Holotype. Adult female, dissected and mounted onto one slide, collected from type locality on 30 July 2015 (PSUZC-PK2002-01). Allotype. One adult male, dissected and mounted onto one slide, (PSUZC-PK2002-02), all collected from type locality on 20 January 2016. Paratypes. One undissected adult ovigerous female paratype mounted onto one slide (PSUZC-PK2002-03), and one undissected adult male paratype, mounted onto one slide (PSUZC-PK2002-04), coll. C Boonyanusith and K Wongkamheng; type locality on 30 July 2015 and 20 January 2016, respectively.

One adult ovigerous female and one adult male, both collected from type locality on 2 April 2015 and stored in 70% ethanol, deposited in the collection of the first author (CB).

Prawattisart cave, Muang district, Satun Province, southern Thailand, 6°42'55.82"N, 100°05'19.17"E. The cave is in an isolated, wedge-shaped, limestone hill. It is surrounded by irrigation canal and paddy fields, but there are no connections between water in and outside cave. Beyond entrance of the cave there is a long horizontal gallery, with approximately 5–10 m high and approximately 2–3 m wide. At some place there are two openings, which are large enough for entry, indicating a complex tunnel system. The collecting point is a part of a large water body inside the cave, fulfilled by approximately 10–50 cm depth of water. Water depth varies according to season, but it has never dried out. Water was turbid and flowed slowly. Water temperature was 25.5 °C, pH 7.8, conductivity 360 µS cm^-1, dissolved oxygen 4.9 mgL^-1.

The specific name satunensis is derived from the name of Satun Province, where the species was collected. The name is a noun in the genitive singular, masculine.

Laophontidae. Body gradually tapering posteriorly. Cephalothorax with internal sausage-like structure, and internal rounded structures. Posterior margin of cephalothorax and body somites (except penultimate and anal somite) with sensillum-bearing tubercles. Second and third urosomite partly fused ventrally in female, representing genital double-somite. Caudal rami approximately 2.5 times as long as wide, with one transverse inner row of spinules near insertion of dorsal seta. Caudal seta IV and V fused. Allobasis of antenna without abexopodal seta. Endopodal lobe of P5 with three, and exopod with four pinnate setae. Male P3 enp-2 with apophysis on outer distal corner, reaching tip of enp-3. Exopod of male P5 with three setae, outer seta as long as supporting segment. Male P6 reduced, with outer seta and inner bipinnate seta apically; inner seta approximately twice as long as outer one.

Body (Fig. 2A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 401–445 µm (mean 419 µm, n = 3; 445 µm in holotype); preserved specimen colourless. Body covered with setules (Fig. 8E, F), cylindrical; gradually tapering posteriorly, with maximum width at posterior part of cephalothorax. Prosome approximately 1.3 times as long as urosome (including caudal rami) (Fig. 2A). Rostrum small, completely fused to cephalothorax. Cephalothorax as long as wide, approximately 0.5 times the length of prosome, just under integument with anterior internal structure comprising three parts; each of which sausage-like, and with internal rounded structures near distal margin (Figs 2A, B, 8A, B). Cephalothorax and all free thoracic somites with sensillum-bearing tubercles along posterior margin (Fig. 2A, B). Second and third urosomite fused ventrally (Fig. 2D), distinct dorsally and laterally (Fig. 2A–C), original division between sixth thoracic somite and first abdominal somite, with dorsal sensillum-bearing tubercles (Fig. 2C). Genital field ribbon-shaped, with seta representing P6 at outer distal corner (Fig. 2D). The remnant of first abdominal somite (posterior half of genital double-somite) with lateral sensillum-bearing tubercles (Fig. 2C, D). Posterior margin of genital double-somite and fourth urosomites with outer sensillum-bearing tubercles; posterior half of genital double-somite and fourth urosomite with posterior setules dorsally (Fig. 2C) and small spinules ventrally (Fig. 2D) between sensillum-bearing tubercles. Penultimate urosomite with posterior setules dorsally and laterally, with one posterior row of spinules ventrally. Anal somite as long as wide, with arch row of long spinules posterior to anal operculum (Figs 2A, C, 3A, 8D), with ventrolateral row of minute spinules near insertion of caudal rami (Fig. 2D). Anal operculum poorly developed, with minute spinules along posterior margin (Figs 2C, 3A, 8D).

Caudal rami (Figs 2C, D, 3A). Slightly convergent, 2.5 times as long as wide, with one transverse row of inner spinules near insertion of caudal seta (VII) (Figs 2C, 3A). Anterolateral accessory seta (I) minute, close to anterolateral seta (II), both subapical. Posterolateral seta (III) inserted on minute pedestal. Outer terminal seta (IV) slender, fused at base with inner terminal seta (V), the latter longest, without fracture plane, approximately 0.6 times as long as body length. Inner accessory seta (VI) slender. Dorsal seta (VII) tri-articulate, inserted at quarter of rami. Length ratio of caudal setae to ramus length, from seta I to seta VII of the holotype: 0.4 : 0.8 : 1.3 : 0.7 : 9.7 : 0.7 : 1.1.

The ovigerous female bears one oval egg sac with eight eggs, located ventrally between pair of fifth legs.

