Holotype: Adult male (FU, 2018-01-15-01), tl 66.2 mm, cl 18.8 mm, rl 9.4 mm; a stream near the bamboo park, the Zaomu Mountain Forest Park, Guangdong Province China (22°43'60"N, 112°47'10"E, alt. 182 m, stn. 7), 15 January 2018. Paratypes: 7 males (FU, 2018-01-15-02) tl 45.0–61.1 mm, cl 11.8–16.4 mm, rl 7.2–9.3 mm. 14 females, 2 ovigerous females (FU, 2018-01-15-03), tl 39.8–61.5 mm, cl 9.9–17.3 mm, rl 6.5–9.3 mm, same data as for holotype. Paratypes: 2 males (FU, 2018-01-15-04), tl 32.1–48.8 mm, cl 8.0–14.2 mm, rl 5.0–8.0 mm. 1 female, tl 40.0 mm, cl 11.0 mm, rl 5.9 mm, a small stream near the Luohan hill, Heshan, Jiangmen City, Guangdong Province China (22°41'10"N, 112°43'33"E, alt. 140 m, stn.11), 12 May 2018. Paratypes: 8 males (FU, 2018-01-15-05), tl 43.3–51.2 mm, cl 11.8–14.4 mm, rl 8.5–10.1 mm. 14 female, 10 ovigerous females, tl 39.2–60.1mm, cl 10.1–16.5 mm, rl 6.5–9.6 mm, Longquan Gorge near Heshan, Jiangmen City, Guangdong Province China (22°41'6"N, 112°44'59"E, alt. 180 m, stn. 10), 12 May 2018.

Rostrum 0.51–0.71 of cl, tip slightly bent downwards, reaching to end of third segment of antennular peduncle. Rostral formula: 3-4+5-8/2-3 (usually 3), teeth equally spaced. Cephalothorax, abdomen, and second pereiopods smooth, without microspinules. Second pereiopods shorter than tl in both sexes; merus 1.0–1.2 times as long as the ischium; carpus 4.5–5.2 times as long as width, 1.2–1.4 times as long as merus and 0.8–1.0 times as long as palm. Palm not inflated, 4.8–5.3 times as long as wide. Movable finger 0.66–0.86 times as long as palm, without any gape when crossed. Fixed finger with one proximal tooth; moveable finger with two proximal teeth. All segments smooth, with only a small amount of spines along the lateral surfaces of the palm. Eggs large; size 1.1–1.4 × 1.5–1.8 mm diameter.

Rostrum (Fig. 2A, B) rl 0.51–0.71 of cl, high, reaching downward to end of third segment of antennular peduncle. Dorsal margin with 8–12 teeth, three or four equally spaced teeth behind orbit; ventral margin with two or three teeth (usually three).

Carapace (Fig. 2A) smooth; antennal spine well developed, situated below lower orbital angle. Hepatic spine slightly larger than antennal spine, and slightly above level of antennal spine.

Antennule (Fig. 2A, B) bearing sharp stylocerite, reaching end of eye; anterior margin of basal segment distinctly convex, second segment 0.46 times as long as basal segment, 0.83 time as long as distal segment. All segments with submarginal plumose setae.

Antenna (Fig. 2A, B) bearing scaphocerite large, rectangular, 2.4–2.6 times as long as wide. Outer margin almost straight, ending with a strong spine, overreached by lamella.

Mandibles, maxillulae, maxillae, first maxillipeds, second maxillipeds and branchial formula typical for genus. Third maxillipeds with robust endopod and ischiomerus slightly bow-shaped, with rows of long simple setae on distal inner and outer margins. Carpus 0.70 times length of ischiomerus, with row of long simple setae on inner margin and sparse row of simple setae on outer margin; distal segment 0.83 times of penultimate segment, with long simple setae on inner margin. Exopod reaching distal end of ischiomerus, with plumose setae distally, basal with well-developed oval lateral plate; two arthrobranchs, one rudimentary, obscured by the larger one.

First pereiopods (Fig. 2C) slender, overreaching antennal scale by carpus; carpus 1.6–2.0 times as long as chela; fingers shorter than palm, 0.80–0.90 times as long as palm.

Second pereiopods (Fig. 2D, E) shorter than tl. Shape and segment ratios of left and right second pereiopods similar in both sexes, extending beyond the antennal scale by 1/2 of carpus; merus 1.0–1.2 times as long as ischium; carpus 4.5–5.2 times as long as wide, 1.2–1.4 times as long as merus, 0.80–1.0 times as long as palm; palm not inflated, 4.8–5.3 times as long as wide, movable finger 0.66–0.86 times as long as palm; fingers not gaping when crossed; fixed finger with one tooth at proximal, moveable finger with two proximal teeth; all segments smooth, only small amount of spines along lateral surfaces of palm.

