The specific epithet "loko" is the name of the type locality. It is used here as a noun in apposition. The gender is masculine.

Male. Body length 2.46 mm from head anterior margin to telson distal margin. Carapace reaching up to fourth pedigerous somite. Ocular scales present, broadly rounded, with longest dimension in medial third. Ocular scale 1.3x as long as broad, overlapping base of antenna I. Carapace extending to pereonite 2.

Antenna 1 (Fig. 3) 1.2x body length, biramous, peduncle of 3 segments, lined with setae on dorsal and ventral sides. Primary flagellum with 20 segments, each with a distoapical seta; terminal segment on each flagellum with 2 apical setae, accessory flagellum with 12 segments.

Antenna 2 (Fig. 3) uniramous, 0.3x body length, peduncle of 5 segments, distomedian margin of segment 1 with one simple seta (type IIA1). Flagellum with 7 segments, terminal segment with 2 apical setae.

Labrum round, 2.0x as long as broad, smooth, distal margin with fine, short microsetae. Labrum and labium without peculiarities.

Labium deeply cleft, margined with fine setae, with cleft margined with microsetae.

Mandible (Fig. 4, LMD and 4RMD) with palp of 3 segments, ratio of segments 1–3 as 1:2:1; segments 1 subtriangular and unarmed; segment 2 elongate, medially extended, naked; segments 2 subcylindrical, lateral surface, distal portion bearing 6 macrosetae; segments 3 subcylindrical, subterminal bearing a plumose seta and distal bearing 2 plumose setae. Corpus mandibula left pars incisiva 6-dentate, right pars incisiva 5-dentate; left lacinia mobilis 4-dentate; right lacinia mobilis 6-dentate.

Maxilla I (Fig. 4, MX1) with coxal endite margined with a row of 5 plumose setae and 6 simple setae, respectively; basal endite with 2 rows of 6-toothed macrosetae and 3 simple setae;endopod unsegmented with one medial simple seta and apically lined with 4 plumose setae, 4x as long as proximal segment; exopod vestigial, represented by long plumose seta.

Maxilla 2 (Fig. 4, MX2) coxal endite medial surface lined with 19 long plumose macrosetae, distal surface with 3 plumose setae and 5 simple setae; basipod with 2 endites, proximal endite bilobed, each apex margined with 2 rows of plumidenticulate macrosetae, basipodal endite 3 apex margined with longer plumidenticulate macrosetae. Endopodite of 2-segments; proximal segment inerm; distal segment with 4 simple setae.

Maxilliped (Fig. 4, MP) exopod reduced, longer than broad, with 2 elongate pectinate macrosetae. Endopod vestigial, represented by isolated seta. Basal endite broad, lateral margin straight, apical fifth of anterior surface and distal margin with stout pectinate spines and macrosetae; more macrosetae medially than at apex, apex with more spines.

Gnathopod (Fig. 3, GN) uniramous, basoischium elongate, 4Í as long as broad, innerm. Merus subequal to basoischium. Carpus expanded distally, bearing a longitudinal row of 5 simple setae macrosetae on medial margins; each seta shorter than carpus. Propodus suboval, bearing longitudinal row of 5 simple setae on medial margin; each seta subequal to the length of propodus. Dactylus suboval, distal margin convex. Unguis formed by 3 curved spines at distolateral corner. Dactylus distomedial angle inerm.

Pereopod II (Fig. 3) coxa not pronounced, rounded. Exopod 0.8x length of endopod, composed of 2 segments: basal segment suboval, inerm. Segment 4-6 distal corner with plumose macrosetae. Basis subcylindrical, with one medial seta and one distal seta, 3Í as long as wide. Ischiomerus rectangular, twice as long as width, bearing one medial seta and one distal seta. Carpus subcylindrical, not expanded distally, 1.7x longer than width, bearing 2 medial and one lateral distal spiniform macrosetae. Propodus anterioposteriorly flattened, approximately 4x longer than width, medial margin bearing 2 setae and distal corner with one seta. Propodus lateral margin bearing 2 subapical macrosetae distally, each about as long as segment’s width. Dactylus anterioposteriorly flattened, margins converging into truncated apex; apex half as wide as base. Dactylus apex with single elongate spine (unguis), twice as long as dactylus.

