Holotype: THAILAND • ♀ (adult), 0.73 mm long; Satun Province, Khay Cave; 6°53'40"N, 99°46'44"E, 17 m a.s.l.; 17 December 2014; C. Boonyanusith leg.; hand net; completely dissected and mounted on two slides in glycerol and sealed with nail vanish; PSUZC-PK2004-01–02. Allotype: THAILAND • ♂ (adult), 0.67 mm long, collection data as for holotype; PSUZC-PK2004-03. Paratypes: THAILAND • 1 ♀ (adult) and 1 ♂ (adult); same data as for holotype; PSUZC-PK2004-04–05.

THAILAND • 2 ♂♂ (adult); same data as for holotype; preserved in 70% ethanol; retained in collection of the first author (CB).

The species is named after Prof. Dr. La-orsri Sanoamuang (Khon Kaen University) in honour of her great and invaluable contribution on the knowledge of the planktonic fauna in Thailand. The name of species is a feminine noun in genitive singular.

The Khay Cave is in La-Ngu district, Satun province, ca. 760 km south of Bangkok (Thailand) (Fig. 2A). The cave is in an isolated, limestone hill of the Nakhon Sri Thammarat Mountain range, at an elevation of 17 m a.s.l, ca. 6.5 km from the Andaman Sea, (Fig. 2A–C). The cave has two entrances. The first one is located ca. 3 m over the hill floor and the second is at the base of the hill. Beyond the entrance is a horizontal gallery, which is ca. 20 m high. Occasionally, the gallery is inundated by freshwater during the rainy season. There is no permanent route connecting water in the cave and the sea; however, ca. 40 years ago, the cave was probably inundated by the sea water during the rising up of the sea water level (personal communication). The type locality is a small pool hidden under the cave wall with a small opening (Fig. 2D). It is ca. 10 m far from the first entrance and is seasonally filled by rain. The water temperature was 24.6 °C, pH 8.93, conductivity 450 µS cm^-1, DO 5.7 mg L^-1, and salinity 0.2 ppt.

Female: Pseudocyclopidae. Fourth and fifth pedigerous somites completely fused. Postero-lateral corners of cephalosome and first three pedigerous somite rounded. Genital double-somite barrel-shaped, ornamented with hyaline membrane all around the posterior margin; hyaline membrane with large medial notch ventrally. Genital pores paired, located ventrolaterally. Hyaline membrane of preanal somite expanded dorso-medially to form trapezoidal double-pointed flap. Caudal ramus with triangular pointed projection on distal margin. Antennule relative short, not reaching beyond distal margin of prosome. Apical spine on female P4Exp-3 elongated, ca. 3 × as long as outer terminal spine. Apical spine on female P5Exp-3 ca. 1.8 × as long as outer terminal spine. Male: The left P5Exp-3 highly transformed, bearing three irregular lobes; Endp oval-shaped, much shorter than right P5Endp, ca. 1.6 × as long as wide. The male right P5Exp with minute inner spiniform seta; distal outer spine elongated, ca. 3.4 × as long as proximal outer one and ca. 2.7 × as long as apical spine; subapical spine vestige ca. 0.7 × as long as apical spine.

Body (Fig. 3A) with a total length of 0.68 and 0.73 mm (measured from anterior margin of cephalosome to tip of projection of caudal rami, mean: 0.71 mm; N = 2). Prosome 5-segmented, elliptical, ca. 70 % of body length and 2.5 × as long as urosome, with greatest width at posterior end of first pedigerous somite; greatest width ca. 43 % of prosome length. Cephalosome and first three pedigerous somites free; postero-lateral corners rounded. Fourth and fifth pedigerous somites completely fused (Fig. 3A); postero-lateral corners rounded, symmetrical. Naupliar eye not discernible. Urosome 4-segmented, comprising genital double-somite, two free abdominal somites and very short anal somite (Fig. 3A–D). Genital double-somite barrel-shaped, ca. 45 % of urosome length, with greatest width at mid-length of double-somite, with hyaline membrane all around the posterior margin; hyaline membrane with large medial notch ventrally. Genital pores paired, located ventrolaterally (Fig. 3C). First and second free abdominal somites subequal in length, bearing hyaline membrane; hyaline membrane of the first free abdominal somite with serrulate margin, that of the second expanded dorso-medially to form a trapezoidal double-pointed flap, representing a pseudoperculum. Anal somite very short, telescoped within the preceding urosomite (Fig. 3B, D).

