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Sunfish (Lepomis spp.) are among the most common piscine inhabitants of freshwater lakes and ponds in North America. Lepomis spp. breed at the same time creating hybrid zones, where genetically distinct populations mate and produce mixed offspring that are sexually viable hybrids. One aspect of hybridization that may have important consequences is parasitism and its patterns of recruitment in the hybrid sunfish. This study investigated these patterns both at the level of the individual parasite species as well as in the parasite communities infecting the fish. Two sample sites possessing hybrid sunfish populations were investigated: 1 system had bluegill sunfish (Lepomis macrochirus), redear sunfish (Lepomis microlophus), and their hybrids, while the other system had bluegill sunfish (L. macrochirus), green sunfish (Lepomis cyanellus), and their hybrids. The hybrids were infected by mostly generalist parasites that commonly infect all Lepomis spp. Most of the individual parasite species followed a dominance pattern (59.1%) of infection, where parasite abundance in hybrids resembled at least one of the parental species, with the remainder exhibiting intermediate levels of parasitism, supporting an additive pattern of parasite recruitment (40.1%). At the community level, the patterns of parasite recruitment differed in L. macrochirus × L. microlophus hybrids, which showed a dominance pattern, and L. macrochirus × L. cyanellus hybrids, which showed an additive pattern of parasite recruitment. These differences in parasite recruitment between hybrid groups may be attributed to varying degrees of dietary and niche overlap between the parental species in the 2 study systems.

Duoculture has been reported to increase growth rates of some fishes when reared in combination, due to “shading” effects between the species. Two experiments, one involving outdoor cage-rearing in a reservoir, and the other, indoor tank-rearing, were conducted within each of three temperatures ranges (means of ~18.0°C, ~22.0°C and ~26.5°C), to determine whether duoculture of bluegill (BG) Lepomis macrochirus and yellow perch (YP) Perca flavescens would lead to improved growth relative to when the two species were reared separately. Juvenile bluegill and yellow perch were reared in triplicated groups each involving monoculture sets of 100% BG and 100% YP, and a duoculture set of 50% BG + 50% YP. Experiments in cages (Exp. 1) ran for 150 days while those in tanks ran for 126 days (Exp. 2). In Experiment 1, bluegill exhibited significantly greater ( P <0.05) mean weight ( P <0.05) in duoculture than in monoculture, under the high summer-like range of temperature (~26.5°C) over most of the experiment, whereas yellow perch showed no significant difference in mean weight in duoculture versus monoculture. By the end of a 150-d experiment, bluegill in duoculture outweighed those in monoculture by 62.5%. In Experiment 2, yellow perch in duoculture grew significantly larger than in monoculture ( P <0.05) under the warm thermal regime (mean of ~22°C), while no significant differences were detected in mean weight of bluegill in monoculture versus duoculture. Yellow perch in duoculture outweighed those in monoculture by 33.1% at the end of the experiment. Yellow perch performed better in duoculture than in monoculture under the low thermal regime (mean of ~18°C) in both experiments. A significantly greater reduction of CV wt was observed for both bluegill and yellow perch in duoculture than in monoculture in Experiment 1, while no differences in CV wt reduction were detected for bluegill in Experiment 2. Feed conversion ratios (FCR) of bluegill and yellow perch reared in duoculture were significantly lower than for both fishes reared in monoculture in Experiment 1, while there were no significant differences in FCR among the three groups throughout most of Experiment 2. Findings indicate that duoculture of yellow perch and bluegill holds good potential to improve growth and FCR, and to reduce size variation by diminishing social interaction costs.