Pereopods III (Fig. 5) through IV similar to that of pereopod II. Pereopod V coxa not pronounced, rounded. Exopod 0.8x length of endopod, composed of 2 segments; proximal segment subrectangular, not expanded medially, inerm. Distal segment lanceolate, submargined in long, plumose macrosetae. Basis subcylindrical, inerm, twice as long as width. Ischiomerus subcylindrical, 3x as long as width, inerm. Carpus subcylindrical, expanded distally, 1.5x longer than width, inerm. Propodus anterioposteriorly flattened, approximately 6x longer than width, medial margin bearing a longitudinal row of well-spaced, spiniform macroseta, each subequal to segment’s width. Propodus lateral margin bearing 2 subapical macrosetae, each longer than segment’s width. Dactylus anterioposteriorly flattened, margins converging to a truncated apex; apex 0.7x as wide as base. Dactylus apex serrate, innerm.

Pereopod VI (Fig. 5) coxa not pronounced, rounded. Exopod 0.8x length of endopod, composed of 7 segments: proximal segment suboval, medially expanded, with single macroseta at distolateral corner. Segment 2-7 subrectangular, distal corner with plumose macrosetae. Basis suboval, inerm, twice as long as width. Ischiomerus subcylindrical, 2.3x as long as width, bearing 2 medial setae. Carpus subcylindrical, not expanded distally, 1.4x longer than width, bearing 2 macrosetae medially. Propodus anterioposteriorly flattened, approximately 6x longer than width, medial margin bearing a longitudinal row of spiniform macroseta, each subequal to segment’s width. Propodus lateral margin bearing 2 subapical macrosetae, each about twice as long as segment’s width. Dactylus anterioposteriorly flattened, margins converging to a truncated apex; apex 0.7x as wide as base. Dactylus apex serrated, innerm.

Uropod (Fig. 3) Endopod ovate, one articulate, longer unsegmented than protopod; lateral margin bearing 18 cuspidate setae; distomedial margin bearing long, plumose macrosetae, equal in length to endopod; distolateral margin bearing seven elongate plumose macrosetae. Exopod 2-segmented, first segment longer than second segment; proximal segment with straight lateral margin armed with row of stout spines; medial margin convex, widest at middle, with row of plumose macrosetae; Distal segment subovate, lateral margined with elongate, plumose macrosetae; medial edge margined with stout spines, 0.8x length of segment.

Telson (Fig. 3) longer than broad, 1.8x longer than basal broad, tapering, distally, terminal concave, anal lobes protrude beyond the terminal stretch. Left and right subterminal margin with 9 and 12 cuspidate setae.

Theosbaena loko sp. n. is the third species of the genus reported from Thailand. Theosbaena loko sp. n. can be distinguished from its congeners in having a telson 1.8x longer than its breadth, maxilla 1 palp distal segment 4x as long as proximal palpomere and a maxillopodal exopod twice as long as its basal width. It shares some characteristics with T. cambodjiana in having: mandibular palp segment 1:2:1 ratio 9:1.5:7/ segment 2 with 6 plumose setae; ocular scale evenly arcuate and rounded; gnathopod dactylus subrectangular, with 3 long, stout, arcuate macrosetae, each bearing a ventral membrane and uropod distal segment of the exopod and the distal margin of the endopod both bearing elongate, plumose macrosetae and endopod lateral edge is margined with a row of scaliform macrosetae. However, T. loko sp. n. differs from T. cambodjiana in the absence of pleopod 1. Theosbaena loko sp. n. is similar to T. kiatwongchai in having: a mandibular palp segment 1:2:1 ratio 9:1.5:7/ segment 2 with 6 plumose setae (vs. palp segment 1:2:3 ratio 9:1.5:7/ segment 2 with 6 microsetae); pereopod 1-4 exopod contains more than 2 segments (vs. 2 segments); ocular scale evenly arcuate and rounded (vs. transverse); and uropod endopod lateral edge is margined with a row of scaliform macrosetae (vs. endopod medial margin is inerm, except for 2 filiform macrosetae, midway along its length). Diagnostic morphological characters and their variation for each population/species are given in Table 1 and the identification key of the genus Theosbaena is provided.

