Petersius
Myers (1929) restricted Petersius to its type species, P. conserialis, a moderately large species from East Africa, because eof its lack of fontanel. The absence of a fontanel in the adults stahe was later rejected, as a generic character, at leats in one group of American Characiformes – the Bryconinae (Howes, 1982). Petersius, the first described (and largest) form of the Petersiini wou id be characterized only by its large size . lt is known only from 2 specimens, the holotype (described in 1894) and another specimen collected in 1922, which is figured by Poll (1967a). lt is therefore likely that the type species is restricted to a few localities, or that only juvenile specimens have been collected but, owing to their small size and the presence of a parietal fontanel, they were not recognized as P. conserialis.
If the latter is true, then a number of forms with a fontanel, but resembling P. conserialis and now regarded as belonging to the genus Rhabdalestes, should be included, or better re-included, in Petersius (Géry, 1995).
Observations of the alestid taxon Petersius conserialis, was known only from two small river systems in East Africa taht were restricted to alcohol-preserved specimens and radiographs. Therefore, many characters involving internal features are unknown.
Boulenger (1909: 231) reported that Petersius conserialis differs from the other species that he assigned to that genus in its relatively large body size and ‘”the complete closing up of the parietal fontanelle”. This account of the species was based on the holotype of the species that had a total length 145 mm (Boulenger, 1909: 231). Myers (1929: 5), Hoedeman (1951: 5), Poll (1967a: 28) and Géry (1995: 40) followed Boulenger in attributing the lack of a parietal fontanel to P. conserialis. Alternatively, Murray & Stewart (2002: 1892) noted that “smaller specimens” of P. conserialis of unspecified body size have the fontanel present between the parietals and posterior part of the frontals, but apparently did not examine adults of the species in which the extent of the opening is significantly reduced.
According to Zanata & Vari (2005), a parietal fontanel is present, albeit not extensive, in all specimens of P. conserialis examined in their study, some of which reach a total length (approximately 135 mm TL) comparable to that of the holotype of the species (145 mm TL). More significantly, there is minimal difference in the proportional extent of the parietal fontanel across the size range of specimens of P. conserialis examined in their study (97.3-111.4 mm TL). These observations reduce the likelihood that the aperture would be completely absent in the only somewhat larger holotype of the species notwithstanding Boulenger’s (1909) statement to the contrary.
Nevertheless, according to Zanata & Vari (2005), Petersius is relatively basally positioned in the cladogram of the Alestidae and is phylogenetically distinctly separated from clade constituted by the lineage composed of the alestid species of small body size that traditionally constituted the bulk of species in the Petersiini. Petersius furthermore lacks all of the unambiguous synapomorphies amenable to examination on whole specimens or radiographs of such material characterizing clade “small alestids”. The preponderance of the evidence thus indicates that the association of Petersius and the name Petersiini under previous classifications with the species forming clade “small alestids” in their study is inappropriate.
As a consequence, the association of Petersius with members of clade “small alestids” as implied by the traditional concept of the Petersiini is unwarranted (Zanata & Vari, 2005).
Prior to Myers (1929), Petersius included P. conserialis and a number of alestids of small body size that had in common cuspidate teeth, the absence of the inner symphyseal dentary tooth, and a complete lateral line. Myers (1929: 5) restricted Petersius to its type species, P. conserialis, a species of moderately large body size and the other species previously assigned to that genus were shifted to other genera. Those taxa along with Petersius, nonetheless, formed the tribe Petersiini of later authors (e.g. Poll, 1967a; Géry, 1977; Paugy, 1990b). The information obtained in the study carried out by Zanata & Vari (2005) indicates that Petersius is more basally positioned in the phylogeny than are the other alestid species traditionally included in the Petersiini, and that this tribe as traditionally defined constitutes a nonmonophyletic assemblage (see figure below).
Hypothesis of relationships within the Alestidae, indicating the drift-vicariance hypothesis and the age of the oldest fossil alestids assigned to particular clades in this study (redrawn from Zanata & Vari, 2005).For these authors, Bryconalestes is recognized as a genus assigned to the “longipinnis-group” within Brycinus.
The validity of the genus Petersius seems to be doubtful because all known studies are based on the few preserved specimens of the only species belonging to this genus.Thus, several characters, like the internal features, could not be measured on these specimens. As a result, it seems that gender discrimination can be artificial because it is mainly based on lack of data and not on specific and discriminant characters. Until the capture of new specimens belonging to the species P. conserialis allows for further analysis, like many other authors we accept, at the moment, the validity of the genus Petersius.
No synonym
Type species
Petersius conserialis Hilgendorf, 1894 by monotypy.
Currently, one species is assigned to the genus Petersius.
Bibliography
Petersius conserialis Hilgendorf, 1894
Type and type locality
Petersius conserialis Hilgendorf, 1894: 172. Type locality: “Kingani River”. Holotype: ZMHU 13535.
Common name
Estonian: Petersikala
Finnish: Kynätetra
Swahili: Kasa (Tanzania)
Description
Diagnosis: body depth 3.1 (2.75) and head length 3.75 (4) in standard length. Prominent lower jaw. Parietal fontanel absent. Dental formula: [4-5/8 (upper jaw), 8 (lower jaw)], [type: [4/8 (upper jaw), 8 (lower jaw)] ; without any lower internal teeth. Cuspids number: upper jaw, external row: 3; internal row: 4-6; lower jaw, external row: 4-7. Unpaired fins non-filamentous. Dorsal fin: II,8 (II,8). Anal fin: III,19 (III,19). Gill rakers on the lower limb of the first gill arch: 20 (18). Longitudinal line scales: 32+2 (33); transversal line scales: 6½-7½ (above)/3½ (below) (7½/3½); scales around caudal peduncle: 10. Lateral line complete: 34 tubes.
Maximum reported size: 145 mm SL.
Colour: a large black mark on caudal peduncle extending on median caudal-fin rays.
Distribution
Eastern Africa: Rufiji River basin (Tanzania) and Chambezi system (Zambia) (see also Faunafri).
A relatively widespread species with no known major widespread threats.
Major threats: no information available.
Bibliography