Caudal rami (Fig. 3D) subrectangular, ca. 1.8 × as long as wide (measured from base to level of insertion of setae V), with triangular pointed-projection on distal margin dorsally (Fig. 3B); projection 0.4 × as long as ramus length; caudal seta II to VII present, caudal seta I absent; seta II spiniform, with setules along inner margin; seta III plumose, approx. mid-length of seta IV; seta IV shorter than seta V, with breaking planes and plumose; seta V longest, with breaking plane and plumose, sub-equal to urosome length; seta VI slim and plumose. Seta VII inserted dorso-medially near insertion of seta V and seta VI (Fig. 3D). Length ratio of caudal setae to ramus length, from seta II to seta VII: 0.6 : 2.3 : 4.3 : 5.5 : 3.7 : 1.0. Length ratio of caudal setae from seta II to seta VII: 1.0 : 3.7 : 6.9 : 8.9 : 6.0 : 1.5.

Rostrum (Fig. 3E) weakly developed 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 at middle third of rostrum.

Antennule (Fig. 4A–C) symmetrical, 24-segmented, reaching to distal margin of prosome; ancestral segments II-IV and ancestral segments XXVII–XXVIII completely fused, representing evident segments 2 and 24, respectively. Segments 8 and 9 partly fused, with remnant of ancestral articulation of ancestral segment X and XI, penultimate and ultimate segments sub-equal in length. Armature formula as follows (Roman numeral corresponds to ancestral segment): 1+ae (I), 6+ae (II–IV), 2+ae (V), 2 (VI), 2+ae (VII), 2 (VIII), 2+ae (IX), 2+2ae (X–XI), 1 (XII), 1+ae (XIII), 1+ae (XIV), 1+ae (XV), 1+ae (XVI), 1 (XVII), 1+ae (XVIII), 1 (XIX), 1 (XX), 1+ae (XXI), 1 (XXII), 1 (XXIII), 2 (XXIV), 2+ae (XXV), 2 (XXVI), 5+ae (XXVII–XXVIII).

Antenna (Fig. 4D) biramous. Coxa short, bearing one spinulose seta on distomedial corner. Basis with two sub-equal setae on distomedial corner. Exp 9-segmented, apical segment small, setal formula 1, 1, 1, 1, 1, 1, 1, 1, 3. Endp 2-segmented; proximal segment bearing two setae on medial margin, setae inserted in the same place; distal segment bilobed, bearing three medial setae and six apical setae on medial lobe, with seven apical setae on distal lobe.

Mandible (Fig. 4E) with sclerotised gnathobase comprising ten cuspid or simple teeth and one small dorsal seta on cutting edge of coxal gnathobase. Mandibular palp biramous; basis with four setae on inner margin. Exp 5-segmented, ultimate segment minute, setal formula 1, 1, 1, 1, 2. Endp 2-segmented; proximal segment with four setae on distomedial corner; distal segment with ten apical setae.

Maxillule (Fig. 5A) with praecoxal arthrite bearing nine marginal, spinulose spines and one seta on anterior surface, and four setae on posterior surface. Coxal epipodite with nine apical setae; two proximal ones spinulose, other plumose; coxal endite with four apical setae. Basis fused to exopod, proximal and distal endites armed with four and five apical setae, respectively; basal exite with knob-like appearance and one vestigial seta. Exp with ten setae along apical and outer margin. Endp 3-segmented, proximal and middle segments partly fused, setal formula 4, 4, 7.

Maxilla (Fig. 5B) 6-segmented, comprising praecoxa, coxa, basis and 4-segmented Endp. Praecoxa partly fused to coxa, proximal and distal praecoxal endites with five and three apical setae, respectively. Coxa with two endites, each armed with three apical setae. Basis with large basal endite, armed with four strong apical setae; one of which ornamented with spinule row at mid-length of seta. Endp 4-segmented, setal formula 2, 2, 2, 3; ultimate segment with two long and one short setae.

