| Resumo: |
The cynodontid fish Rhaphiodon vulpinus Agassiz, 1829, known as "peixe-cachorro", is the only species of the family oceurring in the Pantanal of Mato Grosso. The initial development of its larvae was characterized, based on material collected with a 500 µm mesh size plankton-net, in several biotopes of the Pantanal of Barão de Melgaço, State of Mato Grosso, Central Brazil. The analysis was carried out with 72 larvae with standard length (SL) between 4.7 and 13.8 mm. The morphometric measurements varied positively with standard length, and body proportions ranged 8.1-17.8% for head length (HL), 6.2-10.1% for body height, and 63.673.5%, 16.8-21.6%, and 61.8-78.2% SL, respectively for pre-dorsal, pre-pectoral and pre-anal distances. The eye diameter varied positively with HL and its proportion varied between 9.1 -22.5% HL. The body relations did not show great variation along the larval development. The morphologicai characterization of larvae from 4.7 to 5.8 mm SL, in the pre-flexion phase, showed lhe presence of anembryonic finfold covering the unraycd fins, exposed branchial arches, absence of exogenous digestive content, unpigmented eyes and round shaped head. Larvae with SL between 8.1 and 13.8 mm SL were in the flexion phase, presenting a pointed head, mouth filled with externai and internai rows of numerous teeth, presence of exogenous food, pigmented eyes, branchial filaments still exposed, evident swimming-bladder, and formation of the first caudal, dorsal and anal fins' rays. Only individuais in the pre-flexion and flexion phases were analyzed, and no completely developed fin was evidenced. A maximum number of 69 myomeres was counted and no evident pigmentation over the body observed. Comparing R. vulpinus larvae to those of other Characiformes, it was observed that this species and Apareiodon affinis Steindachner, 1879 present smaller body proportions, and variation range of body height, while R. vulpinus is characterized by a shorter eye diameter. The myomere number is a relevam feature for the idenlification of R. vulpinus larvae, since it is the Characiform that possesses the highest myomere number, among those species of this family studied so far.
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