Caridina pacbo, Do & Rintelen & Dang, 2020

Caridina pacbo View in CoL , new species

( Figs. 5–7 View Fig View Fig View Fig )

Material examined. Holotype male, cl 4.2 mm ( IEBR–FS 003 ), 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″N, coll. Do Van Tu, 25 May 2017 GoogleMaps . Paratypes: 5 females, cl 5.0– 6.3 mm; 2 males, cl 4.2–4.5 mm ( ZMB 30295 ), same data as holotype GoogleMaps .

Comparative material. Caridina nguyeni Li & Liang, 2002 : male, cl 3.2 mm ( ZMB 30280), Vietnam: Cao Bang, Ha Quang, Truong Ha, Pac Bo   GoogleMaps , a small creek run into Le Nin   GoogleMaps stream, 22°59′4.0″N 106°02′53.7″E, coll. Do Van Tu, 25 May 2017; male, cl 3.3 mm ( ZMB 30283), Vietnam: Cao Bang, Ha Quang, Lung Nam, a small stream runs along the road of Thin Tang village   GoogleMaps , 22°58′33.99″N 106°04′7.06″E, coll. Do Van Tu, 25 May 2017. 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   GoogleMaps , a small stream near the footpath to the one thousand year old tree (in Vietnamese name: chò), 20°21′0.779″N 105°36′11.675″E, coll. Thomas von Rintelen & Do Van Tu, 03 April 2017.

Description. Carapace length 4.2–6.3 mm (median 5.2 mm). Rostrum slightly curved downwards, short, reaching beyond middle or close to end of basal segment of antennular peduncle, 0.25–0.36 (median 0.3) times as long as carapace, rostral formula: 5–12+2–6/0–3 (n = 8), teeth small ( Fig. 5A View Fig ). Suborbital angle acute, completely fused with antennal spine; pterygostomian margin rounded ( Fig. 5A View Fig ). Eyes well developed with globular cornea, anterior end reaching to 0.6 times length of basal segment of antennular peduncle ( Fig. 5A View Fig ). Antennular peduncle 0.35–0.62 (median 0.49) times as long as carapace; basal segment 1.38–1.67 (median 1.67) times as long as second segment, second segment 1.2–1.33 (median 1.33) times as long as third segment ( Fig. 5B View Fig ). Stylocerite mostly reaching beyond the end of basal segment, sometimes to the middle of second segment of antennular peduncle ( Fig. 5B View Fig ). Scaphocerite ovate, reaching beyond distal end of antennular peduncle, 2.64–3.13 (median 2.73) times as long as wide ( Fig. 5C View Fig ).

Sixth abdominal somite 0.37–0.51 (median 0.46) times length of carapace, 1.25–1.77 (median 1.4) times as long as fifth somite, 0.95–1.08 (median 1.0) times length of telson. Telson length 1.77–1.92 (median 1.91) times as long as proximal wide, distal margin round with median projection, with 4–5 pairs of dorsal spinules and one pair of dorso-subdistal spinules; distal end with four pairs of spines, lateral pair slightly longer than intermediate pairs ( Fig. 5D, E View Fig ). Preanal carina high, with few setae, lacking a spine ( Fig. 5F View Fig ). Uropodal diaeresis with 19–20 movable spinules, outermost one as long as lateral angle ( Fig. 5G View Fig ).

Incisor process of mandible ending in one row of six irregular teeth, molar process truncated ( Fig. 5H View Fig ). Lower lacinia of maxillula broadly rounded, upper lacinia elongated, with a number of distinct teeth and setae on inner margin, palp short and stout with few simple setae at tip ( Fig. 5I View Fig ). Upper endites of maxilla subdivided, palp short, scaphognathite tapering posteriorly, with numerous long, curved setae at posterior margin ( Fig. 5J View Fig ). Distal end of palp of first maxilliped triangular, with a short projection; flagellum of the exopod very elongated, endopod high, reaching 0.67 times length of the flagellum of exopod ( Fig. 5K, L View Fig ). Podobranch of second maxilliped incompletely reduce to a lamina with few fingerlike projections ( Fig. 5M View Fig ). Third maxilliped reaching to end of antennular peduncle, ending in single terminal claw, exopod reaching to 0.3 times length of penultimate segment; ultimate segment as long as penultimate segment ( Fig. 5N View Fig ). Branchial formula as is typical for genus, five pairs of pleurobranchs well developed; three pairs of arthrobranchs, two on third maxillipeds, with second pair strongly reduced in size, one pair on first pereopod; one pair of podobranchs on second maxilliped strongly reduced.

