Caridina baitulong, Hung Anh & von Rintelen & Thi Yen & Tu, 2025

Hung Anh, Le, von Rintelen, Thomas, Thi Yen, Phan & Tu, Do Van, 2025, A new freshwater shrimp species of the genus Caridina (Crustacea, Decapoda, Atyidae) from Quang Ninh province, Northeast Vietnam, Zoosystematics and Evolution 101 (4), pp. 1601-1611 : 1601-1611

Caridina baitulong sp. nov.

Figs 1 , 2 , 3

Examined material.

Holotype: • 1 male, cl 3.8 mm, IB -FS 011 , Vietnam, Quang Ninh Province, Van Don District, Ba Mun Island , a small stream, 21°4'27.57"N, 107°35'33.8159"E, coll. Do Van Tu, 9 May 2018.

Paratypes: 14 males, cl 3.1–3.9 mm, 4 females, cl 3.9–4.4 mm, 76 additional specimens, ZMB 31583 , same data as holotype ; • 1 male, cl 3.8 mm, 1 female cl. 4.0 mm, IB -FS 012 , same locality as IB -FS 011 and ZMB 31583, coll. Do Van Tu, 20 March 2023 ; • 1 male, cl 3.5, 1 female, cl 4.1 mm, 105 additional specimens, ZMB 31578 , Vietnam, Quang Ninh Province, Van Don District, Ba Mun Island , a small stream, 21°4'33.36"N 107°35'30.666"E, coll. Do Van Tu, 9 May 2018 ; • 1 male, cl 3.6 mm, 22 additional specimens, ZMB 31586 , Quang Ninh Province, Mong Cai District , a stream near the road to the border, 21°36'38.304"N 107°44'3.75"E, coll. Do Van Tu, 24 January 2018 , coll. Do Van Tu, 8 May 2018; • 1 male, cl 3.9 mm, 1 female cl. 4.1, 105 additional specimens, ZMB 31582 , Quang Ninh Province, Van Don District, Cai Lim Island , a small stream, 21°6'18.6059"N 107°34'17.568"E, coll. Do Van Tu, 8 May 2018 ; • 1 male cl. 3.9 mm, 1 female cl. 4.1 mm, 5 additional specimens, ZMB 29650 , Vietnam, Quang Ninh Province, Cam Pha City , a small stream, 21°08'12.3"N 107°16'15.7"E, coll. Do Van Tu, 2007 ; • 1 male, cl 3.7 mm, 1 female cl. 4.0 mm, 3 additional specimens, ZMB 29651 , same locality as ZMB 29650, coll. Do Van Tu, 3 January 2011 .

Comparative material.

Caridina clinata Cai, Nguyen & Ng, 1999 : • 2 males, cl 3.1 and 3.6 mm, ovigerous female, cl 3.7 mm, ZMB 30664 , Vietnam, Ninh Binh, a small stream in Cuc Phuong National Park , 20°19'11.118"N 105°37'42.168"E, coll. Thomas von Rintelen & Do Van Tu, 4 March 2017. Caridina pacbo Do, von Rintelen & Dang, 2020 : • male, cl 4.2 mm, IB –FS 003 (Holotype), Vietnam, Cao Bang province, Ha Quang District, Truong Ha Commune, Pac Bo Village , small stream in Khuoi Nam , 22°59'1.7"N 106°02'31.2"E, coll. Do Van Tu, 25 May 2017. Caridina tricincta Do, von Rintelen & Dang, 2020 : • male, cl 5.3 mm, IB –FS 001 (Holotype), Vietnam, Tuyen Quang Province, Na Hang District, Khau Tinh Commune , 22°26'42.1"N 105°23'09.1"E, coll. Nguyen Anh Tuan, 11 December 2012. Caridina serrata Stimpson, 1860 : • 10 males, cl 3.1–4.7, 5 females, cl 4.0–4.6, ZMB 30306 , Vietnam, Quang Nam Province, Hoi An City, Cu Lao Cham Island , a small stream running into the reservoir, 15°56'34.9"N 108°31'22.7"E, coll. Pham The Cuong, 10 May 2017.

