Priocharax (Weitzman & Vari, 1987)

Mattox, George M. T. & Toledo-Piza, Mônica, 2012, Phylogenetic study of the Characinae (Teleostei: Characiformes: Characidae), Zoological Journal of the Linnean Society 165 (4), pp. 809-915 : 894-895

RELATIONSHIPS OF PRIOCHARAX , REDUCTIVE CHARACTERS, AND MINIATURIZATION

The inclusion of Priocharax as a terminal taxon in the analysis resulted in six equally most-parsimonious cladograms (L = 696, CI = 0.29, RI = 0.75). The only difference between the topologies of the strict consensus cladograms of the analyses excluding and including Priocharax is the clade containing Heterocharax , Hoplocharax , and Lonchogenys which resulted in a polytomy with Priocharax in the latter analysis ( Fig. 41 , shaded area). This polytomy results from two different hypothesis regarding the position of Priocharax among the other genera: one includes Priocharax as sister-group to a clade formed by Lonchogenys , Heterocharax , and Hoplocharax and the other includes Priocharax among species of Heterocharax . These two hypotheses vary in the optimization of a few ambiguous characters.

In their description of Priocharax, Weitzman & Vari (1987) proposed that the relationships of the genus were with members of the Characinae (sensu Géry, 1977) , more specifically with Acestrocephalus , Charax , Cynopotamus , Galeocharax , Gnathocharax , Heterocharax , Lonchogenys , and Roeboides . Lucena (1998) was the only one to have included Priocharax in a phylogenetic study and hypothesized that it was the sister-group of a clade formed by all those genera (= the Characinae of Lucena & Menezes, 2003). The split of the Characinae (sensu Lucena & Menezes, 2003) into two monophyletic groups not closely related to each other, as proposed by Mirande (2009, 2010) and this study, raised the question of the relationships of Priocharax . Mirande (2009, 2010) did not include Priocharax in his analysis and provisionally kept the genus in the Characinae . Priocharax is herein considered a member of the Heterocharacinae (new definition), more specifically within the Heterocharacini (new definition), based on five synapomorphies related to these taxa. Within the latter clade it is more closely related to Heterocharax , Hoplocharax , and Lonchogenys than to Gnathocharax .

Priocharax possesses a pseudotympanum (character 9) that is restricted to the region anterior to the fifth pleural rib (character 10). It also has the os suspensorium extending to a vertical through the second centrum and aligned in an approximately vertical plane (character 115), a feature originally described and illustrated by Weitzman & Vari (1987), and hypurals 2 and 3 in contact along the entire length of their margins (character 149). All these features are interpreted herein as synapomorphies of the Heterocharacinae (new definition). In addition, the pseudotympanum of Priocharax is dorsally delimited by bundles of the epaxial musculature (character 11), a feature hypothesized as synapomorphic for the clade that includes Heterocharax , Hoplocharax , and Lonchogenys (clade 11).

Weitzman & Vari (1987) mentioned the presence in Priocharax of many anatomical features associated with reductive characters ( Myers, 1958; Weitzman & Vari, 1988) (i.e. few scales on the body, reduced number of pelvic-fin rays, loss of laterosensory canal system on the head and body, and loss of bones of the infraorbital series). These characteristics are usually accompanied by a relative decrease in body size compared with related taxa ( Myers, 1958; Weitzman & Fink, 1983), the extreme of which leads to miniaturization. These processes are often associated with events of ontogenetic truncation ( Britz & Conway, 2009) and are of major evolutionary importance for generating morphological diversity ( Myers, 1958; Weitzman & Fink, 1983, 1985; Weitzman & Vari, 1987; Zanata & Vari, 2005).

Thirteen characters of apparent reductive nature were included herein, and are informative at various levels within the Characinae and the Heterocharacinae , especially within the Heterocharacini . One of the synapomorphies of the tribe Characini ( Charax and Roeboides ), for instance, is the reduction or loss of infraorbital 4 as an autogenous element (character 60, independently acquired in the Heterocharacini under the ACCTRAN optimization). Other characteristics represent autapomorphies or putative synapomorphies for some genera, such as the absence of pre-dorsal scales in Acanthocharax (character 2, independently acquired in Hoplocharax ), loss of the anterior branch of the laterosensory canal on infraorbital 6 in species of Charax (character 62), loss of the posteroventral spine on the pterotic (character 45, independently acquired in Gnathocharax , Heterocharax , Hoplocharax , and Lonchogenys ) and participation of the symplectic in the posteroventral margin of the metapterygoid–quadrate fenestra due to the reduction of the latter two bones (character 89, independently acquired in Heterocharax , Hoplocharax , Lonchogenys , and Gilbertolus ), the latter two observed in Phenacogaster . In the context of the Characinae , it is also worth mentioning the retention of the larval pectoral fin in juveniles of some of the genera (Lucena, 1998; Lucena & Menezes, 2003), a feature that although not examined in the present study could constitute an additional synapomorphy of the subfamily possibily related to ontogenetic truncations.