Epipod present on first four pereopods. First pereopod short, robust, reaching the end of basal segment of antennular peduncle; chela 2.07–2.40 (median 2.2) times as long as wide, 1.45–1.6 (median 1.5) times length of carpus; tips of fingers rounded, without hook; dactylus 0.75–0.95 (median 0.83) times as long as palm; carpus excavated anteriorly, 1.31–1.67 (median 1.5) times as long as wide, 0.75–0.88 (median 0.8) times length of merus; merus 2.75–3.8 (median 2.78) times as long as wide, 1.21–1.29 (median 1.25) times longer than chela, longer than ischium ( Fig. 6A View Fig ). Second pereopod long, slender, reaching to end of antennular peduncle; chela 2.67–3.09 (median 2.82) times as long as wide, 0.77–0.86 (median 0.78) times length of carpus; tips of fingers rounded, without hook; dactylus 1.57–1.78 (median 1.67) times as long as palm; carpus 4.25–5.0 (median 4.89) times as long as wide, as long as merus; merus 5.0–5.86 (median 5.45) times as long as wide, longer than ischium ( Fig. 6B View Fig ). Third pereopod slender, reaching beyond end of scaphocerite by its dactylus, terminating in one claw, with four or five accessory spines on flexor margin, dactylus 2.25–2.5 (median 2.47) times as long as wide (terminal claw and spines on flexor margin included), propodus 8.4–9.33 (median 8.98) times as long as wide, 3.89–4.2 (median 4.19) times as long as dactylus; carpus 4.71–5.54 (median 4.88) times as long as wide, 0.76–0.83 (median 0.78) times as long as propodus, 0.51–0.58 (median 0.54) times as long as merus; merus 5.56–6.36 (median 6.05) times as long as wide, bearing three strong, movable spines on posterior margin of outer surface; ischium with a small spinule ( Fig. 6C, D View Fig ). Fifth pereopod slender, reaching to end of second segment of antennular peduncle, dactylus 2.25–2.5 (median 2.47) times as long as wide (terminal claw and spines on flexor margin included), terminating in one large claw, with 28–45 spinules on flexor margin; propodus 11.78–12.57 (median 12.13) times as long as wide, 4.89–5.3 (median 4.90) times length of dactylus; carpus 4.0–4.6 (median 4.15) times as long as wide, 0.5–0.57 (median 0.54) times as long as propodus, 0.63–0.66 (median 0.64) times as long as merus; merus 5.06–5.47 (median 5.13) times as long as wide, bearing 3–4 strong, movable spines on posterior margin of outer surface ( Fig. 6E, F View Fig ).

Endopod of male first pleopod extending to 0.8 times exopod, rectangular in shape, anterior region folding backwards, 2.86–3.33 (median 3.09) times as long as proximal width, inner margin concave, outer margin slightly convex, rounded distally, long plumose setae on outer and distal margins, medium-length setae on inner margin; with appendix interna distinctly exceeding terminal margin of endopod ( Fig. 6G, H View Fig ). Appendix masculina of male second pleopod slender, reaching to proximal 0.7 times endopod length, 8.0 times as long as distal width, rod-shaped, with some short spinules on outer surface and some long spinules on distal surface; appendix interna at the middle of appendix masculina, extending about 0.6 times length of appendix masculina ( Fig. 6I, J View Fig ).

Habitat. This new species was found in a streamlet with mixed sand, gravel, and bedrock substratum, and clear flowing water from the forest ( Fig. 7 View Fig ).

Colouration (from a colour photograph taken immediately after collection). Few irregular small black spots are present on the ventrolateral parts of the carapace. Five transverse black stripes which are irregularly broken at five abdominal somites ( Fig. 8 View Fig ).

Etymology. The new species is named ‘ pacbo ’ after the type locality name: Pac Bo, Ha Quang, Cao Bang Province, northeast Vietnam. The name is used as a noun in apposition.

