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Leptopanchax opalescens (Myers, 1942) is a small seasonal killifish endemic to coastal drainages in the state of Rio de Janeiro, Brazil. Leptopanchax opalescens was recaptured in temporary open vegetation pools (22°42.35’S, 43°41.59’W) in the hydrographic basin of the Guandu River, state of Rio de Janeiro, Brazil, after eight years without records. Information on habitat and population parameters (Capture per unit area (CPUA), size, sex ratio and length-weight relationship (LWR)) are given to provide subsidies for the conservation of this species. Specimens were captured during the rainy season (January-April 2020) in shallow pools (mean depth = 21 ± 12 s.d. cm), with relatively acidic pH (5.2 ± 0.8) and low concentrations of dissolved oxygen (3.5 ± 1 mg/L). The estimated CPUA of the population was 0.89 fish/m2. The total length (TL) of males ranged from 21.1 to 28.8 cm, and females ranged from 17.0 to 31.2 mm. Males were significantly larger (t-test = 2.04, p < 0.05) than females, which were in greater proportion (2.83Female: 1Male, p < 0.001). A positive allometric coefficient (b = 3.19) was found through length-weight relationship curve, indicating that individuals of the species gain proportionally more increment in weight than in length. Leptopanchax opalescens is a critically endangered species, and the results of this study contribute to the knowledge of the distribution, population structure and conservation of the species and its natural habitats.

Increases in temperatures over the last century, more intensively after the eighties, were recorded in several databases for the south‐eastern Brazil. These increases are likely to change fish communities in aquatic systems by decreasing abundances, biomass and sizes of some species more sensitive to climate change. Reservoirs may be particularly susceptible to the effects of climate change, as they isolate previously connected areas limiting fish dispersal and migration, as well as increasing water temperature and thermal stratification. We assessed temporal changes in the abundance and biomass of the ichthyofauna in an isolate reservoir (inflowing waters from small tributaries in a highland region) aiming to associate changes with climate effects over three decades (1990, 2000, 2010). Two abundant native species, a loricariid Loricariichthys castaneus (Castelnau, 1855) and a cichlid Geophagus brasiliensis (Quoy & Gaimard, 1824), were selected to assess eventual response to climate changes in their geographical distribution in the Southeast Atlantic Hydrographic Region, considering different carbon emission scenarios using ecological niche models. A decrease in the densities and biomass of the ichthyofauna and of the two selected species were observed in both summer and winter over the three decades, coinciding with increases in temperatures. These decreases were correlated with increases in positive anomalies of temperature, which may be an indicator of climatic changes, although with low explanation of the data variation. The mean size of L. castaneus decreased between the nineties and 2010, whereas no significant trend was found for the size of G. brasiliensis. Our predictions of new area for future adequacy indicated a loss in the distribution area for both species (mainly for G. brasiliensis), considering the most pessimistic scenario for 2050 and 2070, possibly due to climatic changes. Both hydroelectric dams and global warming pose threats to freshwater fish diversity, and both will interact. Changes observed in the fish assemblage over the last decades were essential for an assessment of the consequences of global warming in this type of reservoir combining larger scale studies with future projections.

A new approach for visual fish survey in reservoirs using underwater drones (remotely operated vehicle- ROV) is presented. The ROV was applied to identify abiotic gradients and to compare fish assemblages on the steep slopes in a tropical reservoir. The tested hypothesis is that fish are concentrated in the littoral zone due to the better physicochemical and habitat conditions, compared to deep and hypoxic layers. Twelve species were recorded (seven native, five exotic), with all species occurring in the littoral zone, seven species in the transition, and four in the profundal zone. A greater fish abundance and richness was found in the littoral zone corroborating the main hypothesis. The littoral zone was dominated by exotic cichlids ( Cichla spp., Coptodon rendalli ), while native catfish ( Loricariichthys castaneus , Pimelodella lateristriga ) occupied deeper areas. The fish distribution seems to be driven by local factors, such as oxygen availability and habitat structure. The preference for the littoral zone by alien cichlids may have led to the extirpation/decrease of native characids and induced catfishes to occupy deep habitats. Underwater drones can be a valuable tool for the simultaneous collection of abiotic/biotic data, especially in deep reservoirs with complex habitats, resulting in advances in the environmental monitoring. Supplementary Information The online version contains supplementary material available at 10.1007/s10750-021-04790-9.