Theosbaena loko sp. n. is only known from the freshwater pool in the dark zone of Loko Cave, Khao Chaison District, Phatthalung Province. The Cave is 352 metres long. The Cave contains three pools, with T. loko sp. n. found in all three pools, although two of the pools dry out during the dry season.

The new species was found swimming and walking on the clay substrate of the pool in the dark zone of the Cave. The physical factors in the pool were as follows: Temperature (25.1–25.7⁰C); conductivity (217–282 µS); total dissolved solids (146–182 ppm); salinity (108–137 ppm); dissolved oxygen (6.0–8.2 mgO₂/l); pH (7.98–8.22); turbidity (8–12 FAU); water hardness (99–150 mg/l CaCO₃); and CaCO₃ (70.20–85.40 mg/l). The new species co-occurs with stygobiotic isopod Stenasellus sp., three species of Rotifer: Lecane bulla (Gosse, 1851); Lecane hamata (Stokes, 1896); Lecane quadridentate (Ehrenberg, 1830), a species of Daphniidae (Scapholeberis kingi Sars, 1888) and undetermined Cyclopidae. Moreover, two fish species were observed: Barbodes binotatus (Valenciennes, 1842) and Rasbora paviana Tirant, 1885. These fish may be potential predators of this microshrimp. The co-occurrence of stygobiotic fauna in the same area is not exceptional and T. cambodjiana was also reported to live in the same pool with the isopod Stenasellus cambodianus Boutin & Magniez, 1985. Additionally, T. cambodjiana in Khon Kaen, Thailand occurs with five other stygobiotic species in the same pond, i.e. Dugesia deharvengi Kawakatsu & Mitchell, 1989, Heterochaetella glandularis (Yamaguchi, 1953), Aequigidiella aquilifera Botosaneanu & Stock, 1989, Stenasellus rigali Magniez, 1991 and Siamoporus deharvengi Spangler, 1996 (Deharveng and Bedos 2012). Unfortunately, there are no ecological data for T. kiatwongchai. The authors attempted to access the habitat of T. kiatwongchai in August 2020, but unfortunately, the cave access was dangerous and the Forest Park staff claimed that the air was unsuitable, meaning that it was impossible to undertake a fauna and habitat evaluation of the cave.

The researchers herein propose T. loko sp. n. as an endangered species according to the IUCN Standards and Petitions Committee (2019) criteria. This status is proposed because its population size is small (only seven captured specimens from five observations). The new species is highly endemic to the permanent pool in the Loko Cave and the discovery considerably extends the narrow geographic occurrence of the genus (Fig. 1). The karst hill and the Cave are surrounded by agricultural areas, such as paddy fields and rubber and orchard plantations, where agricultural practices and anthropogenic activities have significantly increased. Today the Cave has become a tourist attraction where lights and simple infrastructure inside the Cave have been introduced. The habitat is, therefore, threatened in the face of growing anthropogenic disturbance. Interestingly, this Cave has one of the richest fauna in Thailand, harbouring at least 79 species of cave fauna with many species that are unknown to science (Jantarit et al. 2020). The Cave also hosts Pendlebury's roundleaf bat, Hipposideros pendleburyi Chasen, 1936, which has been assessed as a vulnerable species by the IUCN. Hence, the description of this new species not only emphasises the high level of endemism in this cave, but also has implications for environmental awareness, developing policy for cave conservation strategies, together with promoting ecotourism in the areas.