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(Figures 1-3, Table 1)
Differential diagnosis. Labeo meroensisn. sp. is distinguished from all other Nilotic Labeo species by having only 5.5 scale rows between lateral line and mid-dorsal line (vs. 6.5 or more). It differs from all other African Labeo except L. djourae and L. nunensis by the combination of the following characters: cylindriform body, transverse plicae on the inner surface of the lips, 5.5 scale rows between lateral line and mid-dorsal line, 39 to 41 scales in lateral line and 16 scales around the caudal peduncle. Labeo meroensisn. sp. is distinguished from L. djourae from the Chad and Benue systems and L. nunensis from the Sanaga river in Cameroon by having a larger eye diameter (21.5-27.4% of head length vs. 14.5-20.8% in L. djourae and 14.0% in the type material of L. nunensis ). The dorsal fin of L. nunesis Pellegrin, 1929 bears an anterior filament, which is not present in L. meroensis . Labeo djourae differs furthermore from L. meroensisn. sp. in having a longer snout (46.7-55.9% vs. 32.2-41.4% of head length), larger interorbital width (35.0-50.0% vs. 30.5-38.5% of head length) and a prominent emargination in the dorsal snout region which is absent in L. meroensisn. sp.
Description. Description based on the holotype and 15 paratypes . Labeo meroensisn. sp. is a small species with a maximum size in the type series of 52.5 mm SL. Body cylindriform; inner surface of lips with transverse plicae. Counts and proportional measurements are presented in Table 1. Two pairs of barbels present: maxillary barbels well developed, but mostly hidden with only tip reaching beyond jaw articulation (Fig. 3); rostral barbels small. Anterior nasal opening with well developed tube. Eyes large, in lateral position: visible in dorsal but not in ventral view. Prominent breeding tubercles sometimes developed.
Dorsal fin concave with 4 unbranched and 10 branched rays; without filamentous extension. Three of the unbranched rays supranumerary, at least anterior one only visible in cleared and stained specimens or x-rays. Pectoral fins low, laterally positioned on the body, reaching level of dorsal-fin base. Pectoral fin with 16 to 23 rays. Pectoral and pelvic fins broadly separated. Pelvic fins insert about in middle of dorsal fin and reach beyond anus. Anus 3 to 4 scale rows in front of anal fin. Pelvic fins consist of one unbranched, seven branched and again one unbranched ray. Caudal fin deeply notched with pointed dorsal and ventral lobes, consisting of 10 upper and 9 lower principal rays plus 10 - 11 upper and 7 - 10 lower procurrent rays. 39 to 41 scales in the lateral line, 5.5 scale rows between dorsal-fin origin and lateral line and 3.5 to 4.5 scales between lateral line and origin of pelvic fin. Vertebral column with 38 vertebrae (18 - 19 abdominal; 19 - 20 caudal vertebrae). Dorsal fin inserts after neural spine of vertebra number nine; anal fin inserts after haemal spine of vertebra 26 to 28, usually 27.
Coloration. Live specimens with dark-olive back with silvery reflection (Fig. 2). Towards to horizontal septum the coloration becomes more greenish. A narrow, almost golden reflecting band about one scale row above the horizontal septum. Along flank a broad grey band continuing on to the head. Belly clear white; some melanophores only on ventral side of snout. Fins transparent; only some melanophores in dorsal fin and along upper and lower border of caudal fin. Upper half of eye conspicuously orange (Fig. 2). Fixed specimens with brown back without reflections; belly pale. Grey band along flank hardly visible and completely absent from head. Fins less transparent and orange tint of eye absent.
Distribution and habitat. The species is so far only known from the two collecting sites listed in this study (Fig. 4): the river Nile between the 6th and 5th cataract at Shendi (Fig. 5) and Atbara (city, not river).
Most specimens were found in flowing shallow water over sand; one specimen was collected in submerged shore vegetation. Water parameters for Shendi were 158 µS, 23.2 °C, pH 8.23 at the 12.01.06 and for Atbara 168 µS, 20.5 °C, pH 8.30 at the 14.01.06.
Etymology. Named in reference to the ancient Nubian capital Meroe positioned at the River Nile between Shendi and Atbara, where the types have been collected.
Discussion
The taxonomy of many smaller Labeo species is problematic. Jégu & Lévêque (1984) synonymized several nominal Labeo parvus-like species from West Africa under L. parvus . In the same year, Lévêque & Daget (1984) published a list of all African Labeo species including their synonymies. Only one year later Reid (1985) revised the African Labeo species and his results differed in several aspects from the views of Jégu & Lévêque (1984), most importantly in the context of this study restricting L. parvus to the Zaire-region and south of it and noting that the small Labeo species in West Africa belong to several different species. In a book on West African freshwater fish species, Lévêque (1990) adopted only some points of Reid (1985) and referred mainly to Jégu & Lévêque's (1984) ideas. Lévêque (1990) synonymized L. ogunensis with L. parvus , erroneously contributing this step to Jégu & Lévêque (1984), who kept however, L. ogunensis as a valid species. In the latest edition of the West Africa book, this mistake was retained and the previously listed synonymies were omitted ( Lévêque , 2003). This confusion about the identity of L. parvus and related species can only be resolved by a thorough revision of the taxa involved. Labeo meroensisn. sp. also resembles L. parvus in shape, size and colouration, but it can clearly be separated from the other species in this complex. This initial similarity may be the reason for the record of a L. parvus in the Southern Sudan 1981 (Bianco, 1981, cited in Bailey, 1994). Bailey (1994) assumed that this record may be due to confusion with L. forskalii . This issue can not be further clarified, but it could be an indication for a much broader distribution of L. meroensisn. sp. as known so far.
The maximum size of the L. meroensisn. sp. type series is only 52.5 mm standard length and it is likely that the species may grow larger. A specimen of 50.6 mm SL showed well developed breeding tubercles, but when the gonads of the two cleared and stained specimens were dissected, they did not appear to be mature.