Antennule (Fig. 3B, C). Short, 5-segmented. First segment with medial and distal rows of spinules. Armature formula: I-[1], II-[8], III-[7+(1+aesthetasc)], IV-[1], V-[9+acrothek]. Aesthetasc on third segment robust, fused basally to one seta. Apical acrothek on fifth segment slender, consisting of one aesthetasc fused basally to two slender smooth seta.

Antenna (Fig. 3D). Comprising coxa, allobasis, and 1-segmented endopod and exopod. Coxa without ornamentation. Allobasis with one row of inner spinules, with 1-segmented exopod; the latter with two apical and two lateral bipinnate setae. Free endopodal segment with one strong, sharp spine, and one seta at distal third of segment; the former accompanied by several strong, short spinules; distal end with six elements: one minute seta, one slender, bipinnate seta, two geniculate setae, and two strong, smooth spines.

Mandible (Fig. 3E). Gnathobase with strong, chitinised teeth and lateral pinnate seta. Mandibular palp short, 1-segmented, with five slender setae sub-equal in length.

Maxillule (Fig. 4A). Composed of robust precoxa, coxa, basis, endopod fused to basis, and 1-segmented free exopod. Precoxal arthrite with six strong apical spines and one slender lateral seta. Coxa with cylindrical endite bearing two smooth setae, one of which robust. Basis with cylindrical endite bearing three setae, one of which robust. Endopod incorporated to basis, with three setae. Exopod free, 1-segmented, with two sub-equal apical setae.

Maxilla (Fig. 4B). Composed of syncoxa, allobasis, and 1-segmented endopod. Syncoxa with two endites; proximal endite with three apical pinnate setae, distal endite with two pinnate and one slender apical seta, outer margin with spinules. Allobasis with apical drawn out into claw, with one anterior and one posterior seta. Endopod 1-segmented, with two smooth apical setae.

Maxilliped (Fig. 4C). Subchelate, 3-segmented, comprising syncoxa, basis, and endopod. Syncoxa with one pinnate seta at inner distal corner. Basis with two transverse rows of outer spinules, one of which near base of endopod. Endopod drawn out into strong claw, with one minute seta near its base.

P1 (Fig. 4D). Coxa with longitudinal row of outer setules. Basis with one outer bipinnate spine and one inner spine near insertion of endopod, with longitudinal row of anterior spinules medially, with long setules along inner margin. Both rami 2-segmented. Exopod reaching proximal third of enp-1; exp-1 with one bipinnate outer spine, with one row of outer spinules; exp-2 with three outer spines, two apical geniculate setae, with outer spinules and inner setules. Enp-1 approximately seven times as long as wide, with outer spinules and inner setules; enp-2 with one median strong outwardly curved claw-like smooth spine and one slender inner seta, with few outer spinules.

P2 (Fig. 5A, B). Coxa with oblique row of spinules on anterior surface near outer margin, with row of spinules at distal outer corner. Basis with outer spine. Rami with 3-segmented exopod and 2-segmented endopod; endopod reaching tip of exp-2. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with three outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one plumose seta), and one inner plumose seta. All segments of exopod with row of outer spinules and inner setules. Enp-1 without armature, enp-2 with two apical and two inner plumose setae. All segments of endopod with row of outer spinules and long inner setules.

P3 (Fig. 5C). Coxa, basis, and segmentation of rami as in P2. Outer element of basis one long seta. Endopod reaching tip of exp-2. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with three outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one plumose seta), and one inner plumose seta. Ornamentation of exopod as in P2. Enp-1 without armature; enp-2 with one outer bipinnate seta, two apical, and three inner plumose setae. Outer and inner margins of segments of endopod with setules.

P4 (Fig. 6A). Coxa with row of outer long spinules. Basis with one slender outer seta. Rami with 3-segmented exopod and 2-segmented endopod; endopod reaching tip of exp-1. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with two outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one plumose seta), and one inner plumose seta. Ornamentation of exopod as in P2 and P3. Enp-1 without armature, with long inner subdistal spinules and smaller outer spinules; enp-2 with one outer bipinnate seta, one apical plumose seta, and one inner plumose seta.

Armature formula of P1−P4 as in Table 1.

P5 (Figs 6B, C, 23A). Baseoendopod and exopod separated, with setules as on figures. Baseoendopod with basal seta and three pinnate setae on endopodal lobe, with spinules at base of distalmost seta. Exopod with four pinnate setae; innermost one longest.

P6 (Fig. 2D). Reduced to minute prominence at outer distal corner of genital field, with one short, slender seta.

Body (Fig. 6D). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 352−410 µm (mean 374 µm, n = 3; 410 µm in allotype); preserved specimen colourless. Prosome approximately 1.3 times as long as urosome (Fig. 6D). Cephalothorax as long as wide, 0.5 times the length of prosome, internal sausage-like structure as in female, with two internal rounded structures. All free thoracic somites with sensillum-bearing tubercles along dorsal posterior margin, but fifth thoracic somite (first urosomite) with additional row of posterior setules dorsally (Fig. 6D). Second and third urosomite completely separated. Second urosomite with dorsal sensillum-bearing tubercles along posterior margin (Fig. 6D). Fourth urosomite without lateral protuberances, with lateral sensillum-bearing tubercles, with one posterior row of dorsal setules and ventral spinules. Ornamentation on next three urosomites as in female (Fig. 6E). Anal somite and anal operculum as in female.