Third pereiopods (Fig. 2F) extending to end of third segment of antennular peduncle by distal propodus; propodus 2.5–3.3 times as long as dactylus, with 5–7 spines on posterior margin; dactylus 5.5 times as long as wide, terminating in small claw.

Fourth pereiopods (Fig. 2A) extending to end of third antennular peduncle segment by distal propodus, somewhat similar to third pereiopods.

Fifth pereiopods (Fig. 2G) extending to end of third segment of antennular peduncle; propodus 3.4–6.5 times as long as dactylus, with 5–7 spines on posterior margin; dactylus 3.5 times as long as wide, terminating in small claw.

First pleopods of male with endopod of approximately half-length of exopod, slightly concave at inner margin, tip rounded, without appendix interna.

Second pleopods with well-developed appendix masculina, reaching middle of endopod, approximately twice as long as appendix interna with numerous stiff setae.

Abdomen (Fig. 2A) glabrous, smooth, pleura of first to third somites broadly rounded; pleura of fourth and fifth somites also rounded, but with almost rectangular posterolateral angle; sixth somite 1.2–1.4 times as long as fifth somite, 0.59–0.67 times as long as telson.

Telson (Fig. 2H) smooth, 0.46–0.61 times of cl, longer than sixth abdominal segment; dorsal surface furnished with two pairs of stout, movable, spine; posterior margin tapering regularly to a sharp point with two pairs of posterior a spine; numerous plumose setae present between inner pair of spine.

Uropodal diaeresis with a spine shorter than lateral angle.

Eggs large, size 1.1–1.4 × 1.5–1.8 mm.

The juvenile was yellowish and semi-transparent (Fig. 3E); the adult male had a few indistinct longitudinal yellow stripes on the carapace, with one transverse yellow band on the first abdominal somite. All segments of the second pereiopods were golden (Fig. 3A, B). The ovigerous females had a pale yellow longitudinal stripe on the mid-dorsal surface from the rostrum to the tail, which extended to both sides of the abdominal somites. The palms of the third to fifth pereiopods had black and white rings (Fig. 3D). The eggs were brown (Fig. 3C).

Neighbour-joining (NJ) and maximum likelihood (ML) trees inferred from partial COI sequences (619 bp) from ten species of Palaemonidae, including the new species, are shown in Figure 4. Macrobrachium laevis sp. nov. is clustering with M. maculatum with high bootstrap support (96 % in ML tree and 96 % in NJ tree). Interspecific genetic divergence (K2P) among these ten species is summarised in Table 2. The pairwise distance was 0.15–0.62%. The new species was closest to M. asperulum (0.1475–0.1496) and M. maculatum (0.1095–0.1154), and the morphological characters supported this relationship. Moreover, the genetic divergence between M. laevis sp. nov., M. inflatum, M. nipponense and M. dongaoensis were 0.1558–0.1598, 0.1576–0.1617 and 0.2154–0.2218, respectively, supporting the morphological differentiation of the three species. Of the species analysed, M. dongaoensis was most genetically divergent from the new species (0.2154–0.2218).

Species name is derived from laevis (Latin) in reference to the smoothness of the segments of the second pereiopod, carapace, and abdomen.

