Caridina namdat, Tu & Dong & Rintelen, 2021

Tu, Do Van, Dong, Dang Van & Rintelen, Thomas Von, 2021, Description of one new species of freshwater shrimp of the genus Caridina (Crustacea: Decapoda: Atyidae) from two karst caves of Northern Vietnam, Zootaxa 4999 (3), pp. 228-242 : 231-239

publication ID	https://doi.org/ 10.11646/zootaxa.4999.3.3
publication LSID	lsid:zoobank.org:pub:8EB9C063-2924-4D60-8008-F00638352D8F
persistent identifier	https://treatment.plazi.org/id/D22F87E5-0404-9722-83CD-FE9FFD65FAE5
treatment provided by	Plazi
scientific name	Caridina namdat
status	sp. nov.

Treatment

Caridina namdat View in CoL , sp. nov.

( Figs. 2–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Examined material. Holotype: 1 male, cl 4.5 mm, IEBR-FS 005 , Vietnam, Bac Kan Province, Cho Moi District, Tan Son Commune, Nam Dat Village , a small cave, N 22°5’46.199’’ E 105°56’8.688’’, 11.iii.2021, Do VT GoogleMaps . Paratypes: 19 females, cl 3.1–6.0 mm, 17 males, cl 3.0–5.0 mm, IEBR-FS 006 , same data as holotype GoogleMaps ; 4 males, cl 4.1–4.5, 5 females, cl 4.6–5.0, ZMB 30341, same location as the holotype GoogleMaps , 6.vi.2019, Do VT; 3 males, cl 3.9–4.1 mm, 3 females, cl 4.3–4.5 mm, ZMB 30342, Vietnam, Bac Kan Province, Cho Moi District, Tan Son Commune, Nam Dat Village , a small cave, N 22°5’19’’ E 105°54’58’’, 6.vi.2019, Do VT GoogleMaps .

Comparative material. Caridina cucphuongensis Dang in Dang, Thai & Pham, 1980: male, cl 3.8 mm; 2 females, cl 4.2–4.4 mm, ZMB 30234, Vietnam, Ninh Binh, Cuc Phuong National Park, a small stream near the foot path to the one thousand years old tree (Vietnamese name: chò), N 20°21’0.779’’ E 105°36’11.675’’, von Rintelen T, Do VT, 03.iv.2017. Caridina clinata Cai, Nguyen & Ng, 1999 : male, cl 3.1 mm, ZMB 30664, Vietnam, Ninh Binh, a small stream in Cuc Phuong National Park , N 20°19’11.118’’ E 105°37’42.168’’, von Rintelen T & Do VT, 3.iv.2017. Caridina pseudoserrata Dang & Do, 2007 GoogleMaps : 1 male, cl 4.5 mm, ZMB 33788, Vietnam, Cao Bang Province, Nguyen Binh District , a small stream near the road no. 202, N 22°38’20.820’’ E 105°58’33.318’’, 8.x.2020, Do VD. Caridina pacbo Do, von Rintelen & Dang, 2020 GoogleMaps : 1 male, cl 4.2 mm, IEBR – FS 003 (Holotype), Vietnam, Cao Bang Province, Ha Quang District, Truong Ha Commune, Pac Bo Village , small stream in Khuoi Nam , N 22°59′1.7″ E 106°02′31.2″, 25.v.2017, Do VT. Caridina tricincta Do, von Rintelen, Dang, 2020 GoogleMaps : 1 male, cl 5.3 mm, IEBR – FS 001 (Holotype), Vietnam, Tuyen Quang Province, Na Hang District, Khau Tinh Commune , N 22°26′42.1″ E 105°23′09.1″, 11.xii.2012, Nguyen AT, 11. Caridina thachlam Do, Cao & von Rintelen 2021 GoogleMaps : 1 male, cl 4.2 mm, IEBR –FS–004, Vietnam, Thanh Hoa Province, Thach Thanh District, Thach Lam Commune, Bo Village , N 20°21’24.0779” E 105°30’27.347”, 10.v.2019, Do VT, Hoang AT & Dang VD GoogleMaps .