Cephalothorax and cephalic appendages.

Carapace length 3.1–4.4 mm (median 3.6 mm, n = 20). Rostrum slender, straight, sometimes downward-curving, reaching to end of second segment of antennular peduncle, 0.41–0.59 (median 0.47) times as long as carapace, rostral formula 3–4 + 6 – 10 / 2 – 4 (Fig. 1 A ). Suborbital angle acute, completely fused with antennal spine; pterygostomian margin rounded, slightly produced forward (Fig. 2 A ). Eyes well developed with globular cornea, anterior end reaching to 0.7 times length of basal segment of antennular peduncle (Fig. 1 A ). Antennular peduncle 0.5–0.7 (median 0.58) times as long as carapace; basal segment 1.62–2.38 (median 1.86) times as long as second segment, second segment 1.02–2.17 (median 1.37) times as long as third segment (Fig. 1 B ). Stylocerite reaching to end of basal segment of antennular peduncle (Fig. 1 B ). Scaphocerite ovate, reaching beyond distal end of antennular peduncle, 2.16–4.89 (median 3.7) times as long as wide (Figs 1 A, C ).

Abdominal somites, telson and uropods.

Sixth abdominal somite 0.36–0.51 (median 0.44) times length of carapace, 1.1–1.84 (median 1.49) times as long as fifth abdominal somite, 0.62–1.06 (median 0.79) times length of telson. Telson length 2.51–3.53 (median 3.26) times as long as proximal wide, distal margin triangular, terminating in a short median projection, with 5–6 pairs of dorsal spiniform setae and one pair of dorso-subdistal spiniform setae; distal end with 3–5 pairs of spiniform setae, lateral pair longer than intermediate pairs (Figs 1 D, E ). Preanal carina low, slightly bent backwards, with few setae, lacking a spine (Fig. 1 F ). Uropodal diaeresis with 16–22 (median 20) movable spiniform setae, outermost shorter than lateral angle (Fig. 1 G ).

Mouthparts and branchiae.

Incisor process of mandible ending in one row of 6–7 irregular teeth, molar process truncated (Fig. 1 H ). 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. 1 I ). Upper endites of maxilla subdivided, palp short, scaphognathite tapering posteriorly, with numerous long, curved setae at posterior margin (Fig. 1 J ). Distal end of palp of first maxilliped triangular, with a short projection; flagellum of the exopod very elongated, endopod high, reaching 0.7 times length of flagellum of exopod (Figs 1 K, L ). Podobranch of second maxilliped incompletely reduced, with few finger-like projections (Fig. 1 M ). Third maxilliped reaching to end of antennular peduncle, ending in single terminal claw, exopod reaching 0.6 times length of penultimate segment; ultimate slightly shorter than penultimate segment; epipod present on the coxa (Fig. 1 N ). Branchial formula as typical for genus Paracaridina Liang, Guo & Tang, 1999 , 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 slightly reduced, arthrobranch on first pereiopod absent ( Liang et al. 1999).

Pereiopods.

