Holotype : Adult male (FU, 2019-01-25-01), tl 16.7 mm, cl 4.8 mm, rl 1.5 mm; a cave river at Pengjiaao, Tangtou Town, Sinan County, Guizhou Province, southwestern, China (27°44'10"N, 108°11'58"E, alt. 294.7 m), 25 Jan. 2019. Paratypes: 1 male (FU, 2019-01-25-02) cl 5.4 mm; 1 male (FU, 2019-01-25-03) cl 6.8 mm; 1 male (FU, 2019-01-25-04) cl 4.8 mm; 2 males (FU, 2019-01-25-05), cl 4.2–6.2 mm; 20 females (9 ovigerous) (FU, 2019-01-25-05), cl 4.9–6.6 mm, sampled together with the holotype.

Caridina semiblepsia Guo, Choy & Gui, 1996. Adult male (FU, 1994-05-17-01), tl 17.5 mm, cl 4.5 mm, rl 0.7 mm; a cave river at Tongpatong, Baojing County, Hunan Province, China, 17 May 1994. Paratypes: 4 males (FU, 1994-05-17-02) cl 4.8–5.6 mm; 5 females (2 ovigerous) (FU, 1994-05-17-03), cl 4.7–6.3 mm, sampled together with the holotype.

Caridina ablepsia Guo & Jiang, 1992. Adult male (FU, 1989-05-23-01), tl 26.8 mm, cl 6.5 mm, rl 1.8 mm; a cave river at Xiaolongtong, Yunshun County, Hunan Province,China, 23 May 1989. Paratypes: 5 males (FU, 1989-05-23-02) cl 5.4–6.7 mm; 6 females (FU, 1989-05-23-03), cl 5.7–6.9 mm, sampled together with the holotype.

Rostrum short, slightly sloping downwards, usually reaching to the end of the 2^nd segment, occasionally reaching to the end of the 1^st segment or the end of the 3^rd segment of antennular peduncle, rostral formula 4–10+10–16/3–11. 1^st pereiopod carpus 0.77–0.83× as long as chela, 1.6–1.7× as long as high; chela 1.9–2.2× as long as broad; fingers 1.2–1.3× as long as palm. 2^nd pereiopod carpus 1.2–1.3× as long as chela, 4.7–6.1× as long as high; chela 2.2–2.9× as long as broad; fingers 1.6–2.3× as long as palm. 3^rd pereiopod propodus 3.8–4.1× as long as dactylus, with 9–11 thin spines on the posterior and lateral margins. 5^th pereiopod propodus 3.7–4.1 × as long as dactylus, with 11–13 thin spines on the posterior and lateral margins, dactylus terminating in one claw, with 38–44 spinules on flexor margin. Endopod of male 1^st pleopod extending to 0.45–0.50× exopod length, distal half usually curved posteriorly in the natural, occasionally not bent backwards, wider proximally, subrectangular, 2.4–2.7× as long as wide, appendix interna well developed, arising from distal 1/3 of endopod, reaching beyond end of endopod. Appendix masculina of male 2^nd pleopod rod-shaped, reaching to 0.51 length of endopod, appendix interna reaching to 0.93 length of appendix masculina. Uropodal diaeresis with 10–12 movable spinules. Eggs size (without eyespots) 0.67–0.82 × 1.29–1.38 mm, eggs size (containing embryos with eyes) 0.98–1.02 × 1.16–1.47 mm.

Body (Fig. 5A–D): depigmented, slender and subcylindrical, medium-sized, males up to 22.7 mm tl, females up to 26.0 mm tl.

Rostrum (Fig. 3A, B): 0.25–0.47 of cl, reaching to the end of the 2^nd segment of antennular peduncle (75.8%, N =33) in large specimens, or to the end of the 1^st segment (15.2%), or to the end of the 3^rd segment of antennular peduncle (9.0%), straight, slightly sloping downwards; armed dorsally with 14–26 teeth, including 4–10 on carapace, ventrally with 3–11 teeth; lateral carina dividing rostrum into two unequal parts, continuing posteriorly to orbital margin.

Eyes (Fig. 3A–C): small, partly reduced, with short stalk, cornea pigmentation variable, usually with pigment at centre of cornea, or totally absent (only one specimen).

Carapace (Fig. 3A, B): smooth, glabrous; antennal spine acute, fused with inferior orbital angle; pterygostomial angle subrectangular, slightly protrude forward; pterygostomian spine absent.

Antennule (Fig. 3D): peduncle short, reaching slightly short of scaphocerite; stylocerite short, reaching 0.75–0.88 length of basal segment; anterolateral angle reaching 0.20 length of the 2^nd segment; basal segment as long as combined length of the 2^nd and 3^rd segments, 2^nd segment as long as 0.53–0.61× of basal segment, 1.29–1.32× of the 3^rd segment; all segments with sub-marginal plumose setae.