Caudal rami as in female (Fig. 6D, E).

Antennule (Fig. 7A). 8-segmented, geniculate, with three segment distal to geniculation. First segment with proximal and subdistal outer spinules. Armature formula I-[1], II-[9], III-[7], IV-[2], V-[9+(1+aesthetasc)], VI-[1], VII-[1], VIII-[7+acrothek)]. Aesthetasc on fifth segment robust, fused basally to one seta. One pinnate seta on fifth segment. Apical acrothek on eighth segment small, consisting of one aesthetasc fused basally to two slender smooth seta.

Rostrum, antenna, mouthparts and P1, P2 (not shown) as in female.

P3 (Fig. 7B). General shape as in female but both rami 3-segmented. Endopod reaching tip of exp-2. Exp-3 with three outer bipinnate spines, two apical elements (of which outer one spiniform robust seta with outer spinules and inner setules, inner element one smooth spiniform seta), and one inner plumose seta. Enp-2 with inner seta and outer apophysis on outer distal corner reaching tip of enp-3, with long outer spinules and few inner setules; enp-3 with two inner and two apical plumose setae.

P4 (Fig. 7E). General shape as in female but exp-3 relatively shorter and with stronger elements. Exp-3 with two outer bipinnate spines, two apical elements (of which outer one spiniform robust seta with outer spinules and inner setules, inner element one spiniform seta), and one inner plumose seta.

Armature formula of P1–P4 as in Table 1.

P5 (Fig. 6E, F). With outer basal seta arising from long setophore; without endopodal lobe. Exopod with three setae, outermost slender and slightly longer than segment, approximately 1/3 times as long as the middle seta.

P6 (Fig. 6E). Reduced to one minute rectangular protuberance, with outer plumose seta and inner bipinnate seta; inner seta approximately twice as long as outer one and reaching posterior margin of next urosomite.

The right P2 enp-2 lacks the proximalmost inner seta (Fig. 5B), and one additional small inner seta on the P5 exopod was observed (Fig. 6B) in the holotype. The P3 enp-3 of the allotype possesses two inner setae. Also, on the right ramus the setae possess outer and inner setules (Fig. 7B), but those of the left ramus possess inner spinules (Fig. 7C, D).

Onychocamptus satunensis sp. n. is the only species of Onychocamptus with internal sausage−like structure on cephalothorax (Figs 2A, B, 8A–C), differing from other members. O. satunensis sp. n. is most similar to O. tratensis sp. n., both species sharing the following remarkably characters: 1) absence of abexpodal seta on allobasis, 2) presence of 4 setae on exopod of antenna, 3) presence of 2 outer spines on P4 exp-3, 4) presence of 4 setae on exopod of P5 of the female, and 5) presence of 3 setae on exopod of P5 of the male. The southern Thai species (O. satunensis sp. n.), however, can be distinguished from the eastern one (O. tratensis sp. n.) by the presence of internal sausage−like structure, the presence of internal rounded structures, and the relative length of outer seta on P3 enp-2. Comparative study between two Thai new species and their congeners is provided in Table 3.

This species is known from the type locality only.

Holotype. Adult female, dissected and mounted onto two slides (PSUZC-PK2003-01–PSUZC-PK2003-02). Allotype. One adult male, dissected and mounted onto two slides (PSUZC-PK2003-03–PSUZC-PK2003-04). Paratypes. One undissected adult female, mounted onto one slide, (PSUZC-PK2003-05). One undissected adult male, mounted onto one slide, (PSUZC-PK2003-06). One adult female, dissected and mounted onto two slides, (PSUZC-PK2003-07–PSUZC-PK2003-08).

All specimens were collected by S Maiphae and T Saetang from type locality on 9 January 2017.

Ten females and five males, all collected from type locality on 9 January 2017 and stored in 70% ethanol, deposited in crustacean reference collection, Zoological Museum, Kasetsart University (ZMKU_CP).

Samer-rach peat swamp, Trat Province, eastern Thailand, 12°28'04.0"N, 102°21'20.6"E. Water temperature ranged between 28.83 °C, pH of 6.23, salinity 6.91 ppt, total dissolved solids 7.9 mg L^-1, and dissolved oxygen 4.41 mg L^-1.

The specific name tratensis is derived from the name of Trat Province, where the species was collected. The name is a noun in the genitive singular, masculine.

Laophontidae. Body gradually tapering posteriorly. One middle and two lateral rounded integumental window-like structures on cephalothorax. Second and third urosomite fused ventrally in female forming genital double-somite. Caudal rami cylindrical, both sides parallel, approximately 2.5 twice as long as wide, with one longitudinal row of minute spinules on inner margin near insertion of dorsal seta and horizontal row of minute spinules near insertion of inner terminal seta. Outer terminal seta (seta IV) fused at base with inner terminal seta. Allobasis of antenna without abexopodal seta. Endopodal lobe of P5 with two bipinnate and one plumose setae on inner margin and exopod of P5 with four plumose setae. Enp-2 of male P3 with apophysis on outer distal corner; apophysis reaching the tip of enp-3. Exopod of male P5 with three bipinnate setae, outer seta two times as long as supporting segment. P6 of the male reduced, with outer seta and inner bipinnate seta apically; inner seta approximately twice as long as outer one.

