Desenvolvimento inicial do Pacu-branco Myloplus Rubripinnis (Characiformes: serrasalmidae) da bacia do Rio Xingu

Abstract

The species Myloplus rubripinnis, popularly known as the white pacu, has great ecological potential as a seed disperser and represents an important food and economic resource for several riverine families. However, little is known about the bioecology of adult specimens, and there is no research on the early development of this species. In this context, the present study aimed to morphologically characterize the initial phases of the life cycle of M. rubripinnis, captured in the middle stretch of the Xingu River, and identify the main changes in growth patterns through different regression models. The individuals were collected using a plankton net in various habitats present in the Xingu River, during the four phases of the local hydrological cycle (flood, flood, ebb, and drought) between January 2021 and April 2022. After identification, the specimens were classified according to the developmental stage into the larval period (yolk-sac, preflexion, flexion, and postflexion) and the juvenile period. A total of 55 individuals with a standard length ranging from 7.21 to 35.53 mm were analyzed. During the larval period, the eyes are large and spherical, the head size varies from small to large, and the fusiform body ranges from long to moderate, with a convex dorsal profile. The intestine reaches the middle region of the body, and the mouth is terminal. Initially, the larvae are altricial, with pigmentation scarce in the body, limited to a linear band along the lateral line and intensifying towards the back of the caudal peduncle. In the early stages, small clusters of punctate chromatophores appear in the occipital region, on the side of the snout, in the first rays of the dorsal and anal fins, at the base of the anus, and in the rays of the caudal fin. In more developed stages, irregular vertical bands form throughout the body. The total number of myomers ranged from 41 to 42 segments (21 to 22 preanal and 20 postanal). The complete sequence of fin formation and the number of unbranched and branched rays are as follows: caudal (superior iiii+9-7+iiii inferior), dorsal (iii,20), anal (iii,32), ventral (i,5), and pectoral (i,10). The growth analysis indicated greater changes during the transition from the flexion to the post-flexion stages, marking the end of larval development. It is noteworthy that the pattern of pigmentation, along with meristic data, serves as effective distinguishing characteristics for identifying the species among its congeners. This emphasizes the importance of morphological, meristic, and morphometric aspects for the accurate identification of the species in its naturals environment. Additionally, this information can aid in understanding the locations and spawning periods, as well as contribute to the management, conservation, and sustainability efforts of this Neotropical fish.