Cephalothorax and cephalic appendages. Carapace length 3.0–6.0 mm (median 4.5 mm). Rostrum very short and slender, straight, reaching to middle of basal segment of antennular peduncle, 0.1–0.2 (median 0.2) times as long as carapace, without teeth ( Fig. 2A View FIGURE 2 ). Suborbital angle acute, completely fused with antennal spine; pterygostomian margin rounded ( Fig. 2A View FIGURE 2 ). Eyes degenerated with cornea pigmentation smaller than normal, eyestalk reduced, anterior end reaching to 0.4 times length of basal segment of antennular peduncle ( Fig. 2A View FIGURE 2 ). Antennular peduncle 0.4–0.5 (median 0.5) times as long as carapace; basal segment 1.6–1.9 (median 1.8) times as long as second segment, second segment 1.1–1.4 (median 1.2) times as long as third segment ( Fig. 2B View FIGURE 2 ). Stylocerite short, reaching to 0.8 times length of basal segment of antennular peduncle ( Fig. 2B View FIGURE 2 ). Scaphocerite ovate, reaching beyond distal end of antennular peduncle, 2.3–2.9 (median 2.6) times as long as wide ( Fig. 2A, C View FIGURE 2 ).

Abdominal somites, telson and uropods. Sixth abdominal somite 0.3–0.4 (median 0.4) times length of carapace, 1.2–1.5 (median 1.4) times as long as fifth abdominal somite, 0.7–1.0 (median 0.9) times length of telson. Telson length 2.9–2.4 (median 2.1) times as long as proximal wide, distal margin triangular, terminating in a short median projection, with 4–5 pairs of dorsal spiniform setae and one pair of dorso-subdistal spiniform setae; distal end with 3–4 pairs of spiniform setae, lateral pair slightly shorter than intermediate pairs ( Fig. 2D, E View FIGURE 2 ). Preanal carina low, with few setae, lacking tooth or spine ( Fig. 2F View FIGURE 2 ). Uropodal diaeresis with 15–18 movable spiniform setae, outermost slightly shorter than lateral angle ( Fig. 2G View FIGURE 2 ).

Mouthparts and branchiae. Incisor process of mandible ending in one row of 6–7 irregular teeth, molar process truncated ( Fig. 2H View FIGURE 2 ). Lower lacinia of maxillula broadly rounded, upper lacinia elongated, with a number of distinct teeth and setae on inner margin, palp stout with few simple setae at tip ( Fig. 2I View FIGURE 2 ). Upper endites of maxilla subdivided, palp short, scaphognathite tapering posteriorly, with numerous long, curved setae at posterior margin ( Fig. 2J View FIGURE 2 ). Distal end of palp of first maxilliped triangular, with a very short projection; flagellum of the exopod very elongated, endopod high, reaching 0.9 times length of flagellum of exopod ( Fig. 2K, L View FIGURE 2 ). Podobranch of second maxilliped incompletely reduced, with few finger-like projections ( Fig. 2M View FIGURE 2 ). Third maxilliped reaching near the end of antennular peduncle, ending in single terminal claw, exopod reaching 0.3 times length of penultimate segment; ultimate segment as long as penultimate segment; epipod present on the coxa ( Fig. 2N View FIGURE 2 ). Branchial formula as typical for genus, five pairs of pleurobranchs well developed; two pairs of arthrobranchs on third maxillipeds, with second pair strongly reduced in size; one pair of podobranchs on second maxilliped strongly reduced, arthrobranch on first pereiopod absent.

