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

• Published in: Vertebrate zoology Vol. 71; no. 6; pp. 645 - 667
• Main Authors: Mattox, George M. T., Conway, Kevin W.
• Format: Journal Article
• Language: English
• Published: Pensoft Publishers 18-10-2021; Pensoft
• Subjects: Discovery and exploration; Fresh water; Brazil; Argentina; Philippines
• ISSN: 1864-5755; 2625-8498
• DOI: 10.3897/vz.71.e71886

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 of
Tucanoichthys 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 of
Tucanoichthys
is 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 of
Tucanoichthys
include those that develop relatively late in the ossification sequence of the non-miniature characiform
Salminus brasiliensis
, suggesting that their absence in
Tucanoichthys
can be explained by a simple scenario of developmental truncation. A number of the reductions in the skeleton of
Tucanoichthys
are shared with other miniature characiforms, most notably species of
Priocharax
and
Tyttobrycon
, the latter a putative close relative of
Tucanoichthys
based on molecular data.