Body. Total body length, measured from tip of rostrum to posterior margin of caudal rami, 360–450 µm (mean 400 µm, n = 14; 420 µm in holotype); preserved specimen colourless. Body covered entirely with setules, cylindrical; gradually tapering posteriorly, with maximum width at posterior part of cephalothorax. Prosome 1.3 times as long as urosome (including caudal rami) (Fig. 9A). Rostrum small, completely fused to cephalothorax. Cephalothorax as long as wide, approximately 0.5 times the length of prosome length, with one middle and two lateral rounded integumental window-like structures on cephalothorax. Cephalothorax and all free thoracic somites with sensillum-bearing tubercles along posterior margin. Second and third urosomite fused ventrally (Fig. 9C, D), distinct dorsally. Genital field ribbon-shaped, with seta representing P6 at outer distal corner (Fig. 9C). The remnant of first abdominal somite (posterior half of genital double-somite) with lateral sensillum-bearing tubercles (Fig. 9C, D). Posterior margin of genital double-somite and fourth urosomites with outer sensillum-bearing tubercles; posterior half of genital double-somite and fourth urosomite with posterior setules dorsally (Fig. 9D) and small spinules ventrally (Fig. 9C) between sensillum-bearing tubercles. The penultimate urosomite with posterior setules dorsally and laterally, with one posterior row of spinules ventrally. Anal somite approximately 0.6 times longer than wide, with arch row of long spinules posterior to anal operculum (Fig. 9D), with ventrolateral row of minute spinules near insertion of caudal rami (Fig. 9C). Anal operculum poorly developed, with minute spinules along posterior margin (Fig. 9D).

Caudal rami (Fig. 9C, D). Cylindrical, both sides parallel, 2.5 times as long as wide, with one longitudinal row of minute inner spinules near insertion of caudal seta (VII) and horizontal row of minute spinules near insertion of inner terminal seta (V). Anterolateral accessory seta (I) minute, inserted, close to anterolateral seta (II), both subapical. Posterolateral seta (III) inserted on minute pedestal. Outer terminal seta (IV) slender, fused at base with inner terminal seta (V), the letter longest, without fracture plane, approximately 0.9 times as long as body length. Inner accessory seta (VI) slender. Dorsal seta (VII) tri-articulate, inserted at quarter of rami. Length ratio of caudal setae to ramus length, from seta I to seta VII of holotype: 0.5 : 1.7 : 2.5 : 0.7 : 13.3 : 0.7 : 2.0.

Egg sac (Fig. 9E). Ovigerous female with two egg sacs ventrally between pair of P5, each with eight eggs.

Antennule (Fig. 10A). Short, 5-segmented, large aesthetasc on third segment and small aesthetasc on fifth segment. Surface of all segments smooth, except for medial and distal rows of small spinules on first segment. Armature formula I-[1], II-[7+(1+aesthetasc)], III-[8+aesthetasc], IV-[1], V-[9+acrothek]. Aesthetasc on third segment fuse basally to one smooth seta. Apical acrothek consists of one aesthetasc fused basally to two slender smooth setae. Only seta on first segment bipinnate, all other setae smooth.

Antenna (Figs 10B, 22A). Comprising coxa, allobasis, 1-segmented endopod, and exopod. Coxa without ornamentation. Allobasis with one row of inner spinules, with 1-segmented exopod; the latter with two apical and two lateral bipinnate setae. Free endopod with two strong sharp spines accompanied by several strong, short spinules along outer margin, distal end with five elements; one slender seta, two geniculate setae, and two strong spines.

Mandible (Fig. 11A). Gnathobase with strong, chitinised teeth and dorsal pinnate seta along cutting edge. Mandibular palp short, 1-segmented, with five slender setae sub-equal in length.

Maxillule (Fig. 11B). Composed of robust precoxa, coxa, basis, endopod fused to basis, and 1-segmented free exopod. Precoxal arthrite with six strong apical spines, with lateral spine. Coxa with cylindrical endite bearing two smooth setae. Basis with cylindrical endite bearing three setae. Endopod incorporated to basis, with three setae. Exopod free, 1-segmented, with two sub-equal apical setae.

Maxilla (Fig. 11C). Composed of syncoxa, allobasis, and 1-segmented endopod. Syncoxa with two endites; each endite with two apical pinnate setae, outer margin with spinules. Allobasis with apical drawn out into claw, with one anterior and one posterior seta. Endopod 1-segmented, with two smooth apical setae.

Maxilliped (Fig. 11D). Subchelate, 3-segmented, comprising syncoxa, basis, and endopod. Syncoxa with one pinnate seta at outer distal corner. Basis with two transverse rows of outer spinules, one of which near base of endopod. Endopod drawn out into strong naked claw, with one small seta near base.

P1 (Fig. 12A). Intercoxal sclerite naked. Precoxa small and triangular, with one row of spinules at distal margin (not shown). Coxa with one row of long outer spinules. Basis with one outer bipinnate spine and one inner plumose seta near insertion of endopod, with longitudinal row of anterior spinules medially, with long setules along inner margin. Both rami 2-segmented. Exopod reaching proximal third of enp-1; exp-1 with one bipinnate outer spine, with one row of outer spinules; exp-2 with three outer smooth spines and two apical geniculate setae, with row of outer spinules and inner setules. Enp-1 approximately 4.4 times as long as wide, with one row of outer and inner setules; enp-2 with one median strong outwardly curved claw-like smooth spine and one slender inner seta, with few outer spinules.

