Rehabilitation of Bryconaethiops yseuxi Boulenger, 1899 (Characiformes: Alestidae) from the Congo River basin, Africa

Bryconaethiops yseuxi Boulenger, 1899, originally described from “Haut Congo”, is rehabilitated. The species was synonymized, successively with B. microstoma (Günther, 1873) and B. macrops Boulenger, 1920, subsequently rehabilitated, and then again considered as a junior synonym of B. microstoma. Bryconaethiops yseuxi is here considered as a valid species, easily distinguished from all other Bryconaethiops species by its small size (maximum: 72.2 mm SL), 7.5 rows of scales between the dorsal‐fin origin and the lateral line; 11–13/1/8–12 gill rakers on the first gill arch and long dorsal‐fin and anal‐fin bases, respectively, 15.7–17.6% of SL and 22.7–25.9% of SL.


Introduction
The genus Bryconaethiops Gü nther, 1873 is a member of the family Alestidae (formerly classified as Alestinae within Characidae sensu Greenwood et al., 1966) (Paugy 2003;Paugy and Schaefer forthcoming). The most important features of the family are: the presence of a strong oral dentition; the presence of an adipose fin (exceptionally absent) (Paugy 2003); and the presence of cycloid scales (Mamonekene and Teugels 1993). Although Paugy and Schaefer (forthcoming) rejected the subfamily-level classification of the Alestidae, they maintained the present tribal arrangement of the genera as a matter of practical convenience, while recognizing the inadequate, artificial nature of this interim classification. Three major groups can be recognized within the family that are differentiated mainly on the basis of tooth morphology: the genus Hydrocynus Cuvier, 1817 (six species) characterized by strong caniniform, mostly conical, teeth; the Alestiini sensu stricto (45 species) including the genera Alestes Mü ller and Troschel, 1844;Brycinus Valenciennes, 1849, andBryconaethiops Gü nther, 1873, characterized by more modest pluricuspid teeth and molariform teeth in the inner row of the premaxilla; and the Petersiini (16 genera, 59 species), characterized by small size, reduced pluricuspid teeth, and nonmolariform teeth in the inner row of the premaxilla.
The genus Bryconaethiops is distinguished from all other alestid genera by the following unique combination of characters: three rows of premaxillary teeth (Paugy 1990(Paugy , 2003Mamonekene and Teugels 1993); the eyes covered with a well-developed adipose eyelid; and a pronounced sexual dimorphism in the unpaired fins, a convex anterior margin of the anal fin and a filamentous elongation of the dorsal-fin rays in adult males (Paugy and Schaefer forthcoming).
At present, this genus contains four valid species: B. microstoma Gü nther, 1873 known from several river basins of the Lower Guinea province sensu Roberts, 1975 andthe Lower andMiddle Congo basin [(Pellegrin 1928;Daget 1984; Paugy and Schaefer forthcoming); but not in Lake Kivu (Snoeks et al. 1997)]; B. boulengeri Pellegrin, 1900 known from the Congo and Ogowe basin (Daget 1984) [the holotype is the only specimen known to date from the Ogowe River basin; the origin of the holotype is questioned as according to Paugy and Schaefer (forthcoming) the species is absent from Lower Guinea]; B. macrops Boulenger, 1920 known from the Lower Guinea province (Paugy and Schaefer forthcoming) and the Congo basin (Daget 1984), including the Oubangui and Sangha basins (Paugy and Schaefer forthcoming), and B. quinquesquamae Teugels and Thys van den Audenaerde, 1990 present in a few coastal basins of the Lower Guinea province and the Niger delta (Paugy 2003).
While studying a recent collection of fishes from the Djoué River, a right bank affluent of the Lower Congo River, we were unable to attribute some Bryconaethiops specimens to one of these four valid species. We identified them as B. yseuxi, presently considered a junior synonym of B. microstoma. Based on a detailed study of more material, including the types, B. yseuxi is revalidated below.

Material and methods
All available type specimens of the nomino-typical Bryconaethiops species were examined. The data on the holotype of B. microstoma var. habereri Steindachner, 1914 were obtained from the original description of the species and through the help of Dr A. Lamboj who examined the specimen. In total we examined 37 B. microstoma specimens, 23 B. yseuxi specimens, 15 B. boulengeri specimens, 21 B. macrops specimens, and eight B. quinquesquamae specimens mostly from the MRAC collections.
Institutional abbreviations follow Leviton et al. (1985). Other abbreviations are as follows: SL, standard length; HL, head length.
On each specimen eight meristics and 19 morphometrics were taken. A short description is given below. For an illustration of the meristics taken see Tshibwabwa and Teugels (1995). For an illustration of the morphometrics taken see Teugels and Thys van den Audenaerde (1990).

