Labeo niariensis, Liyandja & Stiassny, 2025

Liyandja, Tobit L. D. & Stiassny, Melanie L. J., 2025, Hidden in plain view: A new Labeo (Cyprinidae: Labeoninae) endemic to the Kouilou-Niari River basin in the Lower Guinea ichthyological province, Journal of Fish Biology 107 (2), pp. 592-602 : 596-599

L. niariensis , sp. nov.

Figures 5 and 6; Tables 1 and 2.

Labeo sp. 2 (Liyandja et al., in review).

L. cf. camerunensis ( Liyandja et al., 2022) .

L. annectens , L. lukulae ( Mamonekene & Stiassny, 2012) .

L. camerunenis ( Walsh et al., 2022) .

ZooBank accession numbers: urn:lsid:zoobank.org:act:EC9E- C9E3-F3EF-4871-ADEF-EAB1A99A3BCE and urn:lsid:zoobank.org:pub:.

3.3.1 | Holotype

AMNH 264174 [tissue AMCC 235701, GenBank accession: ON751960 (COX1) and ON778509 (RAG1 )], 118.3 mm SL, main channel of the Léala River , a subtributary of the Niari River (Kouilou-Niari basin), at the bridge with the road P1 , about 1.62 km in a straight line upstream of its confluence with the Louessé River, Niari Department, Republic of Congo, 2.224389, 12.819389, G. Walsh et al., October 2013.

3.3.2 | Paratypes

AMNH 264150 , one individual, 128.9 mm SL, main channel of the Louessé River , a tributary of the Niari River ( Kouilou-Niari basin), at Mayoko village , about 72 km in a straight line north of Massendjo, Niari Department, Republic of Congo, 2.290861, 12.792333, G. Walsh et al., October 2013; AMNH 256425 (tissue AMCC 221811), one, 97.0 mm SL, same locality as holotype, V. Mamonekene, April 2012; AMNH 256506 , one, 121.9 mm SL, same locality as holotype, V. Mamonekene, April 2012; AMNH 256526 , two, 67.8 – 109.1 mm SL, same locality as AMNH 264150, V. Mamonekene, April 2012; AMNH 253942 , one, 113.46 mm SL, main channel of the Niari River at Loudima , Niari Department, Republic of Congo, 4.100278, 13.060556, V. Mamonekene, December 2010. ROM 116816 , one, 100.61 mm SL, same locality as holotype, V. Mamonekene, April 2012. SAIAB 246351 , one, 120.19 mm SL, same locality as holotype, V. Mamonekene, April 2012.

3.3.3 | Diagnosis

L. niariensis is unique among congeners in the possession of a deeply bifurcate posterior process of the kinethmoid bone (Figure 6a). Additionally, it is readily differentiated from all LG congeners, except L. annectens and L. nunensis , by possessing six (vs. five or fewer) pre-dorsal vertebrae (Figure 6). It is further distinguished from L. annectens and L. nunensis by the presence of a prominent anterior notch and only moderately developed posterior process on the first infraorbital (lachrymal) versus absence of an anterior notch and an elongated posterior process, and the presence of a deeply ovoid second infraorbital versus narrow and elongated (Figure 6).

L. niariensis differs from L. annectens by a lower (30 or 31 vs. 31 or 32) total vertebral count with 13 or 14 (vs. 15 or 16) caudal vertebrae, and 34 (vs. 35) pored lateral-line scales (to the end of the hypural plate), by a longer head (23.8% – 26.2% vs. 19.6% – 22.6% SL), shorter (3.4% – 4.6% vs. 4.7% – 7.7% SL) vent – anal-fin distance, lower (25.9% – 35.4% vs. 36.2% – 59.3%) vent – anal-fin distance to greatest CPD ratio and higher (45.8% – 56.8% vs. 31.3% – 45.7%) orbital length to least CPD ratio. L. niariensis can be distinguished from L. batesii , L. lukulae , L. nunensis and L. sanagaensis by having 9 (vs. 10 or 11) branched dorsal-fin rays, and from L. camerunensis (and L. batesii ) by having 12 (vs. 16) circumpeduncular scales. Further, the new species is distinguished from L. batesii and L. nunensis by a shallower body (BD 17.9% – 20.6% vs. 20.9% – 27.5% SL) and caudal peduncle (CPD 12.6% – 13.3% vs. 15.2% – 18.3% SL); from L. batesii , L. nunensis and L. sanagaensis by a shorter (17.1 – 19.7 vs. 19.8 – 23.7% SL) dorsal-fin base and longer caudal peduncle (CPL 105.7% – 116.4% vs. <93% CPD); from L. camerunensis , L. lukulae and L. sanagaensis by a shorter (3.4% – 4.6% vs. 5.1% – 8.5% SL) vent – anal-fin distance; from L. lukulae by higher (30 – 31 vs. 28 – 29) vertebrae counts, a longer (55.7% – 58.7% vs. 49.9% – 55.6% SL) pre-pelvic distance and lower (22.8% – 32.7% vs. 51.9% – 66.9%) vent – anal-fin distance to greatest CPD ratio; from L. camerunensis by a smaller (35.8% – 47.5% vs. 49.8% – 54.2% HL) mouth gape; and from L. nunensis by lower (30 – 31 vs. 32 – 33) total vertebrae counts with 17 (vs. 18 or 19) abdominal vertebrae.