P2 (Fig. 12B). Intercoxal sclerite and precoxa as in P1. Coxa with two oblique parallel rows of long outer spinules (one on anterior and others on posterior surface), with one row of spinules at distal margin, and few spinules at inner margin. Basis with outer bipinnate spine, with spinules at base of spine, with one row of long spinules between exopod and endopod, and with one row of inner setules. Rami with 3-segmented exopod and 2-segmented endopod; endopod reaching tip of exp-2. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with three outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one plumose seta), and one inner plumose seta. All segments of exopod with several rows of strong outer spinules and inner setules, and only exp-1 with one row of outer setules. Enp-1 without armature, enp-2 with two apical and two inner plumose setae. All segments of endopod with one row of long outer and inner setules.

P3 (Fig. 13A). Intercoxal sclerite and precoxa as in P1. Coxa with two parallel rows of long spinules along outer margin (one on anterior and other on posterior surface). Basis with one smooth outer seta; with spinules at base of seta and one row of inner setules. Segmentation of rami as in P2, endopod reaching to middle segment of exp-2. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with three outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one plumose seta), and one inner plumose seta. All segments of exopod with several rows of strong outer spinules and one row of inner setules, and exp-1 and exp-2 with one row of outer setules. Enp-1 without armature, enp-2 with one outer bipinnate seta, two apical, and three inner plumose setae. Outer and inner of all segments of endopod with long setules.

P4 (Fig. 13B). Intercoxal sclerite and precoxa (not shown) as in P1. Coxa with one row of setules and several outer spinules. Basis with one smooth outer seta; with spinules at base of seta and one row of inner setules. Rami with 3-segmented exopod and 2-segmented endopod, endopod smaller than exopod. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta; exp-3 with two outer bipinnate spines, two apical elements (of which outer one bipinnate spine, inner element one plumose seta), and one inner plumose seta. All segments of exopod with several rows of strong outer spinules and with fewer inner setules. Enp-1 without armature, enp-2 with one outer seta with plumose proximally and bipinnate distally, one apical plumose seta, and one inner plumose seta. Outer and inner of all segments of endopod with setules.

Armature formula of P1–P4 as in Table 1.

P5 (Figs 13C, 23B). Baseoendopod and exopod separated with setules as figured. Baseoendopod with basal seta and three inner setae on endopodal lobe; one proximal bipinnate seta, one middle bipinnate seta and one apical plumose seta, with one row of spinules at base of each seta, as well as with one distal row of spinules between distal seta of baseoendopod and exopod. Exopod with four plumose setae, with row of inner and outer setules, and with spinules at base of innermost seta.

Body (Fig. 14A, B). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 340–360 µm (mean 350 µm, n = 3; 350 µm in paratype); preserved specimen colourless. Prosome approximately 1.5 times as long as urosome. Cephalothorax as long as wide, 0.5 times the length of prosome. All free thoracic somites with sensillum-bearing tubercles along posterior margin, but fifth thoracic somite (first urosomite) with additional row of posterior setules dorsally. Second and third urosomite completely separated. Second urosomite with dorsal sensillum-bearing tubercles along posterior margin. Fourth urosomite without lateral protuberances, with one posterior row of dorsal setules and ventral spinules. Ornamentation on next three urosomites as in female. Anal somite approximately 0.5 times longer than wide. Anal operculum as in female.

Caudal rami (Fig. 14A, B). As in female.

Antennule (Fig. 14C). 8-segmented, large aesthetasc on fifth segment and small aesthetasc on eighth segment. First segment with proxim al setules and subdistal outer spinules, second segment with one row of outer spinules, other segments smooth. Armature formula I-[1], II-[9], III-[3], IV-[2], V-[9+(1+aesthetasc)], VI-[0], VII-[1], VIII-[7+acrothek]. Aesthetasc on fifth segment robust. Apical acrothek consists of one aesthetasc fused basally with two slender smooth setae.

Rostrum, antenna (coxa, allobasis, and endopod), mouthparts, and P1 as in female.

P2 (Fig. 15A). Intercoxal sclerite naked. Precoxa small and triangular, with one row of spinules at distal margin. Coxa with two rows of outer spinules. Basis with one outer bipinnate seta, with spinules at base of spine, and with one row of inner setules. Rami with 3-segmented exopod and 2-segmented endopod, endopod reaching to middle of exp-3. Armature and all ornamentation of endopod as in female. Enp-1 without armature, enp-2 with two apical plumose setae and two inner plumose setae. All segments of endopod with one row of outer spinules and one row of inner setules.

P3 (Fig. 15B). Intercoxal sclerite and precoxa as in P2. Coxa with two rows of outer spinules. Basis with one outer plumose seta; with spinules at base of seta and one row of inner setules. Both rami 3-segmented, endopod reaching to middle of exp-2. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta, inner seta much shorter than in female; exp-3 with three outer bipinnate spines, two apical elements (both spiniform seta with outer spinules and inner setules), and one inner plumose seta, inner seta much shorter than in female. All segments of exopod with several rows of strong outer spinules and one row of inner setules except exp-3. Enp-1 without armature, enp-2 with outer distal apophysis exceeding the tip of enp-3, one inner plumose seta, and with one row of outer spinules and fewer inner setules; enp-3 with two inner plumose setae and two apical plumose setae, and with one row of spinules at base of outer seta.

