Aim This study aimed to assist in guiding research with eggs and larvae of continental fish in Brazil, mainly in the knowledge of the early development, as well as to present the state of the art and to point out the gaps and future directions for the development of researches in the area. Methods The literature review was carried out in the electronic platforms and studies were examined to investigate whether any addressed morphological description of fish eggs and/or larvae. Results We found 121 studies, including scientific articles, theses, dissertations, chapters of books and books. This bibliographic production characterized 140 species, representing only 4.1% of the total species present in the Brazilian basins. Most of the species studied belong to the orders Characiformes and Siluriformes, which are of commercial interest and native to the Paraná, Amazon, Paraguay and Uruguay basins. For some very species-rich families in the Brazilian rivers, such as Characidae and Loricariidade, only a fraction of their species was characterized for the early development (2.6% and 0.7% of the total, respectively) and 56% of the families did not present any study addressing the early ontogeny. Regarding the content of the descriptions, for 73 species, there is characterization of the embryonic period and for 62, there is description of the whole larval period. For only 41 species there were complete descriptions (embryonic and larval) of the early development. All of these studies were conducted mainly with material derived from induced spawning and material obtained in the field (regressive series). Conclusions The characterization of eggs and larvae, still presents few studies, limited mainly to species from the rivers of the Plata and Amazon basins, to studies of fish of commercial importance and the incompleteness of most of these studies is still outstanding.

The Neotropics represent a hotspot for freshwater biodiversity with vast number of fish species of scarce ecological knowledge. This hold true for the Uruguay River, where fish assemblages and their diets remain unexplored. Fish assemblages were surveyed in 14 sites along its main course, from headwaters to mouth (approximately 1800 km), with the aim to identify the trophic roles of fish and to describe trophic structure of these assemblages. Following standardized samplings, diet was determined to perform a trophic classification of species. One hundred species (2309 gut contents) were analysed and classified into four trophic groups subdivided into eight lower-level groups: Piscivore, piscivore-invertivore, detritivore, omnivore-detritivore, omnivore-invertivore, omnivore-planktivore and omnivore-herbivore. The trophic structure of the assemblages varied along the river, with the relative species richness of fish consuming terrestrial invertebrates increasing towards the middle river section, probably driven by the large floodplains in that areas, supporting global theories such as flood pulse concept. This study describes the feeding habits of fish along the Uruguay River, being the first dietary description for 29 species. This knowledge is essential for management and conservation, serving as baseline in the context of future environmental changes and generates novel evidence about the functioning of ecosystems in this scarcely studied climatic region.

Diversity patterns and their causes remain important questions, especially for ecosystems that preserve natural conditions. This is the case of fish diversity in large Neotropical rivers. In this context, we investigated fish diversity patterns along an extensive unregulated river section (ca. 450 km) in the Middle Uruguay River Basin. Sampling was conducted seasonally between May 2017 and March 2018, at six sites (patches) with contrasting environmental conditions. We collected 3,008 individuals belonging to 90 species. Nine were migratory, which summed relevant biomass in local assemblages (20 to 60%). We observed spatial variation in assemblage structure, but weak seasonal effects. Overall, biomass was similar among sites, but richness tended to increase downstream. Migratory fishes showed opposed trends, with higher richness and biomass upstream, particularly catfishes (Siluriformes). Ordination analyses separated sites in three groups based on variations in composition and abundance, and revealed associations between these groups and specific hydro-geomorphic conditions (i.e., flow, depth, channel width and the presence of riparian vegetation). Our study revealed that fish diversity distributes heterogeneously along the main channel, where hydro-geomorphic patches select for distinct assemblages along riverine gradients.

Evidence indicates that migratory fish reproduce in the middle Uruguay River, but the location of spawning sites remains unknown. To identify spawning sites in the basin, fish eggs and larvae were sampled monthly between October 2016 and January 2017. The sampling was carried out in three sections along the middle Uruguay River, covering two environments: the main channel and the mouth of tributaries (Comandaí, Ijuí, and Piratinim rivers). A total of 11,519 eggs and 3,211 larvae were captured, belonged to ten migratory species. Eggs and larvae of migratory fishes, were widely distributed, with predominance of segmented eggs, and larvae in yolk-sac and pre-flexion stages, with higher densities near the confluence with the Piratinim River. Larvae assemblages showed spatial variation, indicating that spawning sites differ among migratory species. This study provides new information about fish reproduction in the middle Uruguay River, revealing that migratory species spawn in different localities along the main channel and tributaries. This region may function as a critical site for fish reproduction, although it is currently threatened by the risk of hydropower expansion, emphasizing the need for measures that preserve their environmental conditions, hydrological connectivity and ecological functions.

