Theosbaena loko sp. n. a new stygobiotic microshrimp (Thermosbaenacea: Halosbaenidae) from southern Thailand

Abstract Background Thermosbaenaceans are subterranean crustaceans, widespread and occur in freshwater, oligohaline or anchialine caves or thermal springs. Currently, four families, seven genera,and 45 species are recognised worldwide. During our studies of the isolated karst, Tham Loko (Loko Cave) in Khao Chiason District, Phatthalung Province, we found an undescribed thermosbanacean species in the genus Theosbaena. Theosbaena is the only genus reported from freshwater in the Oriental Region. Previously, there were only two known species, Theosbaena cambodjiana Cals & Boutin, 1985 from Kampot Province, southern Cambodia and Khon Kaen, Thailand and T. kiatwongchai Rogers & Sanoamuang, 2016 discovered in a cave of Takhli District, Nakhon Sawan, central Thailand. Our new species is the third species recorded in the Oriental Region. New information Theosbaena loko sp. n. differs from its congeners by having a telson 1.8x longer than its breadth, maxilla 1 palp distal segment 4x longer than the proximal palpomere and the maxillopodal exopod twice as long as its basal width. This microshrimp is the third described species of the genus. A key to the species is given and suggestions for the conservation status of the new species are discussed.

In the Oriental Region, only Theosbaena has been recorded in freshwater (Boutin and Magniez 1985). The species in this genus are highly endemic and have a narrow distribution range. All are found in limestone caves with only two known species so far. Theosbaena cambodjiana Cals & Boutin, 1985 is reported from the caves of Kampot Province, southern Cambodia and from a cave in Khon Kaen Province, north-eastern Thailand (Rogers and Sanoamuang 2016). The second species is T. kiatwongchai Rogers & Sanoamuang, 2016, discovered in a cave of Takhli District, Nakhon Sawan Province, central Thailand (Fig. 1).
Here, we present a third species of the genus Theosbaena. The species was discovered in a limestone cave in Phatthalung Province, southern Thailand. We provide a key to the species and discuss its conservation status.

Materials and methods
Specimens were discovered in the dark zone of an isolated limestone of Tham Loko (Loko Cave), Khao Chiason District, Phatthalung Province (Fig. 1). This karst hill has been developed from Permian carbonate rock (286-245 mya) of the Ratchaburi group (Jantarit et al. 2020). The isolated limestone is relatively small, approximately 2.68 km long and 0.81 km wide (Fig. 2). The specimens were collected by hand and stored in 95% ethanol. Specimens were examined and dissected in 70% ethanol. All appendages were embedded in glycerine medium and mounted on a series of glass slides. Morphological characters were examined and drawn using a drawing tube attached to an Olympus CH30 light microscope. The pencil drawings were scanned and digitally inked using a WACOM Bamboo CTH-970 graphics board, following the method described in Coleman (2003). Photos of the habitus were taken by an Olympus Tough TG-5. Setae terminology follows Cals and Boutin (1985), Wagner (1994) and Rogers and Sanoamuang (2016).

Nomenclature
The specific epithet "loko" is the name of the type locality. It is used here as a noun in apposition. The gender is masculine.
Labrum round, 2.0x as long as broad, smooth, distal margin with fine, short microsetae. Labrum and labium without peculiarities.

Diagnosis
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.

Distribution
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.   Magniez, 1991and 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.

Conservation
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 0 2 3 3 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.

Identification keys
Key to species of the genus Theosbaena 1 Uropod endopod with elongate plumose macrosetae shorter than exopod, lateral edge margined with small spiniform macrosetae and two filiform plumose setae at mid-length T.