P4 (Fig. 15C). Intercoxal sclerite and precoxa as in P2. Coxa with one row of spinules along outer margin. Basis with one smooth seta on outer distal corner; ornamented with spinules at base of seta and one row of setules along inner margin. Rami with 3-segmented exopod and 2-segmented endopod. Exp-1 with one outer bipinnate spine; exp-2 with one outer bipinnate spine and one inner plumose seta, inner seta much shorter than in female; exp-3 with two outer bipinnate spines, two apical elements (of which outer one spiniform seta with outer spinules and inner setules, inner element one spiniform seta), and one inner plumose seta. Endopod reaching tip of exp-1; enp-1 without armature; enp-2 with one outer bipinnate seta, one apical plumose seta, and one inner plumose seta. All segments of exopod with several rows of strong spinules along outer margin, and with one row of setules along inner margin except exp-3. Armature and all ornamentation of endopod as in female, but presence of one row of spinules at base of apical seta.

Armature formula of P1–P4 as in Table 1.

P5 (Fig. 14B). With outer basal seta arising from long setophore; without endopodal lobe. Exopod with three plumose setae, outermost shortest, 2.5 times as long as segment, approximately 0.5 times as long as the middle seta.

P6 (Fig. 14B). Reduced to one minute rectangular protuberance, with one outer and one inner bipinnate seta; inner seta approximately twice as long as outer one and reaching posterior margin of next urosomite.

In male, variability was observed in the exopod of antenna, four specimens with four setae and one specimen with five setae (Fig. 10D, E). In female antenna, two strong spines of distal end of endopod not fused in six specimens and fused in only one specimen (Fig. 10C).

This species is known from the type locality only. It was found in two months, January and September 2017.

Two females and two males from Khao Thanan cave, Satun Province, southern Thailand, 07°03'43.2"N, 99°41'42.7"E; coll. C Boonyanusith and K Wongkamhaeng; 12 December 2014. One female from Samer-rach peat swamp, Trat Province, eastern Thailand, 12°28'04.0"N 102°21'20.6"E.; coll. S Maiphae and T Saetang; 25 May 2017.

Laophontidae. Caudal rami more than four times as long as wide in female and approximately three times as long as wide in male. Female P5 exopod and baseoendopod fused, endopodal lobe and exopod with three setae each. P4 exp-3 with three outer spines.

Female (Fig. 16A). Total body length measured from tip of rostrum to posterior margin of caudal rami, 530–550 µm (mean 536 µm, n = 3); preserved specimen colourless. Body covered with setules, cylindrical, gradually tapering posteriorly, with maximum width at posterior part of cephalothorax. Prosome 1.6 times as long as urosome. Rostrum small, completely fused to cephalothorax, with pair of apical sensilla. Cephalothorax as long as wide, approximately 0.5 times the length of prosome (Fig. 16A). Cephalothorax and all free thoracic somites with less developed posterior sensillum-bearing tubercles (Fig. 16A, B). Second and third urosomite fused ventrally forming genital double-somite, with dorsal and lateral remnant of original division (Fig. 16C). Posterior half of genital double-somite and subsequent somites with lateral sensillum-bearing tubercles. Other characters on urosomite as in O. tratensis sp. n.

Caudal rami (Fig. 16C, D). Cylindrical, parallel, 4.3 times as long as wide, with seven setae of different lengths. Anterolateral accessory seta (I) minute, inserted close to anterolateral seta (II) at distal third of rami. Posterolateral seta (III) inserted on minute protuberance. Outer terminal seta (IV) slender, fused at base to inner terminal seta (V), the latter longest, without fracture plane, approximately 0.6 times as long as body length. Inner accessory seta (VI) slender. Dorsal seta (VII) tria-rticulate, inserted at quarter of ramus. Length ratio of caudal setae to ramus length from seta I to seta VII: 0.2 : 0.5 : 0.8 : 1.1 : 9.7 : 0.3 : 0.8.

Antennule and mouthparts as in O. satunensis sp. n. and O. tratensis sp. n., except for allobasis of antenna with one bipinnate abexopodal seta (Figs 17A, 22B).

P1 (Fig. 17B), P2, P3 and P4 (Fig. 17C) as in O. satunensis sp. n. and O. tratensis sp. n., except for P4 exp-3 with three outer spines. Armature formula of P1–P4 as in Table 2.

P5 (Figs 17E, 23C). Baseoendopod and exopod fused; exopod rectangular. Baseoendopod with basal seta, endopodal lobe with three pinnate setae and with row of spinules at base of each seta. Exopod with three bipinnate setae, innermost longest and with spinules at its base.

P6 (Fig. 16C). Reduced to minute prominence at outer distal corner of genital field, with short slender seta.

Body (Fig. 18A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 450–470 µm (n = 2); habitus smaller than female; preserved specimen colourless. Prosome approximately 1.1 times as long as urosome. Cephalothorax as long as wide, 0.5 times as long as prosome. Other characters as in O. satunensis sp. n. and O. tratensis sp. n.