Antenna (Fig. 3E): peduncle about 0.53× as long as scaphocerite; scaphocerite 3.0–3.1× as long as wide, outer margin straight, asetose, ending in a strong subapical spine, inner and anterior margins with long plumose setae.

Mandible (Fig. 3F): without palp, with well-developed incisor and molar processes; left and right mandible of similar size but differing in shape; left incisor process with single sharp tooth and a marginal transparent slice followed by patch of long setae, molar process strongly produced, ridged; right mandible incisor process with two long outer teeth and single short inner tooth, margin leading to molar process with 12 curving setae, followed by patch of long setae, molar process stout and with triturative surface.

Maxillula (Fig. 3G): lower lacinia broadly rounded, with several rows of plumose setae; upper lacinia elongate, medial edge straight, with 36–42 strong spinules and simple setae; palp simple, longer than wide, slightly expanded distally, with four long simple setae.

Maxilla (Fig. 3H): Scaphognathite well developed, tapering posteriorly, distally with regular row of long plumose setae and short marginal plumose setae continuing down proximal triangular process, furnished with numerous long plumose setae; upper and middle endites with marginal simple, denticulate and submarginal simple setae, distally with plumose setae; lower endite with long simple marginal setae; palp slightly shorter than the cleft of upper endite, wider proximally than distally, setose.

First maxilliped (Fig. 3I): Palp broad with terminal plumose setae; caridean lobe broad, with marginal plumose setae; exopodal flagellum well developed, with distally marginal plumose setae; ultimate and penultimate segments of endopod indistinctly divided; medial and distal margins of ultimate segment with marginal and sub-marginal rows of simple, denticulate, and plumose setae; penultimate segments with marginal long plumose setae.

Second maxilliped (Fig. 3J): endopodite ultimate and penultimate antennomeres fused, slightly concave, reflected against basal antennomeres, inner margin of ultimate, penultimate and basal segments with long setae of various types; exopod flagellum long, slender, with marginal plumose setae distally. Podobranchium is comb-like.

Third maxilliped (Fig. 3K): endopod three-segmented, reaching slightly beyond scaphocerite; penultimate segment 0.87–0.92× of basal segment; distal segment as long as penultimate segment, ending in a large claw-like spine surrounded by simple setae, preceded by about 6–9 spines on distal third of posterior margin, proximally with a clump of long and short simple and serrate setae; exopod flagellum well developed, about a third the length of penultimate segment of endopod, distal margin with long plumose setae.

First pereiopod (Fig. 4A): short, reaches end of eyes; chela length 1.9–2.2× breadth, 1.2–1.3× length of carpus; movable finger length 2.7–2.9× breadth, 1.2–1.3× length of palm, setal brushes well developed; carpus excavated disto-dorsally, length 1.6–1.7× breadth, 0.90–0.93× length of merus.

Second pereiopod (Fig. 4B): reaches about end of 3^rd antennular peduncle segment, more slender and longer than first pereiopod; chela length 2.2–2.9× breadth, 0.79–0.85× length of carpus; movable finger length 3.8–4.4× breadth, and 1.6–2.3× length of palm, setal brushes well developed; carpus length 4.7–6.1× breadth, slightly excavated distally, 1.0–1.1× length of merus.

Third pereiopod (Fig. 4C, D): reaches beyond end of scaphocerite; dactylus length 4.0–4.2× breadth, ending in prominent claw-like spine surrounded by simple setae, behind which bears 7–9 spines; propodus length 3.8–4.1× of dactylus, bearing 9–11 spinules on posterior margin, 11.2–12.2× breadth; carpus length 0.60–0.78× of propodus; merus length 1.9–2.1× of carpus, with about three large spines on the posterior margin.

Fourth pereiopod (Fig. 4E): reaches end of 3^rd segment of antennular peduncle; dactylus length 4.0–4.2× breadth, ending in prominent claw-like spine surrounded by simple setae, behind which bears 7–8 spines; propodus length 3.9–4.3× of dactylus, bearing 11–16 spinules on posterior margin, 13.5–14.2× breadth; carpus length 0.53–0.62× of propodus; merus length 1.5–1.7× of carpus, with about three strong spines on the posterior margin.

Fifth pereiopod (Fig. 4F, G): reaches the end of the 3^rd segment of antennular peduncle; dactylus length 4.9–5.4× breadth, ending in prominent claw-like spine surrounded by simple setae, behind which bears a comb-like row of 38–44 spines; propodus length 3.7–4.1× of dactylus, bearing 11–13 spinules on posterior margin, 16.6–17.6× breadth; carpus length 0.50–0.61× of propodus; merus length 1.4–1.5× of carpus, with about three strong spines on the posterior margin.

First four pereiopods with epipod. Branchial formula typical for genus.

