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During early development, fishes undergo significant changes that influence external morphology and the functioning of internal organs and systems. This often results in gradual variation of the morphological traits of individuals across developmental stages. The investigation of larval and juvenile fish development and growth patterns has pertinent implications for the systematic and ecological elucidation of species. Bryconops gracilis is a medium-sized fish, omnivorous that inhabits lotic and lentic environments with acidic and transparent waters in the Amazon basin. In this study, the early development of B. gracilis is described, until recently a practically unknown species. In terms of development, we used morphological, meristic, and morphometric data to characterize the larvae and juveniles. The individuals were collected in the Curuá-Una River, Amazon basin, Brazil. Fifty-four specimens were examined. Samples include individuals with 3.39–21.79 mm SL. Yolk-sac larvae have two attachment organs on the dorsal surface of head and body. The larvae of B. gracilis are considered altricial, with a fusiform body, and the intestine reaches the median region of the body. Initially, the mouth is subterminal and becomes isognathic from the postflexion stage on. During the postflexion stage, the most relevant morphological changes occur (e.g., presence of all fins, mouth position similar to adults, increased body pigmentation), making individuals more specialized to explore new habitats and diets and maximize their chances of survival. Furthermore, vertebrae and myomeres are compared and assist with differentiating some Bryconops species at early life stages that occur in sympatry in the Amazon basin. Our results contribute to knowledge about the external morphology of neotropical freshwater fishes, enabling the identification of larvae and juveniles through traditional taxonomy and broadening the perspective on the ontogenetic study of the adipose fin in Characoidei. • The description of Bryconops gracilis larvae and juveniles enhanced our understanding of freshwater fish early life stages. • Dispersal of B. gracilis larvae may be maximized by the presence of two adhesive organs. • Morphological and morphometric changes during early ontogeny of B. gracilis reflect species' ecological and functional needs. • A new perspective on studies of the origin of the adipose fin in Characoidei is provided. [ABSTRACT FROM AUTHOR]

Spear-like collagen complexes, known as actinotrichia, underlie the epidermal cell layer in the tip of teleost fins and are known to contribute toward fin formation; however, their specific role remains largely unclear. In this study, we investigated of actinotrichia in the role of caudal fin formation by generating collagen9a1c (col9a1c)-knockout zebrafish. Although actinotrichia were initially produced normally and aligned correctly in the knockout fish, the number of actinotrichia decreased as the fish grew and their alignment became disordered. Simultaneously, the fin tip gradually shortened in the dorsal-ventral direction and the entire fin became oval-shaped, while the fin-rays rarely bifurcated and instead underwent fusion, suggesting that actinotrichia are essential for spreading fins dorsoventrally. Furthermore, the epithelial cells that are usually thinly spread in normal fish became spherical in the knockout fish, reducing the area covered by each cell and thus the area of the fin tip. Together, these findings suggest that the tight alignment of actinotrichia provides physical support in the dorsal-ventral direction that allows caudal fins to expand in a triangular-shape. • The physical role of collagen fibers called actinotrichia in caudal fin shape formation was investigated. • Actinotrichia are arranged in parallel to form a two-dimensional scaffold. • When the scaffold is lost, an extreme contraction of epithelial cells occurs. • The contraction of the epithelial cells is thought to shorten the fins along dorsal-ventral axis. [ABSTRACT FROM AUTHOR]

The vertebrate embryo undergoes a series of dramatic morphological changes as the body extends to form the complete anterior-posterior axis during the somite-forming stages. The molecular mechanisms regulating these complex processes are still largely unknown. We show that the Hippo pathway transcriptional coactivators Yap1 and Wwtr1 are specifically localized to the presumptive epidermis and notochord, and play a critical and unexpected role in posterior body extension by regulating Fibronectin assembly underneath the presumptive epidermis and surrounding the notochord. We further find that Yap1 and Wwtr1, also via Fibronectin, have an essential role in the epidermal morphogenesis necessary to form the initial dorsal and ventral fins, a process previously thought to involve bending of an epithelial sheet, but which we now show involves concerted active cell movement. Our results reveal how the Hippo pathway transcriptional program, localized to two specific tissues, acts to control essential morphological events in the vertebrate embryo.

The adipose fin is a structure with questionable function, for a long time even regarded as a rudimentary element without any function. A comparative study of adipose fin development revealed two different modes: a salmoniform-type where the adipose fin develops directly from the larval fin fold simultaneously with the other median fins. And a characiform-type where the adipose fin develops anew after the reduction of the larval fin fold. Compared to the first mode the latter starts late in ontogeny, when all other median fins already have their shape, fin rays and colouration. The characiform-type of adipose fin development contradicts the view that this structure is just a larval fin fold remainder. It also disputes the hypothesis of the adipose fin as a structure without any function. Such a hypothesis cannot explain a systematically widely distributed structure which even exhibits different modes of ontogenetic development in different taxa. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]