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“El Aerolito” is the most biodiverse cenote cave globally, home to Poecilia velifera and Gambusia yucatana . However, their trophic niches and interactions remain undescribed. A total of 53 P. velifera and 38 G. yucatana were analyzed for stable isotopes, along with water samples for chlorophyll, silicates, and physical–chemical parameters. Data were collected during the norths, rainy, and dry seasons in the year 2023. Isotopic niches were calculated, and a PCA analysis was used to assess environmental factors. P. velifera showed δ[sup.13] C values of −25.4‰, −25.7‰, and −24.5‰, whereas G. yucatana exhibited −24.8‰, −24.0‰, and −24.2‰ across seasons. Niche size varied, with the largest during the norths and smallest during the rainy season, indicating seasonality. PCA linked environmental parameters (e.g., temperature and dissolved oxygen) to resource availability, suggesting niche shifts due to rainfall. Mangroves and insects contributed to the isotopic signals, with shifts observed during the dry and rainy seasons. The highest water parameter values occurred during the dry and rainy seasons, while the lowest was during the norths. The system, nutrient-poor and lacking chlorophyll production, relies on external sources. The study concludes that isotopic niches are dynamic and seasonally influenced by abiotic factors, especially rainfall.

Livebearing fishes in the family Poeciliidae have been essential to testing life history theory. These species are remarkable because males internally inseminate females, and females give birth to free‐swimming young, making these fishes amenable to investigating the evolution of a variety of life history traits, including the timing and nature of maternal reproductive investment, timing of maturity, strategies for maternal provisioning of embryos, and several other classic life history traits. However, researchers vary in the methods that they use to measure these traits, making it difficult to compare findings across studies. Here, I present a standardized approach to studying life history traits in livebearing fishes. I describe methods for preserving samples in the field, for collecting data on a standard set of life history traits, and for processing data in ways that will allow comparisons among studies. I highlight different options in preservation techniques and in data collection that are dependent on the specific questions being addressed. Finally, I argue for a standard approach moving forward to make it possible to complete large‐scale comparative studies to reveal how life history traits have evolved in this important model system. Research on livebearing fishes has been instrumental in the development of several elements of life history theory through empirical studies. We posit that a standardized approach to life history data collection within this singular system is needed to address the current important questions in life history evolution through large comparative studies.

Sperm storage is a post‐copulatory strategy by which females can improve their fecundity by creating asynchrony between mating and fertilization. Sperm storage duration varies across vertebrate species, wherein longer sperm storage is thought to coincide with better reproductive success. Among the vertebrates, live‐bearing fishes of the family Poeciliidae are generally assumed to store sperm for extended periods of time, but the temporal dynamics of this process remain unknown for most species. To date, research suggests that superfetatious poeciliids—which give birth to more frequent, temporally overlapping broods—may be able to store sperm longer than non‐superfetatious species. However, robust empirical data for superfetatious poeciliids is very limited. Here, we assess the maximum duration of sperm storage and usage in the superfetatious poeciliid Poeciliopsis gracilis by comparing offspring production over time for both sexually isolated (single) and paired fish. We found that (a) the majority of P. gracilis females can store sperm for a maximum of 5 months with a smaller fraction of individuals able to extend this period to nearly 7 months, likely by “skipping broods,” and (b) the number of offspring produced decreases over time post‐isolation. With this study, we expand our knowledge of post‐copulatory strategies by providing an assessment of both sperm storage longevity and its impact on offspring production over time in a superfetatious, live‐bearing fish from the family Poeciliidae. We aim to encourage further research to generate and publish data on sperm storage longevity across the family Poeciliidae to elucidate how sperm storage varies across species with different reproductive strategies. Among the vertebrates, live‐bearing fishes of the family Poeciliidae are generally assumed to store sperm for extended periods of time, but the temporal dynamics of this process remain enigmatic for most species. With this study, we assess the maximum duration of sperm storage and usage in the superfetatious poeciliid Poeciliopsis gracilis by comparing offspring production over time for both sexually isolated (single) and paired fish. We found that (a) the majority of P. gracilis females can store sperm for a maximum of 5 months, with a smaller fraction of individuals able to extend this period to nearly 7 months, likely by “skipping broods” and (b) the number of offspring produced decreases over time post‐isolation.

Hybridization can promote phenotypic variation and often produces trait combinations distinct from the parental species. This increase in available variation can lead to the manifestation of functional novelty when new phenotypes bear adaptive value under the environmental conditions in which they occur. Although the role of hybridization as a driver of variation and novelty in traits linked to fitness is well recognized, it remains largely unknown whether hybridization can fuel behavioral novelty by promoting phenotypic variation in brain morphology and/or cognitive traits. To address this question, we investigated the effect of hybridization on brain anatomy, learning ability, and cognitive flexibility in first- and second-generation hybrids of two closely related fish species (Poecilia reticulata and Poecilia wingei). Overall, we found that F1 and F2 hybrids showed intermediate brain morphology and cognitive traits compared to parental groups. Moreover, as phenotypic dispersion and transgression were low for both brain and cognitive traits, we suggest that hybridization is not a strong driver of brain anatomical and cognitive diversification in these Poeciliidae. To determine the generality of this conclusion, hybridization experiments with cognitive tests need to be repeated in other families.