Macrobrachium laevis sp. nov. shows close similarity with M. maculatum Liang and Yan 1980 regarding the ratios of various segments of the second pereiopods and in the rostral shape. Macrobrachium laevis sp. nov. can be distinguished from M. maculatum by the smooth second pereiopod whose margin of the palm has scattered microspinules (versus second pereiopod with microspinules on its whole surface); the second tooth of the movable finger placed on the proximal one-quarter (versus on the proximal one-fifth); the lack of papillae along the cutting edges (versus numerous papillae along the cutting edges); the finger slightly longer than the merus (versus the finger distinctly shorter than merus); the wider scaphocerite (2.4–2.6 times as long as wide) (versus 3.5 times); and ovigerous females carrying smaller eggs (1.1–1.4 × 1.5–1.8 mm) (versus larger egg sizes, 1.60–1.68 × 2.12–2.36 mm). Macrobrachium laevis sp. nov. is morphologically close to M. asperulum von Martens 1868 regarding the form of the rostrum and egg size. Macrobrachium laevis sp. nov. can be distinguished from M. asperulum by its smooth carapace and second chelipeds and lack of denticles on the cutting edges (versus with rough carapace and chelipeds, and the presence of approximately ten denticles on the cutting edges), and the second tooth of the movable finger at about proximal one-quarter (versus second tooth of the movable finger on the proximal two-fifths). Macrobrachium laevis sp. nov. superficially resembles M. inflatum Liang & Yan, 1985; however, Macrobrachium laevis sp. nov. can be distinguished from M. inflatum by its shorter rostrum with fewer dorsal teeth and reaching beyond the end of the third antennular peduncle segment, with 8–12 dorsal teeth (versus rostrum reaching beyond the scaphocerite, with 12–17 dorsal teeth); the palm of male second pereiopod being not inflated (versus inflated) and 4.8–5.3 times as long as wide (versus 3.5–3.6 times); the finger distinctly longer than merus (versus the finger as along as the merus); the ischium shorter than the merus (versus the ischium distinctly longer than the merus); and the ovigerous females bearing larger-sized eggs (1.1–1.4 × 1.5–1.8 mm) (versus 0.53–0.59 × 0.62–0.69 mm). Macrobrachium laevis sp. nov. is closely related to M. fukienense Liang & Yan, 1980. It is possible to distinguish Macrobrachium laevis sp. nov. from M. fukienense by the presence of more dorsal and postorbital teeth (8–12 dorsal and 3–4 postorbital teeth) (versus 7–8 dorsal and 1–2 postorbital teeth); the second tooth of the movable finger of the male second pereiopods on the proximal one-quarter (versus on the proximal half). Macrobrachium laevis sp. nov. is also closely related to M. nipponense De Haan, 1849. Macrobrachium laevis sp. nov. can be distinguished from M. nipponense by morphological characters of the male second pereiopods. The second pereiopods of Macrobrachium laevis sp. nov. are distinctly shorter than those of M. nipponense; the finger are distinctly longer than the merus (versus the finger shorter than the merus), and without setae on the cutting edge (versus the cutting edge with the long dense setae) (Fig. 3A, B versus Fig. 6A). It is possible to distinguish living Macrobrachium laevis sp. nov. from other congeners by its striking colour pattern (Fig. 3). Morphological differences between these congeneric species are presented in Table 3.

Specimens of Macrobrachium laevis sp. nov. were collected from two streams and a river. The stream was near bamboo park, the Zaomu Mountain Forest Park, Foshan City (22°43'60"N, 112°47'10"E, alt. 182 m, stn. 7) (Fig. 5A). This stream runs through land covered with a secondary forest, with beds of rock and patches of gravel. The stream width and depth were 2.0–3.5 m and 0.6–0.9 m, respectively, with fast flowing water. The water parameters of the stream at the time of collection (15 January 2018) were: temperature 13.8 °C, pH 7.0, dissolved ammonia nitrogen 0.2 mg/l, and dissolved oxygen 4.0 mg/l. The prawns were found at the bottom of the streams together with an atyid shrimp, Caridina cantonensis Yu 1938. The specimens were also collected from another small stream near the Luohan hill, Heshan, Jiangmen City, Guangdong Province (22°41'10"N, 112°43'33"E, atl. 140 m, stn. 11). The environmental conditions were very similar to the first stream. The water parameters of the stream at the time of collection (12 May 2018) were: temperature 25.6 °C, pH 6.5, dissolved ammonia nitrogen 0.2 mg/l, and dissolved oxygen 4.5 mg/l. Additional specimens were collected from the Longquan Gorge, near Heshan, Jiangmen City (22°41'6"N, 112°44'59"E, atl. 180 m, stn. 10) (Fig. 5B). It is a small river, with a total length of 6 km. The total drop of the river is 108 meters. The river resembles a jade belt and is deeply embedded at the bottom of the Zaomu Mountain. The river has flowing water, with rocks interspersed with sand patches at its bottom. The water parameters of the river at the time of collection (12 May 2018) were temperature of 26.1 °C, pH 7.0, dissolved ammonia nitrogen 0.2 mg/l, and dissolved oxygen 6.0 mg/l.

So far only known from the type locality and nearby localities in the Guangdong Province, southern China.