The reproductive strategy of the non-native predator cichlid Cichla kelberi was determined to explain its success after more than 60 years of being introduced into an isolated reservoir in southeastern Brazil. This was one of the first-known translocations of the genus Cichla out of its natural range. Macro-and microscopy characteristics of the gonadal development stages and the maturation phases, along with the reproductive features (size at first maturation size, gonado-somatic index and sex ratio), were described. It was hypothesized that the stable conditions of the reservoir, with low connectivity, weakly defined spatial gradient and slight seasonal changes in environmental variables, favour the equilibrium strategy that enables predators to have high offspring survivorship because of great parental investment in individual progeny. Sex ratio was well balanced, with males and females reaching first maturity between 30.0 and 28.6 cm total length (L T), respectively. The stages of oocyte (primary and secondary growth, vitellogenic and atresia) and spermatocyte (spermatogonia, spermatocytes, spermatids and spermatozoa) development were identified. Five phases of gonadal development (immature, developing, spawning capable, regressing and regenerating) were described for both sexes. A long reproductive season was found, with spawning peaks in August/September and, to a lesser extent, in April/May. Parental care and spawns in parcels (batch spawns) corroborated the raised equilibrium strategy that was effective in this isolated reservoir. This species developed reproductive mechanisms that fit to different environmental conditions, with multiple spawning being associated with lentic environments and asynchronous development of oocytes, which are released over long periods. The reproductive plasticity in reservoirs may be one of the main factors inherent to the successful of colonization and establishment of the peacock bass in the environments in which they were introduced.

Leptopanchax opalescens is a critically endangered small annual fish. Reproductive traits of this species were studied to improve our understanding of the strategies that facilitate the occupation of temporary wetlands. We compiled egg diameter and maximum total length (TLmax) data from 132 neotropical freshwater fish (83 genera, 43 families) to establish comparisons with this species. We used the egg diameter / TLmax ratio to test the hypothesis that annual fish in temporary wetlands have relatively larger eggs than non-annual species from perennial habitats (lakes, rivers). Fish were collected from Guandu River drainages (Brazil). DNA barcoding was employed to confirm the species identity. The phases of gonadal development and spawn type were described using histological techniques. Egg size and fecundity were determined with microscopic analysis. Females with batch spawning and males with continuous spawning were detected. The bath fecundity ranged from 22–32 vitellogenic oocytes (mean 27 ± 7 SD). Annual species presented greater relative egg size than perennial species (p < 0.001), which is an indication of greater reproductive investment at the expense of somatic growth in temporary wetlands. Larger eggs are advantageous for annual fish as their lifespan is limited, and they can allocate a greater amount of yolk reserves for long periods buried in the substrate under embryonic diapause. Large relative oocytes, spawning in batches, synchronous modal development of oocytes, continued production of sperm in males, and a complex process of embryonic diapause are reproductive traits that favor resilience of L. opalescens and other annual fish in temporary wetlands.

Leptopanchax opalescens is a critically endangered small annual sh. Reproductive traits of this species were studied to improve our understanding of the strategies that facilitate the occupation of temporary wetlands. We compiled egg diameter and maximum total length (TLmax) data from 132 neotropical freshwater sh (83 genera, 43 families) to establish comparisons with this species. We used the egg diameter / TLmax ratio to test the hypothesis that annual sh in temporary wetlands have relatively larger eggs than non-annual species from perennial habitats (lakes, rivers). Fish were collected from Guandu River drainages (Brazil). DNA barcoding was employed to con rm the species identity. The phases of gonadal development and spawn type were described using histological techniques. Egg size and fecundity were determined with microscopic analysis. Females with batch spawning and males with continuous spawning were detected. The bath fecundity ranged from 22-32 vitellogenic oocytes (mean 27 ± 7 SD). Annual species presented greater relative egg size than perennial species (p < 0.001), which is an indication of greater reproductive investment at the expense of somatic growth in temporary wetlands. Larger eggs are advantageous for annual sh as their lifespan is limited, and they can allocate a greater amount of yolk reserves for long periods buried in the substrate under embryonic diapause. Large relative oocytes, spawning in batches, synchronous modal development of oocytes, continued production of sperm in males, and a complex process of embryonic diapause are reproductive traits that favor resilience of L. opalescens and other annual sh in temporary wetlands. IntroductionRivulidae (Cyprinodontiformes) is the eighth most speciose sh family in the World, with 471 valid species (Fricke et al. 2022), and the rst in number of threatened species in the Neotropical Region, where they occur. The threat is such that in a megadiverse country like Brazil, a single family of freshwater sh (i.e., Rivulidae) has a higher number of threatened species (130 spp.) than all classes of marine sh combined (Actinopterygii + Chondrichthyes + Myxini = 97 spp.) (ICMBio 2018). Habitat loss is the main threat to rivulids. The swamps and temporary wetlands where many of these animals inhabit have been drastically destroyed, due to deforestation, drainage and land lling resulting from agricultural and urban expansion (Loureiro et al. 2018;Castro and Polaz 2020). Loss of temporary wetlands from land alterations has signi cant ecological and social consequences (Calhoun et al. 2017), but freshwater research, policy, and conservation have historically neglected these habitats, focusing instead on larger water bodies and agship species (