Chalceus (Cope, 1872)

Zanata, Angela M. & Toledo-Piza, Mônica, 2004, Taxonomic revision of the South American fish genus Chalceus Cuvier (Teleostei: Ostariophysi: Characiformes) with the description of three new species, Zoological Journal of the Linnean Society 140 (1), pp. 103-135 : 104-107

MONOPHYLY OF CHALCEUS

Characters discussed below pertaining to the question of the monophyly of Chalceus were derived from a cladistic analysis carried out by Zanata (2000) that focused primarily on the systematics of the characid genus Brycon . The analysis was based on 144 morphological characters and 58 characiform taxa and included one Chalceus species ( C. macrolepidotus ). In that analysis Chalceus is hypothesized as more closely related to African characids and representatives of the Neotropical families Hemiodontidae and Crenuchidae , followed by a clade formed by several representatives of the Neotropical Characidae . The most basal taxa included in that analysis was Xenocharax , proposed as the sister-group of all other characiforms ( Fink & Fink, 1981: 306).

Characters that supported the monophyletic condition of Chalceus in that analysis were, in the present study, checked in all recognized Chalceus species. Some of the characters proposed by Zanata (2000) as derived for Chalceus species are also present in groups previously hypothesized as being closely related to Chalceus (e.g. Brycon and the African characids (= alestids)) and their eventual utility as synapomorphies for Chalceus depends on a more detailed phylogenetic analysis focused on the relationships of these taxa. We chose to include those characters in the discussion below since they constitute additional distinguishing characters for Chalceus .

Supramaxilla ( Fig. 1 )

All Chalceus species possess a supramaxilla situated along the posterodorsal margin of the maxilla. Among characiforms the presence of this ossification has also been reported for Agoniates ladigesi ( Géry, 1963: 278, fig. 7; = A. anchovia Eigenmann ; Zarske & Géry, 1997: 180) and the Chilodontidae ( Roberts, 1969: 416; Vari, 1983: 10, fig. 1). The presence of a supramaxilla in Agoniates has not been confirmed ( Castro, 1984: 80). The maxilla of Agoniates possesses a median stria that extends along the entire length of the ossification and which separates that bone into dorsal and ventral portions. This structure may have been erroneously interpreted by Géry (1963) as a joint between two separate bony elements.

The phylogenetic relationships of the Chilodontidae lie with the Neotropical characiform families Anostomidae , Prochilodontidae and Curimatidae ( Vari, 1983) all of which lack a supramaxilla. The common possession of a supramaxilla in the Chilodontidae and Chalceus is consequently considered to represent convergence and is hypothesized to be a synapomorphy for Chalceus species.

Series of premaxillary teeth ( Fig. 2 )

Chalceus species are characterized by the presence of three series of cuspidate teeth on the premaxilla. This feature is also present in Chilobrycon , Brycon and Triportheus among characiforms, and has been interpreted as evidence of relationship among the latter two genera and Chalceus by various authors ( Regan, 1911; Howes, 1982; Lucena, 1993). However, the arrangement of teeth in the premaxillary series differs among all these genera, raising questions about the homology of the feature.

Chalceus species possess well-defined elongate outer and inner premaxillary teeth series with 6–13 and 5–10 teeth, respectively. A third series is formed by two more widely spaced teeth of intermediate size situated between the outer and inner tooth series.

The premaxillary dentition in Brycon and Chilobrycon is characterized by an outer series with many cuspidate teeth and an inner series of two large teeth. One or two additional series of teeth are situated between the outer and inner series anteriorly and become aligned posteriorly with the inner series (e.g. see illustrations in Howes, 1982). In Triportheus there is variation in the arrangement of teeth on the premaxilla with two or three series present, in a pattern more similar to that of Brycon .

The arrangement of premaxillary teeth in Chalceus is thus a unique pattern among characiforms and is hypothesized to be synapomorphic.

Internal series of dentary teeth

All Chalceus species possess an internal series of dentary teeth formed by a large symphyseal conical tooth (sometimes tricuspid in large specimens) followed by a gap and a series of smaller conical teeth situated along the dorsal margin of the replacement tooth trenches. The size of the gap varies among Chalceus species and is almost absent in C. macrolepidotus in which the series of small conical teeth is almost continuous with the symphyseal tooth.

Internal series of dentary teeth similar to those of Chalceus occur in Brycon and Chilobrycon deuterodon ( Zanata, 2000) . According to this study, the phylogenetic relationships of Chalceus apparently do not lie with either of those genera and the arrangement of the internal dentary teeth of Chalceus is thus hypothesized to be a synapomorphy for the species of the genus, with the condition in Brycon and Chilobrycon representing convergence.

Anterolateral process of the mesethmoid ( Fig. 3 )

In Chalceus spp. the anterolateral process of the mesethmoid (lateral ethmoid wing sensu Weitzman, 1962: 19) is greatly reduced, thereby providing a small region for the support of the posterior region of the premaxilla. In the majority of characiforms, including genera proposed as closely related to Chalceus ( Zanata, 2000) , such a mesethmoid process is well developed, serving as an anchoring site for the premaxilla. In evaluating the form of the lateral wing of the mesethmoid in other outgroup characiforms we confronted some problems with the coding of this character. In Hemiodus , Hepsetus and Xenocharax , the overall forms of the mesethmoid differ significantly from those present in Chalceus , thereby rendering an evaluation of the homology of the components of the mesethmoid between these various taxa problematic. In the present study the condition of this character in Xenocharax was treated as absent and in Hemiodus and Hepsetus as unknown. A reduced anterolateral process of the mesethmoid in Chalceus is herein hypothesized as autapomorphic for the genus.

Size of body scales

In all Chalceus spp. the scales situated dorsal to the lateral line are about twice the size of those situated ventral to it. In the majority of characiforms, including genera proposed as closely related to Chalceus ( Zanata, 2000) , the scales either decrease gradually in size dorsally to ventrally or those below the lateral line are only slightly smaller than those above it. A condition of the scales similar to that of Chalceus occurs only in the African characid Arnoldichthys spilopterus ( Paugy, 1990: 232) among Characiforms. As discussed above, determining whether this feature is either synapomorphic for Chalceus with an independent acquisition in Arnoldichthys or homologous between the two genera necessitates resolution of the relationships of the African characids, which has yet to be determined.

Size of lateral-line scales

The scales along the lateral-line series of all Chalceus species are alternatively large or small in the region from the posterior margin of the opercle to the anterior portion of the caudal peduncle, and then are approximately equal in size. In the majority of characiforms, including genera proposed as closely related to Chalceus ( Zanata, 2000) , the scales along the lateral-line series are all of similar size or gradually decrease in size posteriorly. Only Arnoldichthys spilopterus has an arrangement of lateral line scales similar to that of Chalceus . Determining whether this feature is either synapomorphic for the species of Chalceus with an independent acquisition in Arnoldichthys , or indicates phylogenetic affinity between the two genera, depends, as already mentioned, on the resolution of the phylogenetic relationships of the African characids and is beyond the scope of this study.

Figures

Figure 1. Maxilla and supramaxilla (arrowed) of Chalceus erythrurus, MZUSP 20385, 104.6 mm SL, lateral view, anterior to left.

Figure 2. Premaxilla of Chalceus erythrurus, MZUSP 20385, 104.6 mm SL, right side, medial view, anterior to left.

Figure 3. Mesethmoid of Chalceus spilogyros sp. nov., paratype, MZUSP 76069, 122.7 mm SL, dorsal view, anterior at top.