Morphology, osteology and reductions in the ontogeny of the scaleless characid Gymnocharacinus bergi


Author's address: Víctor Cussac, Consejo Nacional de Investigaciones, Científicas y Técnicas, Centro Regional Universitario Bariloche, Universidad Nacional del Comahue, Quintral 1250, Bariloche, Río Negro, Argentina.


The aim of this study was to analyse cartilage formation and ossification during the early life history of the relic fish Gymnocharacinus bergi. Additionally, changes associated with feeding and swimming were taken into account while considering the origin of the morphological reductions described for adult G. bergi that sets the species apart from the rest of the Characidae. During the ontogeny of G. bergi, pre-metamorphic larvae have a subventral mouth, allowing benthivorous feeding. During metamorphosis, the mouth shifts to a terminal position, teeth change to multicuspidate, and the short, simple gut develops a loop and pyloric caecae. Juveniles shift their diet to feed on periphytic algae. Scale disappearance in juveniles and adults involves the cessation of scale growth and a rapid re-absorption process. Several situations differ from the other species of this family: (a) the rhinosfenoides and the uroneural 2 never appear, even not as cartilaginous pieces, (b) the fourth basibranchial never undergoes ossification, (c) pseudourostyle, uroneural 1 and extrascapulae are reduced in adults, (d) infraorbitals are also reduced in size and number in adults, (e) scales form during metamorphosis but disappear during the juvenile and adult period. These differences are considered to be a consequence of low interspecific competition, low predation and small population size, allowing regression phenomena.