Sarcocheilichthys sciistius (Abbott, 1901) (Figures 2, 3, 4 and 5; Table 2).

Leuciscus sciistius Abbott (1901): 487 (original description); Böhlke (1953): 33.

Chilogobio czerskii Berg (1914): 490; Berg (1949): 661.

Chilogobio soldatovi Berg (1914): 492; Berg (1949): 662.

Sarcocheilichthys nigripinnis sciistius Nichols (1943): 191.

Sarcocheilichthys nigripinnis Bănărescu et Nalbant (1967): 294; Luo et al. (1977): 474; Yue in Chen et al. (1998): 277.

Sarcocheilichthys nigripinnis czerskii Bănărescu et Nalbant (1967): 294; Luo et al. (1977): 476; Zhu (1995): 76; Kim (1997): 207.

Sarcocheilichthys czerskii Bogutskaya et Naseka (1996): 40; Naseka (1996): 156; Reshetnikov et al. (1997): 741; Naseka (1998): 86; Yue in Chen et al. (1998): 279; Bogutskaya et al. (2001): 44; Kim & Park (2002): 112; Bogutskaya et Naseka (2004): 71; Kottelat (2006):88; Bogutskaya et al. (2008): 327; Parin et al. (2014): 92; Zhang et al. (2016): 78; An et al. (2020): 217.

Sarcocheilichthys soldatovi Bogutskaya & Naseka (1996): 42; Naseka (1996): 155; Reshetnikov et al. (1997): 741; Naseka (1998): 86; Bogutskaya et al. (2001): 44; Bogutskaya & Naseka (2004): 72; Kottelat (2006): 49; Ocock et al. (2006): 46; Bogutskaya et al. (2008): 328; An et al. (2020): 217.

Holotype. USNM 49548 (Figure 2 a, b and c), 53.4 mm SL; From Pei-Ho River (Grand Canal), Tien-Tsin (Tianjin), China.

Additional material examined. ASIZB 221731–221744, 221752–221755, 221779–221781, 40.0– 71.5 mm SL, eighteen specimens, from Baihe River, Miyun District, Beijing City, 19 October 2020 (River of type locality); ASIZB 221901–221921, 38.5–44.8 mm SL, twenty-one specimens, from Songhuajiang River, Hulan District, Harbin City, 15 June 2021; ASIZB 220835–220855, 51.2–74.8 mm SL, twenty-one specimens, from Liuhe River, Xinglong County, Chengde City, Hebei Province, 26 September 2020; ASIZB 221766–221777, 27.6–71.6 mm SL, twelve specimens, from Qinhe River, Jiyuan City, Henan Province, 3 September 2020. ASIZB 221759–221761, 50.9–53.1 mm SL, three specimens, from Yihe River, Yichuan County, Henan Province, 4 September 2020. ASIZB 221779–221786, 44.1–49.4 mm SL, eight specimens, from Guanhe River, Shangcheng County, Henan Province, 17 September 2020.

Diagnosis. Sarcocheilichthys sciistius can be separated from S. parvus, S. caobangensis, S. vittatus, S. lacustris and S. sinensis by several irregular black vertical blotches on lateral side of body (vs. a longitudinal black band or four wide black vertical bars on lateral side of body), it can be further separated from S. parvus, S. caobangensis and S. vittatus by 40–42 lateral-line scales (vs. 34–36 in S. parvus, 38 in S. caobangensis, 37–38 in S. vittatus), it can be further separated from S. lacustris and S. sinensis by barbels absent (vs. a pair of short barbels) and last dorsal-fin ray soft (vs. stiff). Sarcocheilichthys sciistius can be separated from S. davidi by dorsal-fin origin closer to snout tip than to caudal-fin base (vs. equal) and 40–42 lateral-line scales (vs. 38–39), from S. kiangsiensis by 12–14 pectoralfin rays (vs. 15–17) and barbels absent (vs. a pair of short barbels), from S. hainanensis by two rows of pharyngeal teeth (vs. one row), from S. nigripinnis by 40–42 lateral-line scales (vs. 38–40), lower lip length 61.1%–72.8% of upper lip length (vs. 73.2%–90.2% of upper lip length), paired fins orange-red in color (vs. dark brown or black) (Figure 4).

Description. Body elongated and slightly compressed, greatest body depth at dorsal-fin origin and lowest anterior to caudal-fin base; dorsal body profile rising smoothly from nostrils to dorsal-fin base, then sloping gradually to caudal-fin base; ventral body concave from lower jaws to the end of anal-fin base and slightly concave from the end of anal-fin base to caudal-fin base. Anus positioned slightly closer to anal-fin origin than to pelvic-fin insertion. Head short and compressed, length about 22.7−26.1% of body depth and depth greater than width. Snout bluntly pointed, shorter than postorbital head length, basically equal to interorbital width. Eye normal sized, diameter about 24.0−30.5% of head length, smaller than interorbital width, placed in dorsal half of head. Nostrils positioned closely, in front of eyes. Mouth sub-terminal, arc shaped; lips well developed, upper lip smooth and thick, lower lip modified to form two relatively lateral lobes confined only to sides of lower jaw, length 61.1−72.8% of upper lip length; postlabial groove with a narrow interruption, width 23.8−29.8% of upper lip length; lower jaw without developed horny sheath on the cutting edge. No barbels.