In addition to the reductive characters at suprageneric levels, among the Heterocharacini it is also worth highlightening reductive features interpreted herein as autapomorphies of some of the genera included in this tribe. Hoplocharax possesses two reductive autapomorphies: absence of pre-dorsal scales (character 2, independently acquired in Acanthocharax ) and absence of supraorbital (character 65, independently acquired in Gnathocharax ). Gnathocharax has four reductive autapomorphies including the loss of infraorbital 6 (character 61), loss of supraorbital (character 65, independently acquired in Hoplocharax ), and loss of postcleithra 1 and 2 (characters 122 and 123, respectively). Gnathocharax and Hoplocharax also have an interrupted lateral line, a reductive feature not incorporated in this analysis.

In a summary of Neotropical miniature fishes, Weitzman & Vari (1988) mentioned that South America houses the highest number of freshwater miniature species of all tropical regions. They listed 85 species considered miniatures in a strict sense (i.e. species whose adults do not grow longer than 26 mm SL), emphasizing the importance of the family Characidae that includes 46 of these species. Currently, more than 100 species of Neotropical miniature fishes are known, of which nearly one-third belong within the family Characidae (e.g. Costa & LeBail, 1999), including the two species of Priocharax ( P. ariel and P. pygmaeus , maximum length 15 and 17 mm SL, respectively). Weitzman & Vari (1988) recognized that this cut-off point for miniatures was arbitrary and they used it as a preliminary guide for future studies on miniature fishes rather than a definitive enumeration of such species. Members of the tribe Heterocharacini , for example, although growing to longer than 26 mm SL ( Gnathocharax = 50 mm SL, Heterocharax = 47 mm SL, Hoplocharax = 30 mm SL, Lonchogenys = 60 mm SL, Lucena & Menezes, 2003) are characterized by putatively reductive features. In this context, the inclusion of Priocharax in the Heterocharacini represents an extreme of ontogenetic truncation in a group with tendencies towards miniaturization, similarly to what was discussed by Zanata & Vari (2005) for the miniature alestid Lepidarchus and de Pinna (1996) in the case of miniature catfishes of the families Erethistidae (Asian) and Aspredinidae (South American).

In their study of the osteology and phylogenetic relationships of Paedocypris , an Asian cyprinid known to be an extreme case of ontogenetic truncation, Britz & Conway (2009) suggested two ways to investigate the phylogenetic relationships of miniature species. The first is searching for characters in structures that did not suffer ontogenetic truncation in the miniature taxon and that are informative regarding its relationships to non-truncated taxa. This line was followed herein and revealed five characters supporting a hypothesis of a close relationship of Priocharax to the Heterocharacinae , particularly to a subgroup of the Heterocharacini : (1) presence of pseudotympanum; (2) pseudotympanum restricted to the region anterior to the rib of the fifth vertebra; (3) pseudotympanum dorsally delimited by bundles of the epaxial musculature; (4) os suspensorium anteriorly elongate and aligned in an approximately vertical plane; and (5) hypurals 2 and 3 juxtaposed along their entire length. The second way suggested by Britz & Conway (2009) to resolve the phylogenetic relationships of taxa with ontogenetic truncation, adopted in their elegant study of Paedocypris , is the comparison of adults of the truncated taxon with early ontogenetic stages of related non-truncated taxa, an approach that may aid in further elucidating the relationships between Priocharax and the remaining Heterocharacini .

Figures

Figure 41. Strict consensus cladogram of the three equally most-parsimonious trees (L = 696, IC = 0.29, IR = 0.75) obtained in the analysis without Priocharax. Shaded area depicts region of strict consensus cladogram obtained in the analysis including Priocharax (six trees, L = 696, IC = 0.29, IR = 0.75) that differs from the former analysis. Number of clades shown above branches. Values of Bremer support shown below branches. Names in larger font represent taxa included in the Characinae by Lucena & Menezes (2003) or Mirande (2009, 2010).