Meristics
Number of gill rakers: total number of gill rakers counted on the first branchial arch [formula: number of gill rakers on the lower part (hypo-and ceratobranchial)/one gill raker on articulation/number of gill rakers on the upper part (epibranchial)]. Caudal peduncle scales: number of scales counted around the caudal peduncle. Lateral line scales: number of pored scales counted along the lateral line, including the scales on the caudal fin. Longitudinal line scales: number of scales along the longitudinal line, including the scales on the caudal fin. Scales above the lateral line: number of scales from the anterior origin of the dorsal fin, following an antero-posterior line up to the lateral line with the lateral line scale not included. Scales below the lateral line: number of scales from the origin of the pelvic fin, following an antero-posterior line up to the lateral line with the lateral line scale not included. Dorsal-fin rays: number of simple fin rays (roman number) + number of branched fin rays. Anal-fin rays: number of simple fin rays (roman number) + number of branched fin rays.

Morphometrics
All measurements are point-to-point measurements unless otherwise noted. Standard length (SL): distance between anterior border of snout and caudal-fin base at articulation. Snout length: distance between anterior border of snout and anterior border of eye (bone to bone). Eye diameter: distance between anterior and posterior border of eye (bone to bone). Interorbital distance: minimal distance between orbits (bone to bone). Head length (HL): distance between anterior border of snout and posterior bony border of operculum (bone to bone). Predorsal distance: distance between anterior border of snout and articulation of first dorsal-fin ray (bone to bone). Prepectoral distance: distance between anterior border of snout and articulation of first pectoral-fin ray (bone to bone). Prepelvic distance: distance between anterior border of snout and articulation of first pelvic-fin ray (bone to bone). Preanal distance: distance between anterior border of snout and articulation of first anal-fin ray (bone to bone). Dorsal-fin base length: distance between anterior and posterior base of dorsal fin. Anal-fin base length: distance between anterior and posterior base of anal fin. Dorsal-adipose distance: distance between posterior base of dorsal fin and anterior base of adipose fin. Dorsalfin length: distance between articulation of first dorsal-fin ray with body and distal end of longest dorsal-fin ray. Anal-fin length: distance between articulation of first anal-fin ray with body and distal end of longest anal-fin ray. Pectoral-fin length: distance between articulation of first pectoral-fin ray with body and distal end of longest pectoral-fin ray. Pelvic-fin length: distance between articulation of first pelvic-fin ray with body and distal end of longest pelvicfin ray. Caudal peduncle length: horizontal distance between posterior base of adipose fin and caudal-fin base at articulation. Caudal-peduncle depth: minimum vertical depth of caudal peduncle. Body depth: maximum vertical depth of fish at anterior base of dorsal fin.

Historical overview
The genus Bryconaethiops was described by Gü nther in 1873 based on the types of B. microstoma from ''the River Congo'' [Boma, Lower Congo River basin (Boulenger 1909)]. A second species, B. mocquardianus (Thominot, 1886) was described based on a single specimen originating from ''San Benito'' (Equatorial Guinea) but placed in synonymy with B. microstoma by Boulenger (1909). Boulenger (1899) described B. yseuxi, originating from ''Haut-Congo''. He distinguished B. yseuxi from B. microstoma by the elongation of the dorsal-fin rays and by the presence of an additional series of scales between the origin of the dorsal fin and the lateral line, i.e. 7.5 in B. yseuxi versus 6.5 in B. microstoma. A fourth species, B. boulengeri was described by  from ''Adoumas (Ogooué)''. Steindachner (1914) described B. microstoma var. habereri based on a single specimen originating from ''Dscha, einem zuflusse des Sanga'' (Cameroon), which Boulenger (1916) synonymized with B. yseuxi.
In his catalogue, Boulenger (1916) noted that B. yseuxi is perhaps not specifically distinct from B. microstoma. According to Roberts and Stewart (1976), Boulenger (1916) realised that sexual dimorphism in Bryconaethiops is expressed by the filamentous elongation of the dorsal fin, one of the key characters to separate B. yseuxi from B. microstoma. Nichols and Griscom (1917) found several Bryconaethiops specimens with filamentous dorsal-fin rays and others with an intermediate character state. They concluded that B. yseuxi is a synonym of B. microstoma.  described B. macrops [from Bafwasende, Avakubi, Bosabangi and Fundi (Democratic Republic of Congo)], characterized mainly by the eye diameter which is always longer than the snout length and comprises 2.5 times the head length. He considered the species to be close to B. microstoma and B. yseuxi.
Holly (1930) recognized B. yseuxi as a valid species. However, is it not clear if Holly (1930) was aware of the synonymy proposed by Nichols and Griscom (1917). In his reference list, Holly (1930) mentioned the paper but nevertheless reported both diagnostic characters of B. yseuxi as given by Boulenger (1899). Indeed, the filamentous elongation of the dorsal-fin rays was still reported as a diagnostic character for B. yseuxi despite the results of Nichols and Griscom (1917).
Without studying the holotype of B. yseuxi (see Roberts and Stewart 1976), Poll (1939) confirmed the synonymy of this species not with B. microstoma but with B. macrops [a literal translation of Poll's (1939) statement reads ''not only with B. microstoma but also with B. macrops'' (according to Roberts and Stewart (1976) ''B. microstoma or B. macrops''); this is probably a linguistic lapsus of Poll (1939) as the synonymy is discussed under his description of B. macrops and in reference to B. macrops] as he had found Bryconaethiops specimens belonging to B. macrops and previously identified as B. yseuxi. Poll and Gosse (1963) rehabilitated B. yseuxi based on numerous specimens deposited at the MRAC. Roberts and Stewart (1976) documented the differences between B. yseuxi and the other known Bryconaethiops species: B. yseuxi had 11-13 gill rakers on the lower part of the first branchial arch (versus .15 in B. microstoma, B. macrops, and B. boulengeri), a maximum size of 70 mm SL (versus .109 mm SL), and a different position and shape of the teeth in the upper jaw. Daget (1978), in his ''Contribution à la faune de la République Unie de Cameroun. Poissons du Dja, du Boumba et du Ngoko'', without having consulted Poll and Gosse (1963) and Roberts and Stewart (1976), referred to B. yseuxi as a synonym of B. macrops. Paugy (1984) also referred to B. yseuxi as a synonym of B. microstoma, without reference to the publication of Roberts and Stewart (1976). Since then, in various reference works such as the Eschmeyer's Catalog of fishes online (www.calacademy.org/research/ichthyology/ catalog/fishcatsearch.html) and Fishbase (Froese and Pauly 2006), B. yseuxi is considered a junior synonym of B. microstoma. As a result of the latter synonymy, at the present, B. microstoma var. habereri is considered a synonym of B. microstoma.