Dorsal fin iii, 7 (18 specimens); posterior edge slightly concave, its origin anterior to vertical line of pelvic-fin origin and closer to snout tip than to caudal-fin base. Pectoral fin i, 12 (1), i, 13 (12), or i, 14 (5), tips extending beyond half of distance between pectoral-fin and pelvic-fin origins. Pelvic fin i, 7 (18), tips extending beyond anus but not reaching anal-fin origin, length shorter than others fins. Anal fin iii, 5 (18) branched rays; posterior edge slightly concave, origin closer to the pelvic-fin origin than to caudal-fin base, its length close to caudal peduncle length. Caudal fin forked with 9 branched rays on upper lobes and 8 branched rays on lower lobes, tips slightly blunt, upper and lower lobes are equal in length.

Body covered with medium-sized scales. Thorax before pectoral fins naked. Lateral line complete, slightly bent ventrally at beginning, then almost straight to the end of caudal peduncle. Lateral-line scales 40 (4), 41 (12), 42 (2); scales above lateral line 4.5 (18); scales below lateral line 4 (18); predorsal scales 12 (15), 13 (3); circumpeduncular scales 16 (18).

Coloration in life. Dorsal and mid-lateral side of head yellow or light gray, ventral side grayish white. Operculum margin red or yellow. A black vertical marking behind the head. Dorsal and lateral side of the body golden yellow with irregular deep gray blotches, black blotches scattered gradually becomes dense from head margin to caudal-fin base, ventral side grayish white. Dorsal fin translucent or orange-red with two rows of black spots, caudal fin translucent or orange-red and sometimes with a large black spot on lower lobe. Pectoral fin, pelvic fin and anal fin having orange-red median part and hyaline distal edge. (Figure 4a, 4b).

Coloration in preservative. Dorsal and mid-lateral side of head light brown, ventral side grayish white. A black or dark gray vertical marking behind the head. Dorsal and lateral side of the body yellow with irregular brown blotches, black blotches scattered gradually become dense from head margin to caudal-fin base, ventral side greyish white. Dorsal fin and caudal fin with brown basal part and white distal margin. Pectoral fin, pelvic fin and anal fin having a yellow median part and a hyaline distal edge. (Figure 3, 5).

Sexual dimorphism. In breeding season, adult male with bead pearls on snout and cheek. Margin of operculum, dorsal, lateral side of body and all fins bright red (lateral side of body fades quickly when caught) (Figure 4a); adult female with elongated ovipositor extending beyond origin of pelvic-fin (Figure 5).

Distribution. Sarcocheilichthys sciistius is widely distributed in the northern China and the border of China and Russia Far East, including Heilongjiang (Amur) River Basin, Yalu River Basin, Liaohe River Basin, Haihe River Basin (including the type locality), middle and lower reaches of Yellow River Basin and Huaihe River Basin (Figure 6).

Habitat and biology. Sarcocheilichthys sciistius usually inhabits slowly flowing rivers with clear, shallow waters with a sand-gravel substrate (Figure 7a). In the Baihe River, coexisting species include Opsariichthys bidens, Zacco platypus, Rhynchocypris lagowskii, Rhodeus sinensis, Hemibarbus labeo, Squalidus wolterstorffi, and Microphysogobio chinssuensis. In addition, species of freshwater mussel like Unio douglasiae can also be found in this habitat. Sarcocheilichthys sciistius usually feeds on the algae growing on the surface of pebbles (Figure 7b).

Genetic comparisons. The molecular phylogenetic results (both BI and ML) based on Cyt b sequence shows that Sarcocheilichthys sciistius, S. czerskii and S. soldatovi form a monophyletic group which is sister to S. nigripinnis, and S. czerskii together with S. soldatovi nested within S. sciistius (Figure 8). The interspecific genetic distances between S. sciistius and its closest two congeners S. czerskii and S. nigripinnis are 0.6% and 6.6% for Cyt b based on K2P model respectively. The intraspecific genetic distance in S. sciistius is 0.6% and in S. nigripinnis is 1.1%, which are much lower than the inter-species genetic distances (6.6%) between these two species; however, the intraspecific genetic distance in S. sciistius is much higher than that of S. czerskii (0.6% vs. 0.03%, Table 3). The only available sequence from S. soldatovi is also located inside the linage of S. sciistius - S. czerskii. The ASAP analysis recognized six molecular operational taxonomic units (MOTUs) among the given nine species, and PTP method for BI tree supported eight MOTUs with the posterior probabilities of 1.00, 1.00, 1.00, 0.90, 1.00, 0.88, 0.57, and 0.95* (* refers to the node of S.sciistius, S. czerskii and S. soldatovi). The mPTP analysis for ML tree supported seven MOTUs with the posterior probabilities of 1.00, 0.98, 0.98, 0.70, 1.00, 0.76 and 0.96* respectively (* refers to the node of S. sciistius, S. czerskii and S. soldatovi). Both methods showed S. sciistius a distinct taxa and the genetic evidence supported S. sciistius to be a possible distinct species and S. czerskii with S. soldatovi may just be geographic populations of S. sciistius based on the current available sequences. The phylogenetic trees reconstructed by the BI and ML methods showed almost the same tree topology and the support values of each method are shown in the tree (Figure 8). The genetic distances of the Cyt b gene amongst nine species of Sarcocheilichthys that distributed in China are given in Table 3.