First pleopod (Fig. 4H–J): endopod of male subrectangular, distal half usually curved posteriorly in the natural, occasionally not bent backwards, wider proximally, length 0.45–0.50× exopod length, 2.4–2.7× proximal breadth, ending broadly rounded; inner margin slightly concave, bearing long spine-like setae, outer margin slightly convex or straightly, proximally 1/3 naked and distally 2/3 bearing nearly equal length spine-like setae; appendix interna well developed, arising from distal 1/3 of endopod, reaching to or beyond end of endopod, distally with cincinulli.

Second pleopod (Fig. 4K): appendix masculina rod-shaped, reaching about 0.51× length of exopod, with numerous long spiniform setae proximally and distally, appendix interna well developed, almost same size as appendix masculina, reaching about 0.93× length of appendix masculina, distally with cincinulli.

Telson (Fig. 4L): 0.34–0.47× of cl, shorter than the 6^th abdominal segment, 0.90–0.96× length of sixth abdominal segment, tapering posterior, with a median projection, dorsal surface with six pairs of stout movable spinules including the pair at poster lateral angles; posterior margin with four pairs of intermedial strong spiniform setae, sublateral pair shorter than lateral and inner pairs. Exopodite of the urpood bears a series of 10–12 movable spinules along the diaeresis, last one shorter than the lateral process.

Female carrying a number of 20–32 eggs, sized eggs 0.67–0.82 × 1.29–1.38 mm (without eyespots), and 0.98–1.02 × 1.16–1.47 mm (with eyespots).

Colouration (Fig. 5A–D): body and appendages translucent white; eyes with black spot at centre of cornea; internal organs (gonads and hepatopancreas) whitish or yellowish; eggs in females yellowish or blackish.

Caridina sinanensis is named after Sinan County, where the type locality is located.

Six Caridina species lacking body pigmentation and having a small black spot on each eye are known from Chinese subterranean aquatic habitats: C. acuta, C. alu, C. demenica, C. longshan, C. semiblesia, and C. sinanensis These taxa can be readily separated into two groups by the rostrum shape and indentation. In the first group including C. acuta, C. demenica, and C. semiblesia, the rostrums are similarly lanceolate and short, with fewer teeth or unarmed. In the second group including C. alu, C. longshan, and C. sinanensis , the rostrums are long, reaching at least to the end of the 2^nd antennular segment, mostly beyond the end of scaphocerite, and armed with dorsal and ventral teeth. Caridina. sinanensis is morphologically close to C. longshan in sharing a similar spination pattern, the anterior region of endopod on the 1^st male pleopod folded backwards, and the variably pigmented cornea. Caridina. sinanensis can be distinguished from C. longshan by the relatively longer appendix interna on the appendix masculina of the 2^nd pleopod (reaching about 0.93 of appendix masculina vs 0.80 in C. longshan), the length of 6^th abdominal segment distinctly longer than the telson (vs same length of telson in C. longshan), and telson with posteromedian projection and lack of spinules on the surface of posterior telsonic spines (caudal spines) (vs lacking posteromedian projection and possessing spinules in C. longshan). Caridina. sinanensis can be easily separated from C. alu by its short rostrum (reaching to the end of the 2^nd antennular peduncle vs reaching to the end of scaphocerite in C. alu), the carpus of 1^st and 2^nd pereiopods are slender (length to breadth ratio 1.6–1.7 and 4.7–6.1 versus 1.3 and 2.6 in C. alu), the telson with posteromedian projection (vs lacking in C. alu), and male with completely different shape of the endopod of 1^st pleopods and appendix masculina of the 2^nd pleopods. Caridina. sinanensis sp. nov. also shows close similarity with C. semiblesia in the ratios of various segments of the 1^st and 2^nd pereiopods, and the shape of endopod of the 1^st pleopod in males. In addition to a longer rostrum that slopes downwards, C. sinanensis also differs from C. semiblesia in having the end of the palp of the 1^st maxilliped being broadly rounded and without a finger-like tip (vs ending in a finger-like tip in C. semiblesia), the smaller eggs (0.98–1.02 × 1.16–1.47 mm vs 1.05–1.15 × 1.37–1.71 mm), and the stout and long appendix interna of the appendix masculina on the 2^nd pleopod (appendix interna almost same size as appendix masculina and reaching about 0.93 of appendix masculina vs distinctly slender and reaching about 0.80 in C. semiblesia).

Including the GenBank sequences, we analysed 22 COI sequences and 22 16S rRNA sequences in total. The new sequencing results are corrected for 621~bp (COI) and 487~bp (16S) for subsequent analysis. Three specimens of Caridina sinanensis were used in the molecular phylogenetic analysis shown in Figures 6, 7. Specimens assigned to C. sinanensis formed a clade distinct from other species. And the tree topologies derived from COI and 16S rRNA analyses were basically congruent. C. sinanensis sp. nov. is well isolated from other nine Caridina with a sequence divergence of 15.3–26.7% (COI) and 7.2–11.2% (16S), respectively. According to Hebert et al. 2003, the genetic distances support the molecular-based description of C. sinanensis as a new species.

