Petersiini

The tribe Petersiini, otherwise known as the dwarf African characids, includes 67 species distributed widely in sub-Saharan and Nilo-Sudanic Africa, the Congo and Zambezi basins and along the east coast north of the Pongolo (or Phongolo) River (Skelton, 2001). The group is characterized by the relatively small size and the absence of molariform pluricuspid dentition, a feature that is plesiomorphic for the Characiformes. The group was last revised by Poll (1967a), who recognised 14 genera and 58 species. Available evidence (Calcagnotto et al., 2005; Zanata & Vari, 2005) indicates that the tribe as traditionally defined is not monophyletic; however, Zanata & Vari (2005) did find evidence for a monophyletic clade of “dwarf alestids” upon exclusion of Petersius (type genus of the Petersiini) plus inclusion of Bryconalestes (species traditionally regarded as the Brycinus ‘longipinnis group’). Furthermore, the majority of the genera is differentiated with great difficulty and often on the basis of broadly overlapping features or by combinations of such features, many of which are variable within and among species. Poll’s classification (Poll, 1967a) was heavily influenced by such characters as number and form of the teeth, extent of the lateral line on the trunk and general form of the body. Géry (1995) discussed problems with Poll’s system, noting that many of the genera are over split and poorly defined. For example, Phenacogrammus Eigenmann, 1907, and Hemigrammopetersius Pellegrin, 1926, are extremely similar in many respects, yet are differentiated in Poll’s classification on the basis of presence/absence of the inner dentary teeth, a character that varies intraspecifically (Amiet & Nzeyimana, 1990; Paugy, 1990b). In a similar vein, within the assemblages defined by the presence/absence of inner dentary teeth, Rhabdalestes Hoedeman, 1959, Alestopetersius Hoedeman, 1951 (teeth absent), Brachypetersius Hoedeman, 1956 and Phenacogrammus (teeth present) are distinguished largely on the basis of depth of the body. In consideration of these problems, Géry (1977), Paugy (1990a) and Paugy (2003) recognised nine genera, each using a slightly different arrangement that emphasised, or gave differential weight to, different combinations of characters.

Ultimate resolution of such problematic issues concerning the classification of the alestid taxa traditionally placed in the tribe Petersiini will require a significant amount of further study. For the purposes of this work, however, we have chosen the more conservative approach to the recognition of genera and follow Poll (1967a) as the most comprehensive systematic treatment to date. Although unsatisfactory in many respects, most notably in the potential that generic assemblages do not comprise closely related species, this approach nevertheless offers a reasonable solution for differentiating the species, while avoiding any further complication to an already unwieldy and problematic classification scheme. All species are characterized by small size, adults typically to 80 mm and the largest species to 110 mm SL. General body form ranges from slender (e.g., Micralestes, Rhabdalestes) to moderately deep-bodied (e.g., Brachypetersius, Nannopetersius). Colouration typically includes a dark horizontal band at mid-body and a more or less distinctive humeral spot, and pronounced sexual dimorphism with males having:

The maxilla is edentulous, adipose eyelids are absent; dorsal fin with two unbranched plus eight branched rays, caudal fin with two unbranched plus 17 branched principal rays.

All Petersiini genera are representd by 67 species divided in 14 genera.

Key to genera

1  Body entirely scales.........................................................................................................................................................2

    Body partially scales......................................................................................................................................................13

2  Teeth multicuspidat in the outer row on the upper jaw (figure left)..........................................................................................3

    More than 15 conical teeth arranged in one row on each jaw (figure right)..........................................................Clupeocharax

3  Teeth arranged in two rows on the lower jaw (both dentaries); the inner row comprised of two conical teeth (figure below)..........4

    Teeth arranged in single row on the lower jaw without inner conical teeth...............................................................................9

4  Teeth of the upper jaw with more than three cuspids (figure left)............................................................................................5

    All teeth of the upper jaw with three cuspids (figure right)..............................................................................Tricuspidalestes

5  4 teeth in the outer row of the premaxilla (external row of the upper jaw (figure left).................................................................6

    At least 6 teeth in the outer row of the premaxilla (external row of the upper jaw (figure right)..................................Micralestes

6  Lateral line complete (figure left)........................................................................................................................................7

    Lateral line incomplete (figure right)...................................................................................................................................8

7  Fewer than 6 (4½-5½) scales in tranverse row between lateral line and dorsal-fin origin (figure left)....................Brachypetersius

    At least 6 (6½-7½) scales in tranverse row between lateral line and dorsal-fin origin (figure right).......................Nannopetersius

8  Body form tends to be very robust, greatets depth less than 2.5 in standard length; caudal spot present..............Bathyaethiops

    Body form tends to be fusiform, greatets depth more than 2.6 in standard length; caudal spot present...........Phenacogrammus

9  8 teeth in the outer row of the premaxilla (external row of the upper jaw); scales distinctly larger above lateral line (figure left)......

    ..................................................................................................................................................................Arnoldichthys

    4-6 teeth in the outer row of the premaxilla (external row of the upper jaw); scales above and below lateral line equal in size (fig.

    right).............................................................................................................................................................................10

10  A well developped fronto-parietal fontanel; lower jaw not very proeminent............................................................................11

     Fronto-parietal fontanel absent; lower jaw very proeminent......................................................................................Petersius

11  A black humeral spot; vertical fins filamentous in adult males...........................................................................................12

    No black humeral spot; vertical fins never filamentous.......................................................................................Rhabdalestes

12  5½-8½ scales in transverse row between lateral line and dorsal-fin origin (figure left).......................................Alestopetersius

    3½-4½ scales in transverse row between lateral line and dorsal-fin origin (figure right)...........................Hemigrammopetersius

13  Back naked, without scales before adipose fin (figure left).....................................................................................Ladigesia

    Back naked, except for one thin scale on either side of anal-fin (figure right)........................................................Lepidarchus

Bibliography