Osteology of Priocharax and remarkable developmental truncation in a miniature Amazonian fish (Teleostei: Characiformes: Characidae)

Abstract

Establishing phylogenetic relationships of miniature fishes is challenging in taxa with developmental truncation. Within the Characiformes, developmental truncation appears to be relatively rare, with the Neotropical genus Priocharax being an example. Priocharax includes three miniature species among the smallest of the order and has been hypothesized to belong to the Heterocharacinae. The pronounced reduction in its skeleton, however, prevented a clearer evaluation of its relationships. The present detailed osteological study was designed to address this question and revealed that 21 bones are absent and nine other skeletal structures are simplified in Priocharax when compared to other characids. Comparison of the skeleton of adult Priocharax with early developmental stages of other characids demonstrated that most of the absences and simplifications can be interpreted as developmental truncations. The most striking developmental truncations are in the pectoral girdle, in which the endoskeleton remains entirely cartilaginous. Other interesting truncations are in the ethmoid region of the skull, infraorbital series, and Weberian apparatus, in which the claustrum is absent. Our study also revealed some unusual sexual dimorphisms in the pelvic girdle. Two cladistic analyses were performed to assess the relationships of Priocharax within the Heterocharacinae. The first consisted of a traditional analysis in which all absences and reductions of Priocharax were coded in the same way as in the remaining taxa. This resulted in three equally most parsimonious topologies, all of which have Priocharax as the most basal taxon of the Heterocharacinae. The second analysis incorporated ontogenetic information, and most absences and reductions of Priocharax were reinterpreted as apomorphic conditions and thus, coded differently from similar conditions in outgroups. This resulted in a single phylogenetic hypothesis with Priocharax and Gnathocharax as sister groups based on seven synapomorphies. Our approach demonstrates the importance of developmental studies to better understand morphological evolution of miniaturized, truncated taxa, and to generate hypotheses of their relationships.

Danionella dracula, an escape from the cypriniform Bauplan via developmental truncation?

“…Unfortunately, no sequence of ossification for any gobioid is available to test whether these absences in Schindleria are of bones that ossify during the late developmental phase of a nontruncated goby, in the way this was done for the cypriniforms D. dracula (Britz et al, ) and Paedocypris sp. (Britz and Conway, 2009), or the moderately progenetic characiform Priocharax (Mattox et al, ). Until a detailed study for Sundasalanx is available to determine the degree of developmental truncation that has modified its skeleton, the two cypriniforms D. dracula and Paedocypris sp.…”

A new miniature of Xenurobryconini (Characiformes: Characidae) from the rio Tapajós basin, Brazil

“…Similarly to its congeners, X. varii is a miniature species (sensu Weitzman & Vari, 1988), with the largest known specimen reaching 17.3 mm SL. In addition to their diminutive size, the lack of infraorbital bones 4 and 5 in X. varii might also be associated with miniaturization events, as suggested by Weitzman & Vari (1988) and further discussed by ToledoPiza et al (2014) and Mattox et al (2016). Comparatively, the infraorbital series is almost complete in X. heterodon, X. macropus, X. pteropus and X. polyancistrus, which present infraorbitals 1 to 5 (Moreira, 2005).…”

Morphological novelty and modest developmental truncation in Barboides, Africa's smallest vertebrates (Teleostei: Cyprinidae)

“…As in the case of the danionine cyprinids Paedocypris (Britz & Conway, ) and Danionella (Britz & Conway, ) and the characiform Priocharax (Mattox, Britz, & Toledo‐Piza, ), we were interested to see if the bones that fail to ossify in Barboides are bones that otherwise form in the later stages of skeletal development in closely related nonminiature cyprinine relatives. Few developmental studies exist for cyprinines and those which are available focus only on the cranium ( Barbus barbus , Vandewalle, Focant, Huriaux, & Chardon, ) or the viscerocranium ( Barbodes semifasciolatus , Block & Mabee, ) and therefore we compiled the ossification sequence of the entire skeleton of the African cyprinine ‘ Barbus ’ holotaenia as our reference sequence (Figure ).…”