The muscle phenotype of fish is regulated by numerous factors that, although widely explored, still need to be fully understood. In this context, several studies aimed to unravel how internal and external stimuli affect the muscle growth of these vertebrates. The pacu (Piaractus mesopotamicus) is a species of indeterminate muscular growth that quickly reaches high body weight. For this reason, it adds great importance to the productive sector, along with other round fish. In this context, we aimed to compile studies on fish biology and skeletal muscle growth, focusing on studies by our research group that used pacu as an experimental model along with other species. Based on these studies, new muscle phenotype regulators were identified and explored in vivo, in vitro, and in silico studies, which strongly contribute to advances in understanding muscle growth mechanisms with future applications in the productive sector.

This study aimed to describe the occurrence, morphology and prevalence of Spirocamallanus krameri, a parasite of Hoplerythrinus unitaeniatus collected in state of Pará, Brazil. The morphological and morphometric characteristics are consistent with the species S. krameri, by having a hexagonal mouth opening, 16 cephalic papillae and two lateral papillae on the amphids. The buccal capsule is internally different in both sexes, with (11-13) spiral ridges in males and (13-17) spiral ridges in females. The basal ring is well developed, and three teeth are present in males and absent in females. Ten sessile caudal papillae and a pair of phasmidial pores occur in males. The spicules are small, subequal, 81-101 µm in length and the tail is tapered. The vulva in females is postequatorial, the tail is conical with lateral phasmidial pores. Among the species of Spirocamallanus that have short spicules, the general morphology of S. krameri most resembles that of S. inopinatus, but differs from that species, which have two cephalic teeth visible (absent in S. krameri) and specimens of S. inopinatus do not have sexual dimorphism in the buccal capsule of present in S. krameri.

Characidium pumarinri n. sp. is described from the affluents of Río Huallaga, a tributary of Río Marañón that drains the eastern slopes of the Andes, in the Peruvian Amazon. The new species has a deep body (24-31% in LS ), 11-17 well-marked dark bars and remarkable sexual dimorphism related to colour pattern, in which males have darker anal, dorsal, pectoral and pelvic fins than females. C. pumarinri resembles some species allocated in Clade C4, mainly by having a high number of transversal bars and a deep body. In addition, a thorough review and further descriptions of the secondary sexual characteristics in Characidium are provided.

A taxonomic review of the Hyphessobrycon panamensis species-group is presented, with recognition of six valid species. In Central America, H. panamensis was restricted to the drainages of both Atlantic and Pacific coasts of Panama; H. savagei was recorded at the río Pírris and río Térraba, Pacific coast of Costa Rica, and Hyphessobrycon bussingi, new species, described from the río Sixaola basin, Atlantic coast of Costa Rica, and from the río Changuinola, río Guarumo and río San San, Atlantic coast of Panama. At trans-andean South America, H. columbianus was recorded at the río Acandí, Colombia, and its geographical distribution was extended into the coastal drainages in San Blas province, Panama; H. condotensis was recorded at the río Atrato, río Baudó, lower río Magdalena and río San Juan basins, Colombia, while H. sebastiani was considered as its junior synonym. Hyphessobrycon daguae was revalidated, erected to the specific level, considered as senior synonym of H. chocoensis, and recorded from the río Dagua, río Patía and río Telembí basins, in Colombia, the río Mira, at the border between Colombia and Ecuador, and the río Cayapas, río Mataje, and río Santiago, in Ecuador. Color pattern and secondary sexual characters (e.g. number, arrangement and shape of hooks in mature males) suggest that the Hyphessobrycon panamensis species-group is a putatively monophyletic lineage.

Rhinopetitia was known for a long time only by R. myersi, its type species, described from the Ilha do Bananal, Rio Araguaia basin, Tocantins, Brazil. Another recently described species, R. potamorhachia, was considered to differ from R. myersi by having the outer and inner rows of premaxillary teeth with seven to nine cusps, a midlateral dark stripe and a dark humeral blotch (versus outer and inner rows of premaxillary and of maxillary teeth with three to five cusps, and the absence of a dark midlateral stripe). Recently collected specimens from the Araguaia and Tocantins river basins are considered to belong to R. myersi, allowing to better characterize this species which is redescribed herein. Four new species are recognized for the genus, all from Brazil: 1) Rhinopetitia paucirastra, new species, collected from the upper Rio Tocantins drainage, state of Goiás, distinguished from all its congeners mainly by the presence of rudimentary and fewer gill rakers in external row on first gill arch; 2) R. oligolepis, new species originating from the Rio Jamanxim, Rio Tapajós drainage, state of Pará, differing from its congeners by having 4 longitudinal scale rows from dorsal-fin origin to lateral line; 3) R. melanohumeralis, new species, collected from small streams tributaries of the upper Rio Tapajós, and the upper Rio Xingu basins, states of Pará and Mato Grosso, that has the body as deep as in R. paucirastra but has well-developed and more gill rakers in the external row on first branchial arch; and 4) R. nigrofasciata, new species, collected in small tributaries of the upper portions of the Rio Tapajós and Rio Xingu basins, states of Pará and Mato Grosso, with the body narrower than in R. paucirastra and R. melanohumeralis.

A taxonomic review of Copella is presented based on the analysis of the type material of all nominal species and extensive material from South American drainages. Six out of ten nominal species are recognized as valid: Copella arnoldi, C. callolepis, C. compta, C. eigenmanni, C. nattereri, and C. vilmae. Copella carsevennensis is a junior synonym of C. arnoldi, C. nigrofasciata and \'Nannostomus\' stigmasemion are junior synonyms of C. callolepis, C. metae is junior synonym of C. eigenmanni, and C. meinkeni is junior synonym of C. nattereri. Species of Copella occur in the rio Amazonas and Orinoco basins, and coastal drainages of Guyana, French Guiana, Surinam, and Venezuela. An identification key is provided.