Pereiopods. Epipod present on first fourth pereiopods. First pereiopod short, robust, reaching beyond end of basal segment of antennular peduncle; chela 1.8–2.4 (median 2.3) times as long as wide, 1.1–1.4 (median 1.2) times length of carpus; tips of fingers rounded, with hook; dactylus longer than palm in males, 1.1–1.3 (median 1.2) times as long as palm, shorter than palm in females, 0.7–0.9 (median 0.8) times as long as palm; carpus excavated anteriorly, 2.0–2.2 (median 2.1) and 1.5–1.8 (median 1.7) times as long as wide in males and females, respectively; carpus 0.8–1.2 (median 1.0) times length of merus; merus 2.4–3.1 (median 2.7) times as long as wide, longer than ischium ( Figs. 3A, B View FIGURE 3 ). Second pereiopod long, slender, reaching to distal end of antennular peduncle; chela 3.1–4.0 (median 3.4) times as long as wide, 0.6–0.8 (median 0.7) times length of carpus; tips of fingers rounded, without hook; dactylus 1.4–1.9 (median 1.6) times as long as palm; carpus 5.0–7.0 (median 6.1) times as long as wide, 1.0–1.2 (median 1.1) times as long as merus; merus 5.5–6.8 (median 6.2) times as long as wide, longer than ischium ( Fig. 3C View FIGURE 3 ). Third pereopod slender, reaching beyond end of scaphocerite by its dactylus, terminating in one claws, with five or six accessory spiniform setae on flexor margin, dactylus 2.5–3.3 (median 3.2) times as long as wide (terminal claw and spiniform setae on flexor margin included), propodus 9.3–10.8 (median 10.0) times as long as wide, 3.8–4.7 (median 4.0) times as long as dactylus; carpus 4.7–5.8 (median 5.4) times as long as wide, 0.7–0.9 (median 0.7) times as long as propodus, 0.5–0.6 (median 0.5) times as long as merus; merus 5.9–7.5 (median 6.8) times as long as wide, bearing three strong, movable spiniform setae on posterior margin of outer surface; ischium with one movable spiniform seta ( Figs. 3D, E View FIGURE 3 ). Fifth pereiopod slender, reaching to end of second segment of antennular peduncle, dactylus 2.5–2.8 (median 2.5) times as long as wide (terminal claw and spiniform setae on flexor margin included), terminating in one large claw, with 53–63 spiniform setae on flexor margin; propodus 11.3–15.0 (median 13.1) times as long as wide, 3.5–5.0 (median 4.6) times length of dactylus; carpus 4.2–5.0 (median 4.7) times as long as wide, 0.5–0.6 (median 0.5) times as long as propodus, 0.5–0.7 (median 0.6) times as long as merus; merus 5.7–7.7 (median 6.5) times as long as wide, bearing 3 strong, movable spiniform setae on posterior margin of outer surface ( Figs. 3F, G View FIGURE 3 ).

Pleopods. Endopod of male first pleopod extending to 0.9 times exopod, sub-rectangular in shape, 3.2–4.0 (median 3.3) times as long as proximal width, both inner and outer margins concave, rounded distally, long plumose setae on outer and distal margins, medium-length setae on inner margin; with appendix interna exceeding terminal margin of endopod by 0.3 its length ( Figs. 3H, I View FIGURE 3 ). Appendix masculina of male second pleopod slender, sub-cylindrical, reaching to proximal 0.8 times endopod length, 7.0 times as long as distal width, finger-shaped, with some short spiniform setae on outer surface and some long spiniform setae on distal surface; appendix interna at the middle of appendix masculina, extending about 0.7 times length of appendix masculina ( Figs. 3J, K View FIGURE 3 ).

Coloration. The body is white to slightly yellowish in color, with many small red spots and small lines of irregular sizes ( Figs. 4 View FIGURE 4 , 5 View FIGURE 5 ).

Reproductive biology. Four ovigerous females carry one, seven, ten and sixteen eggs, respectively ( Figs. 5A–D View FIGURE 5 ); egg size (with developed eyes) 1.1–1.2 × 0.75–0.8 mm (n = 5 eggs). The carapace length of the smallest ovigerous female is 3.6 mm. The male: female ratio is 1:1.

Etymology. The new species is named after the type locality, Nam Dat Village. The name is used as a noun in apposition.

Habitat. This new species was found in two freshwater pools of two small karst caves. The substratum includes sand, gravel, and bedrock in a water depth of 0.4–0.6 m ( Fig. 6A–C View FIGURE 6 ).

Molecular phylogenetic results. Caridina namdat sp. nov. forms a well-supported clade in all analyses ( Fig. 9 View FIGURE 9 ; trees for single gene analyses not shown). Among the Vietnamese taxa its closest relatives are Caridina pacbo and C. pseudoserrata . The minimum genetic divergence (p-distance) to any other atyid (specifically, C. pseudoserrata ) within the dataset is 8.2 % (COI) and 4.1 % (16S), respectively (see supplementary Tables S1, S2). Within the species, genetic divergence is 0 % for COI and 0–0.2 % for 16S.

Remarks. Caridina namdat sp. nov. is characterized by several morphological characters such as the slightly reduced eyes, short eyestalk, short and without teeth rostrum, short stylocerite, slender second pereiopod, elongated endopod of male first pleopod, and long appendix interna of second pleopod.