The spotted surubim Pseudoplatystoma corruscans is a large migratory catfish native to the La Plata and San Francisco basins. Nonetheless, its reproductive dynamics and behaviour remain poorly understood. This brief note provides the first record of the mating behaviour of P. corruscans in natural conditions. This behaviour was observed in the main channel of the Middle Uruguay River (Brazil). A school of approximately 25 fish, 5 females (with protruding abdomen and larger body size) and 20 males, were followed and videotaped. The largest female led the school, followed by other fish. Some males disputed repeatedly the posterior region of the females, whereas other males continuously repelled them. Eventually, some males, apparently exhausted, floated belly up. During this event, males were in a state of pronounced agitation, and some had abrasions in the head. Polygamy characterized the mating behaviour of P. corruscans.

1. Hydrological connectivity is a basic feature of river ecosystems, responsible for important ecological processes. Human activities, however, have progressively modified natural connectivity, threatening the maintenance of aquatic biodiversity.2. Neotropical potamodromous fish depend heavily on fluvial connectivity to complete their life cycles. However, important ecological aspects remain poorly understood, especially in river systems with limited floodplain development (i.e. channelized rivers and drylands), where the environment poses constraints to fish recruitment. The Uruguay River Basin is such a system, with several migratory fish, small floodplain areas, and limited information about recruitment dynamics and nursery grounds. 3. The lack of information on critical habitats limits conservation and management actions, especially in scenarios of increasing environmental degradation and river regulation. This study investigated recruitment dynamics of migratory fishes in the Middle Uruguay River, a long fluvial segment targeted for hydropower development. The study investigated spatial variations in the abundance, composition and size structure of juvenile fish among different sites along an extensive fluvial gradient (ca. 650 km), comparing the main channel with tributaries.4. Juveniles of migratory fish were widely distributed in the region, totalling nine species, with a predominance of Characiformes. Spatial distribution was variable among species and size classes, with a differential use of tributaries, wetlands and the main channel. Juvenile composition differed between sampling sites and environments, and was correlated with different limnological and spatial variables.Greater abundances occurred in the mouth of a tributary where a small floodplain develops during the wet season.5. The results emphasize the importance of fluvial connectivity for fish migration, dispersal and recruitment, providing valuable information about migratory fishes in river systems with limited wetland development. These findings have significance for the management and conservation of migratory fish under scenarios of hydropower expansion, as comprehensive studies on recruitment are rare in free-flowing rivers.

Length‐Weight relationship (LWRs) were estimated for four fish species occurring in the Ibicuí River drainage basin, southern Brazil. Samples were captured monthly during the spring and summer seasons (October 2018 to March 2019). The fish were caught with gillnets of different meshes (15, 20, 25, 30, 40 mm between adjacent nodes) and a trawl net (5 mm mesh between adjacent nodes) Eighteen excursions were realized in three sites, with a length of approximately 100 km, in third‐order flux, all distributed in sub‐basin Ibicuí River. The captured tools were installed at the end of the day (6 p.m.) and removed around 6 a.m. the next day. The present study provides the length‐weight relationship for four species (Astyanax dissensos, Galeocharax humeralis, Hypostomus roseopunctatus e Hypostomus uruguayensis).

The study of the origin of carbon sources for biomass and energy pathways in riverine food chains is essential in understanding of ecosystem functioning (Deng et al., 2018). Particularly, the transformation of organic matter into primary and secondary production is an essential process that promotes biodiversity patterns and associated ecosystem services (e.g. Humphries, Keckeis, & Finlayson, 2014). Within this context, the investigation of energy pathways that support the production of biomass of metazoan organisms has become one of the most important focuses of study in aquatic food web ecology in the recent decades. Models that describe the energy flow include the River Continuum Concept (RCC, Vannote, Minshall,