Remarks. Phylogenetic analysis of Vietnamese atyid shrimps indicates that Caridina pacbo , new species, is closest to C. pseudoserrata Dang & Do, 2007 , both described from Cao Bang Province (see below). Caridina pacbo , new species, can be distinguished from C. pseudoserrata by these characters: the rostrum reaches the middle or close to the end of basal segment of the antennular peduncle (vs. rostrum reaches middle of second segment of the antennular peduncle in C. pseudoserrata ); the teeth on the rostrum are small (vs. moderate size in C. pseudoserrata ); the chela of the first pereopod is 2.07–2.40 times as long as wide (vs. 1.8 times as long as wide in C. pseudoserrata ); the dactylus of the chela is shorter than the palm (vs. equal or slightly longer than palm in C. pseudoserrata ); the chela of the second pereopod is 2.67–3.09 times as long as wide (vs. 2.25 as long as wide in C. pseudoserrata ); and the appendix interna distinctly exceeds the terminal margin of the endopod of the male first pleopod (vs. reaching to anterior margin of endopod or slightly exceeding it in C. pseudoserrata ) ( Figs. 5 View Fig , 6 View Fig ; cf. Dang & Do, 2007a: figs. 1, 2).

Caridina pacbo , new species, can be included in the C. serrata group because of a long stylocerite extending beyond the distal end of the basal antennular peduncle, the presence of dorsal teeth on the carapace, and the endopod of the male pleopod having a distinctive appendix interna (see Cai & Ng, 1999). However, C. pacbo , new species, differs from C. serrata in: the first pereopod chela being 2.07–2.40 times as long as wide (vs. 2.0 times as long as wide in C. serrata ); the first pereopod merus being 1.21–1.29 times longer than the chela (vs. as long as chela in C. serrata ); and the endopod of the male first pleopod being 2.86–3.33 times as long as its proximal width (vs. 2.5 times as long as proximal width in C. serrata ) ( Figs. 5 View Fig , 6 View Fig ; cf. Cai & Ng, 1999: figs. 2, 3).

Caridina pacbo , new species, can be easily distinguished from C. nguyeni Li & Liang, 2002 , which was also described from Cao Bang province, by its shorter rostrum, reaching to the middle or close to the end of the basal segment of the antennular peduncle (vs. reaching to the end of the third segment of antennular peduncle in C. nguyeni ); fewer number of teeth on the rostrum, armed dorsally with 8–17 teeth including 2–6 teeth on the carapace, ventrally with 0–3 teeth (vs. armed dorsally with 20–25 teeth including 9–12 teeth on carapace, ventrally with 4–6 teeth in C. nguyeni ); and the dactylus of the fifth pereopod bearing 28–45 spinules on its flexor margin (vs. 25–27 spinules in C. nguyeni ) ( Figs. 5 View Fig , 6 View Fig ; cf. Li & Liang, 2002: figs. 1, 2).

The type locality of C. pacbo , new species, is close to Bama County, Guangxi Province, China, where C. bamaensis Liang & Yan, 1983 , was described. Caridina bamaensis is similar to C. pacbo , new species, in the shape of the rostrum and the endopod of male first pleopod. However, the new species can be distinguished from C. bamaensis by: the rostrum sometimes reaching near the end of the basal segment of the antennular peduncle, with teeth on both dorsal and ventral margins (vs. not reaching near the end of basal segment of the antennular peduncle, without teeth on both dorsal and ventral margins in C. bamaensis ); the stylocerite mostly reaching beyond the end of basal segment of the antennular peduncle, or sometimes to the middle of second segment of antennular peduncle (vs. only reaching to the end of basal segment of the antennular peduncle in C. bamaensis ); the carpus of second pereopod is as long as the merus (vs. longer than merus in C. bamaensis ); and the inner margin of the endopod of the male first pleopod is slightly concave (vs. strongly concave in C. bamaensis ) ( Figs. 5 View Fig , 6 View Fig ; cf. Liang & Yan, 1983: figs. 1–10).