Epipod present on first to fourth pereiopods. First pereiopod short, robust, reaching end of basal segment of antennular peduncle; chela 1.98–2.59 (median 2.21) times as long as wide, 1.05–1.51 (median 1.37) times length of carpus; tips of fingers rounded, without hook; usually dactylus longer than palm, 0.88–1.88 (median 1.2) times as long as palm; carpus excavated strongly anteriorly, 1.37–2.04 (median 1.75) times as long as wide; carpus 0.83–1.36 (median 1.05) times length of merus; merus 1.99–3.29 (median 2.48) times as long as wide, longer than ischium (Fig. 2 A ). Second pereiopod long, slender, reaching beyond distal end of antennular peduncle; chela 2.4–3.22 (median 2.59) times as long as wide, 0.64–0.85 (median 0.76) times length of carpus; tips of fingers rounded, without hook; dactylus 1.12–2.25 (median 1.55) times as long as palm; carpus 5.0–7.85 (median 5.68) times as long as wide, 1.03–1.34 (median 1.14) times as long as merus; merus 3.91–5.64 (median 4.88) times as long as wide, longer than ischium (Fig. 2 B ). Third pereiopod slender, reaching beyond distal end of antennular peduncle by its dactylus, terminating in one claw, with five accessory spiniform setae on flexor margin, dactylus 2.05–4.78 (median 3.69) times as long as wide (terminal claw and spiniform setae on flexor margin included), propodus 8.06–11.5 (median 10.43) times as long as wide, 3.35–6.72 (median 3.65) times as long as dactylus; carpus 3.9–6.0 (median 4.75) times as long as wide, 0.49–0.85 (median 0.63) times as long as propodus, 0.34–0.61 (median 0.51) times as long as merus; merus 5.12–8.7 (median 6.56) times as long as wide, bearing 3 strong, movable spiniform setae on posterior margin of outer surface; ischium with one small movable spiniform seta (Figs 2 C, D ). Fifth pereiopod slender, reaching to end of third segment of antennular peduncle, dactylus 2.71–5.2 (median 4.14) times as long as wide (terminal claw and spiniform setae on flexor margin included), terminating in one large claw, with 27–34 spiniform setae on flexor margin; propodus 11.3–14.88 (median 12.93) times as long as wide, 3.35–6.45 (median 4.22) times length of dactylus; carpus 3.59–6.29 (median 5.13) times as long as wide, 0.41–0.65 (median 0.48) times as long as propodus, 0.51–0.78 (median 0.6) times as long as merus; merus 5.5–8.29 (median 6.88) times as long as wide, bearing 3 strong, movable spiniform setae on posterior margin of outer surface, ischium without movable spiniform setae (Figs 2 E, F ).

Pleopods.

Endopod of male first pleopod extending to 0.55 times exopod, elongated and rectangular in shape, 2.53–2.85 (median 2.7) times as long as proximal width, inner margin concave, outer margin slightly convex, rounded distally, long pappose setae on outer and distal margins, medium-length simple setae on inner margin; with appendix interna exceeding terminal margin of endopod by 0.36 its length (Figs 2 G, H ). Appendix masculina of male second pleopod stout, reaching to proximal 0.56 times endopod length, 4.44–5.0 (median 4.72) times as long as distal width, thumb-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, narrow, small, extending about 0.7 times length of appendix masculina (Figs 2 I, J ).

Coloration.

The body is slightly yellowish to grey. Some individuals have black spots and stripes of irregular size over the whole body (Figs 3 A, B ).

Reproductive biology.

Egg size (from n = 5 ovigerous females, eggs with developed eyes) 1.0–1.1 × 0.61–0.62 mm. The carapace length of the smallest ovigerous female is 3.9 mm.

Etymology.

The new species is named after the type locality, Bai Tu Long bay. The name is used as a noun in apposition.

Habitat.

This new species was found in a stream with mixed sand, gravel, and rock substratum, and clear flowing water from the forest (Fig. 4 ).

Distribution.

Our survey data throughout Vietnam indicate that this species is restricted to Quang Ninh province. The big eggs are also clearly indicative of a landlocked life cycle.

Molecular phylogenetic results.

Caridina baitulong sp. nov. is well-supported as a distinct lineage (Fig. 5 , Suppl. material 2). The species seems to be closest to the Caridina serrata species group, but this relationship is not supported. The minimum genetic divergence (p-distance) to any other Vietnamese atyid species is 11.3 % (COI; Paracaridina zijinica , (reported as Caridina typus in Phan et al. 2021, pers. obs. )) and 5.2 % (16 S; C. tricincta , C. thachlam ), respectively. The maximum divergence found within C. baitulong sp. nov. is 4.5 % (4.3 % when using the same dataset as for COI) for 16 S and 9.3 % for COI.

Remarks.

Caridina baitulong , new species is characterized by several morphological characters such as short rostrum, reaching to end of second segment of antennular peduncle; rostrum formula 6–10 + 3 – 4 / 2 – 4; stylocerite reaching to end of basal segment of antennular peduncle; subrectangular endopod of male first pleopod with appendix interna exceeding terminal margin of endopod by 0.36 its length; appendix masculina of male second pleopod stout, reaching to proximal 0.56 times endopod length, thumb-shaped; appendix interna extending about 0.7 times length of appendix masculina.