The fish subfamily Poeciliinae (sensu Parenti, 1981) is widely distributed across the Western Hemisphere and a dominant component of the fish communities of Central America. Poeciliids have figured prominently in previous studies on the roles of dispersal and vicariance in shaping current geographic distributions. Most recently, Hrbek et al. combined a DNA-based phylogeny of the family with geological models to provide a biogeographic perspective that emphasized the role of both vicariance and dispersal. Here we expand on that effort with a database enlarged in the quantity of sequence represented per species, in the number of species included, and in an enlarged and more balanced representation of the order Cyprinodontiformes. We combine a robust timetree based upon multiple fossil calibrations with enhanced biogeographic analyses that include ancestral area reconstructions to provide a detailed biogeographic history of this clade. Key features of our results are that the family originated in South America, but its major diversification dates to a later colonization of Central America. We also resolve additional colonizations among South, Central and North America and the Caribbean and consider how this reconstruction contributes to our understanding of the mechanisms of dispersal.

Natural environments vary, and organisms cope with this variation in two general ways: local adaptation and phenotypic plasticity, although these strategies often overlap and interact. This study explored how local adaptation, phenotypic plasticity, and their interactions shaped phenotypic variation in populations of Poecilia mexicana, an extremophile fish living in adjacent but ecologically divergent habitats. By comparing populations from the wild with fish raised in a common-garden environment, we evaluated how genetic differentiation between populations and plasticity contributed to the phenotypic variation observed in nature. We quantified variation in the size of six organs (brain, eyes, gills, heart, liver, and gastrointestinal tract), routine metabolic rate, and body shape. We found evidence for genetic differences between populations impacting the expression of the majority of traits, in addition to or in interaction with phenotypic plasticity and other predictor variables. Overall, our results suggest that trait divergence between populations was at least in part driven by evolutionary change and not just merely by plasticity induced by environmental differences between habitats. Future studies will have to rigorously test whether evolutionary divergence was caused by natural selection and what traits represent adaptations to the different ecological conditions.

Superfetation, the ability to simultaneously carry multiple litters of different developmental stages in utero , is a reproductive strategy that evolved repeatedly in viviparous animal lineages. The evolution of superfetation is hypothesized to reduce the reproductive burden and, consequently, improve the locomotor performance of the female during pregnancy. Here, we apply new computer-vision-based techniques to study changes in body shape and three-dimensional fast-start escape performance during pregnancy in three live-bearing fishes (family Poeciliidae) that exhibit different levels of superfetation. We found that superfetation correlates with a reduced abdominal distension and a more slender female body shape just before parturition. We further found that body slenderness positively correlates with maximal speeds, curvature amplitude and curvature rate, implying that superfetation improves the fast-start escape performance. Collectively, our study suggests that superfetation may have evolved in performance-demanding (e.g. high flow or high predation) environments to reduce the locomotor cost of pregnancy.

Variation in animal behavior can have important consequences for ecological processes like mate choice, resource competition and invasive potential. Characterizing behavioral variation across major environmental gradients can therefore provide insight into key ecological dynamics that govern the distribution and evolution of biodiversity. This study examined variation in a suite of social and non-social behaviors across 17 populations of eastern mosquitofish (Gambusia holbrooki) spanning a broad latitudinal gradient. Multiple axes of behavior varied significantly across the latitudinal cline, though not always in the predicted direction. Three primary axes of behavioral variation emerged putatively representing activity (movement), boldness (risk-taking), and sociability, respectively. There was no latitudinal variation for the activity axis, but fish from low latitudes were bolder and more social. Sex and/or body size affected behavior along all axes. These patterns demonstrate that broad scale geographic variation in the environment influences divergence in behavior, which could impact ecological and evolutionary processes. Although all of the results are consistent to some degree with small scale studies in other systems, I discuss how the patterns observed in this study highlight the importance of correctly interpreting the ecological context of ex situ laboratory studies.

Parasites can negatively affect the reproductive success of hosts. Placental species may be particularly susceptible, because parasite-induced stress during pregnancy could potentially influence embryo development. Here, we examine the consequences of a trematode infestation (black spot disease, BSD) for fetal development and adult behavior in 19 natural populations of the placental live-bearing fish species Poeciliopsis retropinna (Poeciliidae) in Costa Rica. First, we observed substantial variation in parasite infestation among populations which correlated with a number of local environmental conditions (elevation, river width, depth, and flow velocity). Furthermore, we observed substantial variation in parasite infestation among females within populations associated with maternal age and size. We found that the infestation rate significantly influenced embryonic development, with more heavily parasitized females producing smaller and worse-conditioned offspring at birth, possibly, because a costly immune response during pregnancy limits, either directly or indirectly, nourishment to developing embryos. Finally, a behavioral experiment in the field showed that the infestation rate did not affect an individual’s boldness. Our study indicates that in placental live-bearing fish parasite infestation leads to reduced embryo provisioning during pregnancy, resulting in a smaller offspring size and quality at birth potentially with negative implications for offspring fitness.