Osteology of Tucanoichthys tucano Géry and Römer, an enigmatic miniature fish from the Amazon basin, Brazil (Teleostei: Characiformes: Characidae)

Abstract

Miniaturization, the evolution of extremely small adult body size, is a common phenomenon across the lineages of freshwater fishes, especially in the Neotropics where over 200 species are considered miniature (≤26 mm in standard length [SL]). Close to 30% of all miniature Neotropical freshwater fishes belong to the family Characidae, several of which are of uncertain phylogenetic placement within the family. We investigate the skeletal anatomy ofTucanoichthys tucano, a species of uncertain phylogenetic position from the upper Rio Negro basin, reaching a maximum known size of 16.6 mm SL. The skeleton ofTucanoichthysis characterized by the complete absence of ten skeletal elements and marked reduction in size and/or complexity of others, especially those elements associated with the cephalic latero-sensory canal system. Missing elements in the skeleton ofTucanoichthysinclude those that develop relatively late in the ossification sequence of the non-miniature characiformSalminus brasiliensis, suggesting that their absence inTucanoichthyscan be explained by a simple scenario of developmental truncation. A number of the reductions in the skeleton ofTucanoichthysare shared with other miniature characiforms, most notably species ofPriocharaxandTyttobrycon, the latter a putative close relative ofTucanoichthysbased on molecular data.