Caudal rami (Fig. 18A). slightly divergent, three times as long as wide; length ratio of caudal setae to ramus length from seta I to seta VII : 0.3 : 0.7 : 1.2 : 1.5 : 7.6 : 0.4 : 1.2.

Antennule, antenna, mouthparts, P1, P2, P3 as in O. satunensis sp. n. and O. tratensis sp. n., except for P4 exp-3 with three robust outer spines (Fig. 18D).

Armature formula of P1–P4 as in Table 2.

P5 (Fig. 18C). Baseoendopod absent. Exopod with two setae, inner approximately 1.4 times as long as outer.

P6 (Fig. 18B). Reduced to minute, rectangular protuberance with two setae apically; inner seta approximately 1.5 times as long as outer one.

In male, one additional seta on left P1 enp-2 (Fig. 17F). In female, left P4 with 2-segmented, two apical elements of exp-2 fused (Fig. 17D).

Onychocamptus bengalensis is characterised by the relatively long caudal ramus and fusion of exopod and baseoendopod of P5 in the female. The length:width ratio of caudal ramus in female is variable (three times as long as wide in Lang (1948) and Song and Chang (1995), 3.8–4.0 times as long as wide in Lee and Chang (2005), and six times as long as wide in Hamond (1973)). The length:width ratio of the caudal ramus shown in Hamond (1973), is however, approximately 4.5 (Song and Chang 1995). The length:width ratio of the caudal ramus of the Thai specimens is 4.3. Other characters observed from the Thai specimens match well Hamond’s (1973) redescription, except for the basis of P3 and P4 with one row of setules along inner margin, and posterior margin of anal operculum with one row of spinules in the Thai material. The ornamentation of anal operculum is similar to that of the Korean specimens from the estuary of Ssangcheon Stream (Lee and Chang 2005), but it differs from the Australian specimens by lacking ornamentation at posterior margin (Hamond 1973).

This species has been recorded from Calcutta (India) (Sewell 1934), brackish lagoons in northern coastal suburbs of Sydney (Australia) (Hamond 1973), crab burrows in a mud flat a little apart from shore line in Chindo Island (Korea) (Song and Chang 1995), and from Ssangcheon Stream (Korea) (Lee and Chang 2005).

In this study, we found the species in i) Samer-rach peat swamp, Trat Province, eastern Thailand; water temperature, 27.65 °C; pH, 5.38; salinity, 6.06 ppt; conductivity, 11285.2 µS cm^-1; total dissolved solids, 6982 mg L^-1; dissolved oxygen, 3.86 mg L^-1, ii) Khao Thanan cave, Satun Province, southern Thailand; this is a limestone cave which normally dries out during the dry season (April); water pH, 8.85; dissolved oxygen, 2.7 mg L^-1; conductivity, 2.1 µS cm^-1; salinity varies seasonally because of the effect from the sea nearby.

Seven females and four males from Thale-Noi Lake, Pattalung Province, southern Thailand, 07°46'30.47"N, 100°9'31.68"E; coll. S Maiphae and T Saetang; 23 October 2013.

Laophontidae. Caudal rami more than 2.3 times as long as wide in female and approximately 1.8 times as long as wide in male. P4 exp-3 with three outer spines. Female P5 exopod and baseoendopod separated, with three bipinnate spiniform setae on exopod and two on baseoendopod. Exopod of P5 male with two bipinnate spiniform setae.

Body (Fig. 19A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 400–460 µm (mean 423 µm, n = 7); body cylindrical, gradually tapering posteriorly. Prosome 1.5 times as long as urosome. Rostrum small, completely fused to cephalothorax, and with pair of apical sensilla. All free thoracic somites with sensillum-bearing tubercles along posterior margin. Second and third urosomite fused ventrally forming the genital double-somite, remnant of division dorsally and laterally; penultimate urosomite with row of spinules dorsally and laterally. Anal somite approximately 0.6 times as long as wide, with two rows of spinules; anal operculum poorly developed, with minute spinules along posterior border (Fig. 19B).

Caudal rami (Fig. 19B). Cylindrical, slightly convergent, 2.3 times as long as wide, with seven setae of different lengths. Position of caudal setae as in previous species. Length ratio of caudal setae to ramus length from seta I to seta VII: 0.7 : 0.9 : 1.0 : 0.7 : 11.1 : 0.4 : 1.0.

Egg sac (Fig. 19D). Ovigerous female with one rounded egg sac ventrally between pair of fifth legs, with nineteen eggs.

Antennule and mouthparts as those of previous described species, but allobasis of antenna with one bipinnate abexopodal seta (Figs 19C, 22C).

P1, P2, P3 and P4 (Fig. 19E) as those of previous species, except P4 exp-3 with three outer spines.

Armature formula of P1−P4 as in Table 2.

P5 (Figs 19F, 23D). Baseoendopod and exopod separated, covered with surface setules; baseoendopod with outer basal seta, endopodal lobe with two bipinnate spiniform setae, with one row of spinules at base of each seta; exopod with three bipinnate spiniform setae, with spinules at base of innermost seta only.

P6. Reduced to minute, rectangular protuberance, with one naked seta.