Caridina baitulong somewhat resembles C. serrata Stimpson, 1860 , C. clinata Cai, Quynh & Ng, 1999 , C. tricincta Do, von Rintelen & Dang, 2020 and C. pacbo Do, von Rintelen & Dang, 2020 by possessing a short rostrum and the shape of endopod of male first pleopod ( Stimpson 1857; Cai et al. 1999; Do et al. 2020; Phan et al. 2021). However, C. cucphuongensis can be easily distinguished from these species by the characters shown in Table 2 .

The large genetic distances found between the sampled populations of Caridina baitulong sp. nov. seem remarkable, particularly given the rather restricted distribution area of the species (Suppl. material 2). In land-locked species this is a common pattern (see e. g. de Mazancourt et al. 2023), but the intraspecific genetic distance in Caridina baitulong sp. nov. exceeds the minimum genetic distance found among other Vietnamese species of Caridina such as C. cucphuongensis and C. namdat , or C. clinata and C. xuanlien . There is no universal genetic divergence threshold for species delimitation even in closely related groups of crustaceans ( Costa et al. 2007), so this is not necessarily problematic. Despite the large intraspecific variation in the two investigated mtDNA markers, the morphology of the species is quite homogenous across all studied populations and the high genetic distance found within Caridina baitulong sp. nov. might simply reflect ancestral mitochondrial polymorphism. It might be speculated that the separation of populations by sea level fluctuations has played a role in shaping this pattern.

Caridina baitulong sp. nov. has the defining character of Paracaridina Liang, Guo & Tang, 1999 because of its branchial formula (5 pairs of pleurobranchs well developed; 2 pairs of arthrobranchs on third maxillipeds, with second pair strongly reduced in size; 1 pair of podobranchs on second maxilliped slightly reduced, arthrobranch on first pereiopod absent). However, this character is not unique to Paracaridina ( von Rintelen et al. 2008) and genetically the new species does not cluster with Paracaridina but – albeit with weak support – rather with Neocaridina . We have thus chosen the conservative approach of tentatively assigning it to Caridina given the clear need of a taxonomic revision at the genus level for these three genera (see Do et al. 2021 b).

Figures

Figure 1. Caridina baitulong sp. nov. male paratype, cl 4.6 mm (ZMB 31583). A. Cephalothorax and cephalic appendages, lateral view; B. Antennular peduncle; C. Scaphocerite; D. Telson; E. Distal portion of telson; F. Preanal carina; G. Uropodal diaeresis; H. Mandible; I. Maxillula; J. Maxilla; K. First maxilliped; L. Distal end of palp of first maxilliped; M. Second maxilliped; N. Third maxilliped. Scale bars: 1 mm (A); 0.5 mm (B – D); 0.2 mm (E – J).

Figure 2. Caridina baitulong sp. nov., male paratype, cl 4.6 mm (ZMB 31583). A. First pereiopod; B. Second pereiopod; C. Third pereiopod; D. The same, dactylus; E. Fifth pereiopod; F. The same, dactylus; G. Male first pleopod; H. Endopod of male first pleopod; I. Male second pleopod; J. Appendix masculina and interna of male second pleopod. Scale bars: 1 mm (A, B, C, F, G, I); 0.2 mm (D, E, H, J).

Figure 3. A, B. Live coloration of Caridina baitulong sp. nov. paratypes (IB - FS 012), collected in Quang Ninh Province.

Figure 4. Habitat of Caridina baitulong sp. nov. in Ba Mun Province.

Figure 5. Bayesian Inference phylogram showing the relationships of Caridina baitulong sp. nov. to other Vietnamese atyids, based on the analysis of two mitochondrial gene fragments. Numbers on branches show Bayesian posterior probabilities (> 0.7) and ML bootstrap values (> 70). An asterisk indicates nodes with full support (1 / 100) in both analyses. The scale bar indicates the substitution rate. Numbers in brackets give the DNA accession number at MfN (MfN DNA Crust). See Table 1 for information on the sequenced specimens.