Description
Meristics and morphometrics of B. yseuxi are given in Table I and a comparison with the other valid nominal species of the genus is given in Table II. Bryconaethiops yseuxi is a smallsized species (maximum size 72.2 mm SL). A cleared specimen of 50.8 mm SL (MRAC 55237) had its skeleton already entirely ossified. Roberts and Stewart (1976) found a gravid female of only 47 mm SL.
Body deep and somewhat laterally compressed. Morphologically B. yseuxi is closest to small-sized B. microstoma specimens in overall shape of the body and the fins. Nevertheless, B. yseuxi specimens are always deeper bodied than B. microstoma specimens of the same size (Figure 3).
Mouth terminal; upper jaw with three rows of premaxillary teeth: external row composed of two small median teeth with a small lateral cusp at their outer side (exceptionally also an even smaller one at their inner side) (versus monoscuspid teeth in other Bryconaethiops species); a median row with six or seven (mono-, bi-, or tricuspid) teeth; and an inner row  Min-max a n Min-max a n Min-max a n Min-max a n Min-max a n Rehabilitation of Bryconaethiops yseuxi of eight pluricuspid teeth [three (median teeth) to seven cusps] with their cusp on a median row and without an anterior cusp or row of cusps as generally found in the other Bryconaethiops species. Lower jaw with an external series of six large pluricuspid teeth and an inner row of two median caniniform teeth. A bilateral asymmetric growth favouring the left side of the jaws was observed by Roberts and Stewart (1976). However, based on our observations we cannot confirm this statement. There is a well-marked sexual dimorphism in the dorsal and anal fins. In males, all dorsal-fin rays, except the first and the last two, are filamentously elongated and can attain up to 70.7% SL. The distal border of the anal fin is straight in females, juveniles and immature specimens (Figure 1), but slightly convex anteriorly in males ( Figure 2).

Coloration
Head and body of live mature male specimens silvery with a pinkish hue on sides of body. Upper side of head green-olive. Humeral spot present but not well marked. Adipose-fin whitish transparent, blackish at base.
Anterior two-thirds of distal part of anal fin blackish; posterior third of distal part of anal fin transparent; distal part bordered by a whitish submarginal band; basal part of anal fin greyish black, transparent. Pelvic fins greyish black with a black distal border bordered by poorly delimited whitish submarginal band. Dorsal fin greyish white, transparent with blackish filaments. Pectoral and caudal fins whitish transparent. Distal margin of caudal fin black; base of upper and lower caudal fin lobe blackish resulting in a bilobed spot at base. Live colour pattern of females not observed. After preservation, body yellowish white; darker above lateral line with scales above lateral line faintly dark brown on their basal part. Contrast between body parts above and below lateral line seem to fade away after time. Fins whitish, transparent in overall appearance; humeral spot not well distinguishable, or even regularly absent.
In most preserved males, anterior part of distal border of anal fin blackish and bordered submarginally by a whitish band. Pelvic fins dark with poorly delimited blackish distal border. Dorsal fin filaments blackish. Posterior margin of caudal fin black.
In preserved female and juvenile specimens, all fins pale to transparent and without whitish band on anal fin. Sometimes anal-fin base partially bordered by a fine dark brown band.

Distribution
Bryconaethiops yseuxi is known only from the Lower Congo River basin, from Boma to Pool Malebo (formerly Stanley Pool) (Kinsuka, Kinshasa). The species is reported here for the first time from the Djoué River, a right bank affluent from the Lower Congo River (Figure 4).