Body (Fig. 20A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 320–360 µm (mean 349 µm, n = 4); body cylindrical, gradually tapering posteriorly. Prosome 1.4 times as long as urosome. Rostrum small, completely fused to cephalothorax, with pair of apical sensilla. Anal somite approximately 0.8 times as long as wide, anal operculum poorly developed.

Caudal rami (Fig. 20A). As in female.

Antennule, antenna, mouthparts, P1, P2, P3 (Fig. 20E), P4 (Fig. 20F) as in O. satunensis sp. n. and O. tratensis sp. n., except P4 exp-3 with three robust, outer spines.

Armature formula of P1–P4 as in Table 2.

P5 (Fig. 20D, 23D). Baseoendopod absent. Exopod with two bipinnate spiniform setae, outer seta not reaching beyond second urosomite.

P6 (Fig. 20C). Represented by two bipinnate spiniform setae, inner approximately twice as long as outer, and inner seta reaching beyond distal margin of third urosomite.

Thai specimens agree with Shen and Tai (1979), however the inner seta of the male P6 comes beyond the distal margin of four urosomite and the outer seta is approximately 1/3 as long as the inner seta in one specimen of our samples (Fig. 20B).

This species has been recorded from the delta of the Pearl River (Kwangtung Province, south China) (Shen and Tai 1963), from a stream in Okinawa and Ishigaki Island (Japan) (Ishida 1990) and from Hangetsu Lake (Shiribeshi Province, Japan) (Ishida and Kikuchi 2000), and from reed marshes of the lower reaches of Gonyangcheon Stream and Sopocheon Stream (Jindo Island, Korea) (Lee and Chang 2005).

In this study, we found this species only in Thale-Noi Lake, Pattalung Province (southern Thailand) in June, August, and October 2013. Water temperature ranged between 28.3–31.6 °C, pH of 5.22–7.83, salinity 0.1–1.2 ppt, conductivity 209.5–2385 µS cm^-1, transparency 0.05–1.35 m, depth 0.35–1.65 m, chlorophyll a 0.27–37.53, and dissolved oxygen 2.89–5.76 mg L^-1.

Four females from Ta-pom swamp, Krabi Province, southern Thailand, 08°12'50.19"N 98°46'41.24"E, coll. S Maiphae and T Saetang, on 8 December 2016.

Laophontidae. Caudal rami more than 2.2 times as long as wide in female. P4 exp-3 with three outer spines. Baseoendopod and exopod separated, each with three bipinnate spiniform setae.

Body (Fig. 21A). Total body length, measured from tip of rostrum to posterior margin of caudal rami, 410–480 µm (mean 440.50 µm, n = 4); body cylindrical, gradually tapering posteriorly. Prosome 1.3 times as long as urosome. Rostrum small, completely fused to cephalothorax, with pair of apical sensilla. All free thoracic somites with sensillum-bearing tubercles along posterior margin. Second and third urosomite fused ventrally forming genital double-somite; remnant of division dorsally and laterally. Anal somite approximately 0.7 times as long as wide. Anal operculum poorly developed, with minute spinules along upper posterior border.

Caudal rami (Fig. 21B). Cylindrical, parallel, 2.2 times as long as wide, with seven setae of different lengths. Position of caudal setae as in previous species. Inner terminal seta (V) approximately 0.7 times as long as body length. Length ratio of caudal setae to ramus length, from seta I to seta VII : 0.4 : 0.6 : 0.9 : 0.8 : 8.5 : 0.4 : 1.0.

Egg sac (Fig. 21E). Ovigerous female with one oval egg sac with eight eggs ventrally between fifth pair of legs.

Antennule and mouthparts as in previous species, but allobasis of antenna with one bipinnate abexopodal seta (Fig. 22D).

P1, P2, P3 and P4 (Fig. 21C) as in O. satunensis sp. n. and O. tratensis sp. n. except P4 exp-3 with three robust, outer spines.

Armature formula of P1−P4 as in Table 2.

P5 (Figs 21D, 23E). Baseoendopod and exopod separated; both rami densely covered with setules; baseoendopod with outer basal seta, endopodal lobe with three inner bipinnate spiniform setae, with one row of spinules at base of each seta; exopod with three bipinnate spiniform setae, with spinules at base of innermost seta only.

P6. Reduced to minute, rectangular protuberance, with one naked seta.

The length of the baseoendopodal setae of the female P5 is variable. The original description shows the lateral most seta as the longest (Blanchard and Richard 1891). This seta is also the longest in specimens from Japan (Ishida and Kikuchi 2000). However, the middle seta is the longest in specimens from China and Korea (Shen 1962, Lee and Chang 2005), and the lateral most seta is equal to the middle seta in the Thai specimens.

This species has been found in many localities such as Wu-Li Lake (Kiangsu Province, China) (Shen and Tai 1962), in Harutori Lake (Kushiro, Japan) (Ishida and Kikuchi 2000), in Daecheong Lake and Yedang Reservior (Korea) (Lee and Chang 2005), in Vietnam (Ho and Tran 2007), and in central Thailand (Daday 1906, Watiroyram et al. 2015).

In the present study, we found the species in Ta-pom swamp (southern Thailand, Krabi Province) in December 2016, water temperature was 26.27 °C, pH 6.74, salinity 0.27 ppt, conductivity 571 µS cm^-1, and dissolved oxygen 4.32 mg L^-1.