RESUMO
The early development of Tatia neivai (Ihering 1930) was analyzed (i) to characterize its morphological, morphometric and meristic development; and (ii) to examine its possible ontogenic variation as a result of the development of structures and organs necessary for survival. Samples were collected in the Baía River, basin Paraná River, Mato Grosso do Sul State, Brazil, between February 1999 and April 2000. After identification, the specimens were separated according to notochord development, and the morphometric and meristic characters were measured and counted. Forty-seven larvae and one juvenile with standard length ranging from 6.10 to 23.10 mm were analyzed. Larvae exhibit a cylindrical and highly pigmented body, small eyes, short intestine, terminal mouth, one pair of maxillary barbels and two pairs of mentonian barbels (relatively short). The juveniles presented a body shape similar to their adults. The total number of myomeres (33-35, 12 to 14 pre-anal and 20 to 23 postanal). Number of fin rays: dorsal I+4, anal I+9; pectoral I+5 and ventral six. During ontogeny, morphological changes observed in T. neivai have important ecological consequences, especially between flexion and postflexion stages (metamorphosis) when the larvae develop all necessary structures for its survival, such as locomotion and feeding.