Maxilliped (Fig. 5C) 8-segmented, comprising syncoxa, basis, and 6-segmented Endp. Syncoxa with four syncoxal endites, setal formula 1, 2, 2, 3; seta on first endite spinulose, basal seta on second endite strong, spinulose; distal endite with one long seta and two short, slender setae. Basis with three medial setae, with row of spinules on anterior surface. Endp with setal formula 2, 4, 4, 3, 3+1, 4; basal seta on first endopodal segment spinulose.

P1–P4 (Fig. 6A–D) biramous, comprising coxa, basis, and 3-segmented rami. Intercoxal sclerite trapezoidal. Coxa rectangular, with seta on distomedial corner. Basis of all swimming legs with lateral seta but lacking in P2; lateral seta inserted on posterior surface. Basis of P1 with robust seta on distomedial corner, with finger-like process on posterior surface arising near base of Exp; process reaching distal margin of Endp-1. Outer distal corner of all endopodal segments drawn out into triangular projection; projection relatively large in P1 and P2. P1Endp-1 without any outer seta. Outer distal corner of P1Exp-2 drawn out into spoon-like process, ornamented with spinules along outer margin. Outer distal corner of Exp-1 and Exp-2 of P2–P4 extended, forming 2-pointed sclerotised expansion, distal pointed process larger than proximal one. Outer spine of Exp-3 of all swimming legs relatively short. P4Exp-3 ca. 3.2 × as long as wide, with elongated, smooth apical spine, as long as segment bearing it and ca. 3 × as long as outer terminal spine, with row of curved spinules at its tip. Armature of swimming legs as presented in Table 1.

P5 (Fig. 6E) biramous, with 3-segmented Exp and 2-segmented Endp; armament as in Table 1. Coxa and basis as in P3 and P4. Exp-3 ca. twice as long as wide, with apical and outer terminal spines on its tip; apical spine elongated, ca. 1.8 × as long as outer terminal spine, ca. 1.2 × as long as Exp-3 length. Endp much shorter than Exp, reaching level of articulation of Exp-2; Endp-1 as long as wide, without pointed process on outer distal corner; Endp-2 ca.2.6 × as long as wide, with small, pointed process on outer distal corner.

Body with a total length of 0.65 and 0.67 mm (measured from anterior margin of cephalosome to tip of the projection of caudal rami; mean: 0.66 mm; N = 2). Habitus smaller and slenderer than in female (Fig. 7A). Prosome 5-segmented, as in female, ca. 70 % of body length and 2.5 × as long as urosome, with greatest width at posterior end of first pedigerous somite; greatest width ca. 47 % of prosome length. Cephalosome and first three pedigerous somites similar to those in female. Naupliar eye not discernible. Urosome 5-segmented; comprising genital somite, three free abdominal somites and very short anal somite. Genital somite slightly asymmetrical, ca. 25 % of urosome length; posterior margin with hyaline membrane dorsally. First three free abdominal somites similar in length, each with hyaline membrane all around posterior margin; hyaline membrane on third free abdominal somite as in female. Anal somite very short, telescoped within the preceding somite, as in female (Fig. 7B).

Caudal rami (Fig. 7B) relatively shorter than in female, ca. 1.8 × as long as wide. Armament and ornamentation as in female.

Antennule (Fig. 7C–E) asymmetrical. Left antennule non-geniculate, 24-segmented, setal formula as in female. Right antennule geniculate, 22-segmented; armature formula as follows (Roman numeral corresponds to ancestral segment): 1+ae (I), 6+ae (II–IV), 2+ae (V), 2 (VI), 2+ae (VII), 2 (VIII), 2+ae (IX), 1+ae (X), 1+ae (XI), 1(XII), 1+ae (XIII); 1 spiniform seta+ae (XIV), 1+ae (XV), 1+ae (XVI), 1 (XVII), 1 obtuse, fused spine +1+ae (XVIII), 1 obtuse, fused spine +2 (XIX), 1 (XX), 2+ae; one seta spiniform (XXI–XXIII), 4+ae (XXIV–XXV), 2 (XXVI), 5+ae (XXVII–XXVIII).

Antenna, mandible, maxillula, maxilla, maxilliped, and P1−P4 as in female.