Caridina namdat sp. nov. looks similar to two other species, i.e., Caridina pseudoserrata and C. pacbo , in the shape of the rostrum and the endopod of male first pleopod. However, the new species differs by having reduced eyes, short eyestalk and unarmed rostrum (vs. normal eye and eyestalk, and armed rostrum) (fig. 2; cf. Dang & Do 2007, fig. 1; Do et al. 2020, fig. 5).

The new species can be easily distinguished from the species, which similarly possess a short rostrum such as Caridina cucphuongensis , C. clinata , Caridina tricincta by its reduced eyes, short eyestalk and unarmed rostrum (vs. normal eye and eyestalk and armed rostrum) (fig. 2; cf. Dang in Dang et al. 1980, fig. 230; Cai et al. 1999, fig. 1; Do et al. 2020, fig. 5).

The new species somewhat resembles other cave inhabiting atyid species from Vietnam and China, viz. Caridina thachlam , C. ablepsia Guo, Jiang & Zhang, 1992 , C. demenica Cai & Li, 1997 , C. acuta Liang, Chen & Li, 2005 and C. alba Li & Li, 2010 , by possessing reduced eyes and a short rostrum. However, Caridina namdat sp. nov. can be separated from C. thachlam , C. ablepsia , C. demenica and C. alba by its slightly degenerated eyes that have a cornea with moderately developed pigmentation and the still present eyestalk (vs. strongly degenerated eyes that usually lack of cornea pigmentation and no eyestalk) (fig. 2; cf. Do et al. 2021: figs. 2, 5; cf. Guo, Jiang & Zhang 1992: fig. 1; cf. Cai & Li 1997: fig. 1; cf. Li & Li 2010: fig. 1). When compared to C. acuta , the rostrum of C. namdat is very short, reaching to the middle of the basal segment of the antennular peduncle, without teeth (vs. the rostrum is longer, reaching to the extremity of basal segment to the middle of the second segment, with 0–2 teeth on dorsal margin), the carpus of the second pereiopod is 5.0–7.0 (median 6.1) times as long as wide (vs. the carpus of the second pereiopod is 4 times as long as wide) (figs. 2–3; cf. Liang et al. 2005: figs. 15–29).

The genetic results support the distinctness of Caridina namdat sp. nov., as it both forms a distinct clade and the minimum genetic distance to its closest congeners (s. above and supplementary Tables S1, S2) is almost twice as high (8.2 %) than the minimum distance of 4.9 % (COI, DNA barcoding fragment) found among congeneric Decapoda ( Costa et al., 2007) .

We collected females carrying eggs in both March (spring) and June (summer). Our observations agree well with previous studies reported that ovigerous females Caridina present throughout the year ( Dudgeon 1987, Han et al. 2011, Jugovic et al. 2015). The number of eggs in cave-dwelling shrimp species seems to be lower than in epigean species, e.g. 1– 16 eggs in Caridina namdat , 9 eggs in C. thachlam ( Do et al. 2021) , 8– 12 eggs in Gallocaris inermis ( Fage, 1937) (Jubertie–Jupeau 1974) vs. 30 eggs in Caridina clinata , 33 eggs in Caridina nguyeni Li & Liang, 2002 , 59 eggs in Caridina rubropunctata Dang & Do, 2007 , 33 eggs in Caridina haivanensis Do & Dang, 2010 , ca. 50 eggs in C. cantonensis Yu, 1938 ( Dudgeon 1987; species identifed therein erroneously as Neocaridina serrata , see Cai & Ng, 1999), 3742 eggs in Caridina typus H. Milne Edwards, 1837 and 101 eggs in Neocaridina palmata ( Shen, 1948) . We raise our observation here for future confirmation.

Caridina namdat sp. nov. can be considered as a stygobiont based on Sket’s (2008) criteria and is the second atyid shrimp species found in the caves of Vietnam following C. thachlam . A prominent feature of northern Vietnam’s topography are the extensive karst formations. This country’s karst hills and caves are likely rich in endemic species ( Sterling et al. 2006) and a largely unknown fauna in the ecosystems in these caves is awaiting for discovery.

Our surveys throughout Vietnam showed that the new species has a limited distribution in several caves of Ba Kan Province, northeastern Vietnam ( Figs 7A, B View FIGURE 7 ). Twenty years ago, people used to take water from one of these caves for daily use. This practice was ceased, due to the reduced water level in the cave. Deforestation for slash-andburn cultivation must be a major cause of the decreasing water level in the caves ( Fig. 8 View FIGURE 8 ). This is also considered a major threat to the survival of this new shrimp species.