Caridina sinanensis, sp. nov. lives in an aphotic subterranean waterbody where the source of energy may come from allochthonous materials carried or washed into the cave, as there are particulates of vegetable debris in the water. Based on our observation on the shrimp’s feeding behavior and intestinal contents, this species feeds on detritus and microorganisms from the bottom sediments with its brush-tipped chelae and mouthparts, and the full intestine suggests that the foods are relatively abundant (Fig. 5C, D).

Leeches co-occurred with atyid shrimp in the subterranean waters, camel crickets were common on the cave rocks, especially in the dark zone, and a blind unpigmented species of millipede was found crawling along the rocks in the dark zone (Fig. 5E,F). Some trogloxene animals, such as bats and birds, were occasionally encountered in the cave entrance area.

In general, the populations of other cave-dwelling species were very small, while shrimp were moderately abundant. Competition for food and habitat seems insignificant, possibly because the groundwater was enriched with particulate organic matter and predators (such as leeches) were not abundant. Leeches are the natural enemies of the shrimp; they attach to the carapace, branchial chambers and appendages where they feed on the hemolymph of the shrimps. Parasitism certainly confers negative impact on populations of the new species, but accurate population data on the shrimp are lacking.

The sex ratio and reproduction season were preliminary inferred based on three sampling times. On 25 January 2019, 18 individuals were collected, including two adult males, 10 adult females (six ovigerous), and six subadult females. The sex ratio (male to female) is 1:8, and the percentage of ovigerous females is 60%. On 18 March 2019, 18 individuals were caught, including five adult males, 10 adult females (three ovigerous), and three subadult females. The sex ratio is 1:2.6, and the percentage of ovigerous females is 30%. In 18 February 2020, four individuals were caught including one adult male and three adult females (one ovigerous). The sex ratio is 1:3, and the percentage of ovigerous females is 33%. These results showed that the number of males was significantly less than females in the population from January to March. The causes responsible for the skewed sex ratio in favor of females may worth further study.

The ovigerous females comprised 60%, 33%, and 30% of mature females, respectively, in populations from January to March. One male carrying a spermatophore on the intermediate of the fifth walking legs was observed from specimens collected in January (Fig. 4N). This cave dwelling species has the highest number of reproductive individuals recorded during the winter and spring months, suggesting that the peak reproductive period occurs from January to March.

Females carried 20–32 eggs, size of undeveloped eggs (without eyespots) were 0.67–0.82 × 1.29–1.38 mm, size of developed eggs (containing embryos with eyes) 0.98–1.02 × 1.16–1.47 mm. The females of this species carry a small number of large eggs and produce eggs with a large amount of yolk and reduced number of larval stages. It is believed that abbreviated larval development may occurs in this species, larval direct development into benthic hatchlings that resemble miniature adults.

We are trying to understand embryonic development and hatching of this species. On 20 March 2019, five ovigerous females were transported to the laboratory for rearing, but unfortunately, after 7 days, the shrimps died.

Cave ecosystems are an invaluable resource, providing an ideal refuge for cave-dwelling species. Cave shrimp communities are particularly vulnerable to human disturbance, particularly groundwater pollution due to the local agricultural activities (fertilization, herbicide, and pesticide) and overexploitation (domestic usage and agricultural irrigation). These appear to be responsible for the pollution and degradation of subterranean habitat, but the extent of the impact is a little known. If groundwater become contaminated, local aquatic organisms certainly are at risk. Maintaining healthy groundwater shrimp communities requires the reduction of anthropogenic impacts, such as minimizing the use of agricultural pesticides, herbicides, and fertilizers by local farmers. It is suggested that the local government should ration the use of groundwater resources.

The Announcement of the Ministry of Agriculture and Rural Areas of China (CITES Appendix aquatic wild species of China, no. 69, 2018), fails to list freshwater shrimps in the CITES threatened categories. Since Caridina sinanensis is a new species, no conservation status has been assigned. According to the criteria listed in the IUCN Red List categories (IUCN 2019), C. sinanensis should be considered as a Critically Endangered species due to its exceptional rarity, restricted distribution in a single cave system, and the imminent threats from pollution. In order to better protect cave ecosystems, and their associated rare and threatened evolutionary relict fauna, it is critical and of great urgency to collect more baseline data on population and distribution patterns, delineate the importance and threatened status of cave fauna, and to devise corresponding conservation and management measures. Regular monitoring may be necessary to ensure populations are sustained in the face of further anthropogenic disturbances. Furthermore, cave biodiversity protection laws should be enacted as soon as possible.