P5 (Fig. 8A, B) biramous, asymmetrical. Coxae and intercoxal sclerite fused, forming a common base. Basis rectangular, with outer seta on posterior surface. Left leg biramous, with 3-segmented Exp and 1-segmented Endp; Exp-1 with a long robust outer spine; Exp-2 modified, with a long robust outer spine; Exp-3 highly transformed, bearing several flexible and irregular lobes; outer lobe bearing finger-like appendage; middle lobe prominent, bearing scoop-like appendage; inner lobe, with two elements; innermost (uppermost) one curved, strong seta; other one curved, gutter-like, with serrated concave margin at its cutting edge; Endp flat, oval-shaped, ca. 1.6 × as long as wide. Right leg biramous, with 1-segmented Exp and 1-segmented Endp. Exp with two outer spines, inner spiniform seta, and apical spine, plus spine vestige located subapically on anterior surface; distal outer spine elongated, ca. 3.4 × as long as proximal outer one, ca. 2.7 × as long as apical spine; subapical and apical spines machete-shaped, subapical spine vestige ca. 0.7 × as long as apical spine; apical spine ca. 0.4 × as long as distal outer spine; inner spiniform seta minute, located at level of insertion of proximal outer spine; Endp as long as Exp, ca. 3 × as long as wide, armed with two sub-equal spines.

The new species was confidently identified to the genus Boholina based on the combination of the following characteristics mentioned by Fosshagen and Iliffe (1989):

(1) fourth and fifth pedigerous somites fused,

(2) rostrum with a rounded tip,

(3) genital pores paired and separate,

(4) caudal rami with cuticular pointed projection distally,

(5) distal outer corner of all endopodal segments of P1 forming a triangular pointed projection,

(6) P1Endp-3 without outer seta,

(7) P4Exp-3 with modified apical spine,

(8) female P5 with a 2-segmented Endp,

(9) male left P5Exp-3 modified to unique characteristic grasping organ, and

(10) female and male with 4- and 5-segmented urosome, respectively; anal somite very short and sometimes concealed within the preceding somite.

Examination of the structure of the genital double-somite, of P4 and P5 in both males and females, the relative length of subapical spine vestige on the male right P5 and the shape of the male left P5Endp revealed that the new species is most similar to B. crassicephala, which had previously been described from a pool in a cave of Bohol Island, the Philippines. Several characters are shared by both species, especially the structure of the male P5. However, there are also remarkable differences (Table 2). The characteristics which obviously distinguish Boholina laorsriae sp. nov. from B. crassicephala are as follows:

i) Apical spine of the female P5Exp-3 is ca. 1.8 × as long as outer terminal spine in the new species, but it is sub-equal to the outer terminal spine found in B. crassicephala.

ii) Exopodal segment of the male right P5 has medial minute seta in the new species; however, it is absent in B. crassicephala.

iii) Distal outer spine on exopodal segment of the male right P5 is relatively long, and the distal outer spine is ca. 2.9 × as long as the proximal one; however, in B. crassicephala, the spine on exopodal segment of the male right P5 is relative shorter and the distal outer spine is ca. 1.9 × as long as the proximal one.

iv) The male left P5Endp is relatively smaller in the new species than that of B. crassicephala.

The Thai Boholina can be easily distinguished from B. purgata, B. parapurgata, and B. ganghwaensis by the characteristics of the widely separated genital pores, relatively longer subapical spine vestige on the male right P5 when compared to the length of apical spine, the higher length ratio of the apical spine to the outer terminal spine in the female P5Exp-3 and the elongated apical spine of the female P4Exp (Table 2). The genital double-somite of the new species is barrel-shaped, while it is globular in B. munaensis. Additionally, the ii) characteristic is unique for the Thai Boholina. Based on the characteristics used in Moon and Soh (2014) accompanied by additional ones, the morphological characteristics of the six species are presented in Table 2.

Only six specimens were collected from a pool and the new species was not encountered in the other eight caves visited in this research project. Freshwater Cyclopoida belonging to the genera Thermocyclops, Metacyclops, and Mesocyclops, as well as harpacticoids of the genus Schizopera and of the family Ectosomatidae were also collected from the type locality.