Caridina pacbo , new species, also looks similar to another species, Paracaridina zijinica Liang, 2002 , described from Zijin County, Guangdong, China, in the shape of the rostrum and the endopod of male first pleopod. However, the new species differs in the branchial formula being complete with nine pairs of branchials (vs. branchial formula is incomplete with eight pairs of branchials, the base of the first pereopod without arthrobranchiae in P. zijinica ), the rostrum possessing 2–6 teeth on the carapace behind posterior margin of orbital margin (vs. without teeth in P. zijinica ), and the teeth on the rostrum being smaller (vs. moderate size in P. zijinica ) ( Figs. 5 View Fig , 6 View Fig ; cf. Liang, 2002: fig. 3).

The shapes of the rostrum, chela of first pereopod, endopod of male first pleopod, appendix masculina of male second pleopod, and appendix interna of the new species show similarities with Caridina cucphuongensis Dang in Dang, Thai & Pham, 1980. However, compared to C. cucphuongensis , the new species is easily separated by the shorter rostrum, which reaches beyond the middle or close to the distal end of the basal segment of the antennular peduncle (vs. reaching proximal part to sometimes middle of second segment of antennular peduncle in C. cucphuongensis ); the smaller rostral teeth (vs. relatively large in C. cucphuongensis ); the most posterior tooth of dorsal rostrum-carapace teeth row being close to the second most posterior tooth (vs. far from the second tooth in C. cucphuongensis ); the scaphocerite being 2.64–3.13 times as long as wide (vs. 2.50 times as long as wide in C. cucphuongensis ); and the endopod of the male first pleopod being 2.86–3.33 times as long as proximal width (vs. 2.5 times as long as proximal width in C. cucphuonensis ) ( Figs. 5 View Fig , 6 View Fig ; cf. Dang et al., 1980: fig. 230).

Molecular taxonomy of Vietnamese atyid shrimps. All sequenced individuals of Caridina tricincta , new species, cluster within all examined Vietnamese (and Chinese) atyids, which form a rather distinct clade with some Chinese species ( Fig. 9A, B View Fig ). Genetic divergence within and among the three sequenced populations ( ZMB 29641, ZMB 30360, ZMB 30363) ( Fig. 9A View Fig , Table 2 View Table 2 ) of C. tricincta , new species, is rather low with a maximum intraspecific divergence (p-distance) of 1.5% (COI) and 0.4% (16S), respectively ( Tables 3 View Table 3 , 4 View Table 4 ). In contrast, it is well isolated from its congeners with a minimum sequence divergence (p-distance) of 15.1% (COI) and 6.7 % (16S), respectively, supporting that C. tricincta , new species, is distinct from all examined species ( Tables 3 View Table 3 , 4 View Table 4 ).

Caridina pacbo , new species, is possibly sister species to C. pseudoserrata ( Fig. 9A View Fig ). It is well isolated from C. pseudoserrata with a sequence divergence (p-distance) of 8.7–8.8% (COI), and from all other Vietnamese species in the dataset by a minimum divergence of 12.2%, supporting that C. pacbo , new species, is distinct from all examined species. Genetic distances (uncorrected, p-distance) for the COI and 16S dataset are provided in Tables 3 View Table 3 and 4 View Table 4 , respectively.

The phylogenetic trees ( Fig. 9 View Fig ) do provide little to no resolution of the relationships of Vietnamese atyids. Both new species are represented in the COI tree, while Caridina pacbo , new species, could not be sequenced for 16S. However, this was to be expected given the limited taxonomic coverage of the dataset, and that a phylogeny of Vietnamese atyids has not been the aim of this paper. Rather, we intended to use a second independent suite of characters to seek support for the morphological differences found, in line with an integrative taxonomy approach ( Padial et al., 2010). The trees clearly show that the two new species described here are distinct from any of the Vietnamese and Chinese species examined in our phylogeny (and the species that are not represented for lack of samples and thus, sequences are morphologically rather distinct). As outlined in the introduction, no study on Vietnamese atyids thus far has gone beyond using morphological data, which makes it – also considering the lower quality of some older drawings – difficult to reliably identify species. By providing DNA sequences of two gene fragments commonly used for species delimitation purposes, we hope to contribute towards a more sustainable approach to atyid taxonomy in Vietnam and surrounding areas.