3.3.4 | Description

Based on the holotype and eight paratypes. General appearance as in Figure 5; proportional measurements and meristic counts are presented in Tables 1 and 2, respectively. Small-bodied species, maximum observed size 128.9 mm SL (AMNH 264150), with elongated cylindrical body form. Genital opening well in advance of anal-fin origin, vent – anal-fin distance 3.4% – 4.6% SL. Head moderately large, length 23.8% – 26.2% SL, interorbital space slightly convex. Snout prominent, length 49.2% – 59.7% HL, more-or-less rounded, often with shallow ethmoid furrow, poorly developed fleshy appendage with numerous minute tubercles. Eyes large, orbital diameter 24.3% – 29% HL, dorsolateral not visible in ventral view. Mouth inferior, relatively large with plicate lips; anterior barbels absent, posterior barbels small, deeply embedded in lip fold, rarely visible.

Dorsal fin concave, inserted at mid body, four simple and nine branched rays. Anal fin, with three simple and five branched rays. Caudal fin deeply forked, 9 or 10 upper procurrent, 8 or 9 lower procurrent and 19 (2 unbranched and 17 branched) principal rays. Pectoral fin lateroventral, longest ray 19.8% – 23.3% SL. Ventral fin slightly shorter than pectoral fin, longest ray 16.9% – 19.1% SL, one procurrent, one simple, eight branched rays. Anal fin relatively short (base 6.9% – 8.9% SL), not reaching caudal-fin base, longest ray 18.4 – 20.3% SL.

Scales cycloid, 34 pored scales in lateral line; 4 or 5 rows between lateral line and dorsal-fin origin; 3 rows between lateral line and ventral-fin origin, 12 circumpeduncular scale rows.

3.3.5 | Osteology

Presence of a deeply bifurcate (bi-lobed) posterior process of the kinethmoid bone (Figure 6a), absent in congeners (e.g., Figure 6b,c) and interpreted here as a diagnostic for the species. Infraorbital series includes a broad first infraorbital (lachrymal) with a prominent deep, anterior notch but only a moderately developed posterior process (compare Figure 6a with b,c), second infraorbital deeply ovoid and followed by three additional elements. Pharyngeal bones (fifth ceratobranchial) triangular in overall shape and bear long-necked teeth with ovoid, bevelled cusps inserted in three rows, from outer to inner, with two, four and five teeth, respectively (Figure 6a). Urohyal robust, lacking a collum, slightly longer than deep with a somewhat fenestrated lamellar body (Figure 6a). Axial skeleton, exclusive of modified Weberian vertebrae, comprises 30 – 31 vertebrae with 17 abdominal (6 of which are pre-dorsal) and 13 – 14 caudal vertebrae (exclusive of terminal pleural centrum). Number of pre-dorsal vertebrae (6) is constant, but unusually for Labeo species, the number of supraneurals varies from four to six among specimens examined.

3.3.6 | Colouration

Immediately postmortem (Figure 5a) colouration varies from dark brown to dark grey above and light grey to whitish below. Prominent patterns of black pigmentation encircling flank scales present in postmortem specimens, barely visible in preservation. Preserved specimens (Figure 5b), dark brown above and brown to beige below, with faint lateral stripe and ovoid black spot on caudal peduncle extending to scaly caudal-fin base.

3.3.7 | Distribution

Known only from the KNR system, Niari Department, Republic of Congo. Figure 1 shows the distribution of L. niariensis in the KNR system.

3.3.8 | Biology and ecology

All specimens of L. niariensis have been collected in the upper and middle Niari River basin. According to Walsh et al. (2022), the upper basin of the KNR, where most L. niariensis were collected, is characterized by high gradient habitats. Mamonekene and Stiassny (2012) reported that at Loudima, where additional L. niariensis specimens were collected, the Niari flows with a strong current. These observations, combined with dorso-ventral body compression and an inferior mouth placement, suggest that L. niariensis is likely associated with fast-flowing benthic habitats. Nevertheless, little is known about the ecology and habitat requirements of this species, and further ecological studies are needed.

3.3.9 | Etymology

L. niariensis refers to the Department of Niari in the Republic of Congo ( Central Africa), where the entire type series was collected. Species name to be treated as an adjective derived from the toponym Niari ( Department of Niari).

3.3.10 | Comparative material

A total of 46 specimens from the LG ichthyofaunal province were included in our analyses. Apart from the 9 specimens of the new species, 37 other specimens comprise the following:

7 L. annectens [ lectotype: BMNH 1902.11.12.137; paralectotype: BMNH 1902.11.12.138: one individual; topotype: ANSP 92427, one (CT-scanned), BMNH 1906.5.28.57: one, BMNH 1904.2.29.29 – 31: two of three examined]; 3 L. batesii [ holotype: BMNH 1912.6.23.3; AMNH249817: one (CT-scanned); AMNH 249818: one]; 3 L. camerunensis ( holotype: BMNH 1973.5.14.324; paratypes: BMNH 1973.5.14.322 – 323: two); 16 L. lukulae [ topotype: AMNH 276342: seven, AMNH 276343: six; ANSP 38553: one, AMNH 274961: one (CT-scanned), AMNH 274962: one]; 3 L. nunensis Pellegrin, 1929 [ syntype: MNHN-1928-0303: one; AMNH 249816: one (CT-scanned), AMNH 251713: one individual]; and 6 L. sanagaensis [AMNH 249854: two, AMNH 249853: one (CT-scanned), CU 93456: four].