Five females, tl 48.5–52.8 mm, cl 14.2–16.3 mm, 4 males, tl 51.3–65.9 mm, cl 18.0–25.6mm, Sibao Reservoir, Heshan, Jiangmen City (22°44'14"N, 112°50'17"E, alt. 84 m, stn. 1), 3 September 2017; 2 females, tl 48.5–50.4 mm, cl 15.2–16.4 mm, 1 male, tl 51.3 mm, cl 17.3mm, Lingshan Garden, Gaoming, Foshan City (22°45'42"N, 112°44'39"E, alt. 44.9 m, stn. 2), 17 May 2018; 3 females, tl 46.3–49.2 mm, cl 14.1–15.3 mm, 2 males, tl 51.3–61.4 mm, cl 18.2–23.4 mm, Xikong Reservoir, Gaoming, Foshan City (22°42'35"N, 112°43'25"E, alt. 22.4 m, stn. 3), 17 May 2018; 2 females, tl 42.5–44.1 mm, cl 13.2–14.6 mm, 4 males, tl 48.3–59.4 mm, cl 17.4–22.9 mm, Yangmei River, Gaoming, Foshan City (22°45'18"N, 112°46'04"E, alt. 49 m, stn. 5), 9 September 2017; 4 females, tl 41.4–50.3 mm, cl 12.3–16.3 mm, 3 males, tl 47.2–65.5 mm, cl 17.5–24.5 mm, stream near Hengkong Village, Gaoming, Foshan City (22°44'49"N, 112°47'04"E, alt. 72 m, stn. 6), 9 September 2017; 1 female, tl 42.5 mm, cl 13.2, 1 male, tl 48.3 mm, cl 16.7 mm, a stream near Datian Village, Gaoming, Foshan City (22°44'22"N, 112°46'36"E, alt. 56 m stn. 9), 17 May 2018.

Macrobrachium nipponense were found in reservoirs, streams, rivers, and ponds of the Zaomu Mountain Forest Park. The species is native and broadly distributed throughout East Asia (i.e. China, Japan, Korea, Vietnam, and Myanmar), (Cai and Ng 2002; Li et al. 2007). Macrobrachium nipponense was introduced into Singapore, Philippines, Uzbekistan, Iraq, Russia, Belarus, Moldova, and Iran (Chong et al. 1987; Alekhnovich and Kulesh 2001; Mirabdullaev and Niyazov 2005; Cai and Shokita 2006; De Grave and Ghane 2006; Salman et al. 2006). Macrobrachium nipponense is commercially important in Guangdong Province where it is sold live in local fish markets, and is locally consumed at home and in restaurants as a special dish.

The body has a lighter green and transparent colour, and the carapace has an M-shaped mark on the side (Fig. 6A).

China, Japan, Korea, Myanmar, and Vietnam.

Three females, tl 45.8–54.0 mm, cl 12.0–18.3 mm, 4 males, tl 35.6–75.8 mm, cl 9.6–19.8 mm, Yangmei River, Gaoming, Foshan City (22°45'18"N, 112°46'04"E, alt. 49 m, stn. 5), 9 September 2017.

The present specimens are consistent with the original description and illustration by Liang and Yan (1980) and Liu et al. (1990). This species is widely distributed in the southeastern China. Macrobrachium maculatum has an economic importance and is usually found in the same habitat with M. nipponense. Macrobrachium maculatum inhabits freshwater and has been found in rivers, reservoirs, and streams. This species seeks shelter among aquatic vegetation.

The body is very dark brown, the cephalothorax has diagonal yellow stripes, and the abdomen has large spots (Fig. 6B).

Southeastern China (Anhui, Hunan, Fujian, and Guangdong Provinces).

Two females, tl 46.8–50.2 mm, cl 13.8–15.3 mm, 1 male, tl 52.1 mm, cl 14.2 mm, Qianlonggu, Gaoming, Foshan City (22°42'48"N, 112°44'54"E, alt. 124 m, stn. 4), 9 September 2017; 2 females, tl 45.5–51.0 mm, cl 13.4–16.1 mm, 3 males, tl 46.5–60.1 mm, cl 15.2–21.3 mm, Yangmei River, Gaoming, Foshan City (22°45'18"N, 112°46'04"E, alt. 49 m, stn. 5), 9 September 2017; 3 females, tl 40.5–54.3 mm, cl 12.5–20.6 mm, 2 males, tl 41.5–65.2 mm, cl 16.2–23.7 mm, Sibao Reservoir, Heshan, Jaingmen City (22°44'14"N, 112°50'17"E, alt. 84 m, stn.1), 17 August 2017.

Specimens were confidently assigned to the present species due to their inflated palm, the upturned rostrum and the rostral formula, as well as the ratio of the segments in the male second pereiopods. Macrobrachium inflatum is usually found together with M. nipponense.

The body is translucent and light green. The rostrum is transparent to almost colourless. The cephalothorax has blue-black diagonal strips, and the abdomen shows blue-black transverse strips. The second pereiopods have transversal yellow bands on the merus and carpus. All joints of third to fifth pereiopods have transversal yellow bands. The eggs are yellow (Fig. 6C).

Southeastern China (Jiangsu, Anhui, Hunan, Guangdong, and Yunnan Provinces).