in length, 2.5 times as long as spine, outer seta shortest, 1.5 times longer than spine. P6 (Figure 11F, H) reduced to simple plate, represented by three unequal pinnate setae.

Bryocyclops muscicola is the most widely distributed species within the genus Bryocyclops, which is mainly found in Southeast Asia with one record finding from North America (Florida, Reid 1999). To date, the species has been found in Indonesia (Java, Sumatra) and Thailand, including the north eastern region (Loei and Chaiyaphum provinces), the eastern region (Sa Kaeo Province), the southern region (Chumphon, Nakhon Si Thammarat, and Satun provinces) (Menzel 1926; present study, see Figure 1).

Bryocyclops muscicola belongs to Bryocyclops group II sensuLindberg (1953) (Dussart and Defaye 2006). All specimens of B. muscicola collected from caves in Thailand are consistent in terms of the details of their legs, anal operculum, and body ornamentation as observed by Menzel (1926) and Reid (1999). Some differences are noted on caudal rami and prosome: (1) the caudal ramus of Thailand’s specimens have tiny spinules at the insertion of the dorsal seta (VII) instead of the lateral seta (III) in both sexes, and the males have an obviously longitudinal keel on their rami that is absent in the Florida’s specimens; (2) the cephalothorax and pedigers II-IV of Thailand’s specimens have serrated dorsal frills above their posterior margins, a frill only present on pedigers II-IV in the Florida specimens.