The shy-bold behavioral continuum is an element of animal behavior which is often studied for its ecological relevance, particularly in the context of predation risk. How individuals respond to various predation cues is well studied at the individual level, but relatively little is known about how these responses can differ among closely related species. We exposed individual wild-caught juvenile bluegill (Lepomis macrochirus) and pumpkinseed (L. gibbosus) to kairomones of a common predator (Northern pike, Esox lucius), conspecific alarm cues, or a lake water control in a Z-maze trial commonly used to assess relative levels of bold and exploratory behaviors. Neither species exhibited any significant behavioral responses to either predation cue, but bluegill consistently displayed more bold-type behaviors than pumpkinseed. Although the lack of a behavioral response to predation cues in this study is equivocal, we identify clear differences in boldness between these two congeners and discuss the possibility of ecological niche differentiation driving these behavioral differences.Significance StatementClosely related species exposed to similar predator guilds may differ in their behavioral responses to predation risk, particularly during different life-history stages. We exposed juveniles of two co-occurring sunfish congeners (Lepomis spp.) to chemical predation cues (predator kairomones or conspecific alarm cues) in a Z-maze assay to assess relative levels of bold and exploratory behaviors. Bluegill (L. macrochirus) were consistently bolder and more proactive than pumpkinseed (L. gibbosus). Ontogenetic ecological niche differentiation between these species may be the cause of these observed differences and life-history trajectories should be considered in the design of future research questions.

Predation is a pervasive force that structures food webs and directly influences ecosystem functioning. The relative body sizes of predators and prey may be an important determinant of interaction strengths. However, studies quantifying the combined influence of intra‐ and interspecific variation in predator–prey body size ratios are lacking. We use a comparative functional response approach to examine interaction strengths between three size classes of invasive bluegill and largemouth bass toward three scaled size classes of their tilapia prey. We then quantify the influence of intra‐ and interspecific predator–prey body mass ratios on the scaling of attack rates and handling times. Type II functional responses were displayed by both predators across all predator and prey size classes. Largemouth bass consumed more than bluegill at small and intermediate predator size classes, while large predators of both species were more similar. Small prey were most vulnerable overall; however, differential attack rates among prey were emergent across predator sizes. For both bluegill and largemouth bass, small predators exhibited higher attack rates toward small and intermediate prey sizes, while larger predators exhibited greater attack rates toward large prey. Conversely, handling times increased with prey size, with small bluegill exhibiting particularly low feeding rates toward medium–large prey types. Attack rates for both predators peaked unimodally at intermediate predator–prey body mass ratios, while handling times generally shortened across increasing body mass ratios. We thus demonstrate effects of body size ratios on predator–prey interaction strengths between key fish species, with attack rates and handling times dependent on the relative sizes of predator–prey participants. Considerations for intra‐ and interspecific body size ratio effects are critical for predicting the strengths of interactions within ecosystems and may drive differential ecological impacts among invasive species as size ratios shift.

North American sunfishes (Family Centrarchidae) are among the most popular sportfish throughout the United States and Canada. Despite the popularity of sunfishes, their ecological importance, and their extensive stocking and aquacultural history, few molecular studies have examined the evolutionary relationships and species boundaries among members of this group, many of which are known to hybridize. Here, we describe a chromosome-scale genome assembly representing Bluegill (Lepomis macrochirus), one of the most widespread centrarchid species. By combining long-read, Oxford Nanopore sequencing data with short-insert, whole-genome and HiC sequence reads, we produced an assembly (Lm_LA_1.1) having a total length of 889 Mb including 1,841 scaffolds and having a scaffold N50 of 36 Mb, L50 of 12, N90 of 29 Mb, and L90 of 22. We detected 99% (eukaryota_odb10) and 98% (actinopterygii_odb10) universal single-copy orthologs (BUSCOs), and ab initio gene prediction performed using this new assembly identified a set of 17,233 genes that were supported by external (OrthoDB v10) data. This new assembly provides an important addition to the growing set of assemblies already available for spiny-rayed fishes (Acanthomorpha), and it will serve as a resource for future studies that focus on the complex evolutionary history of centrarchids.

Microplastics are ubiquitous in Earth’s ecosystems and many efforts have begun to understand their distributions. Large rivers, like the Mississippi River, provide a unique system in which to look at large-scale patterns of microplastic distribution. In this study, we sampled four species of widely-distributed fishes from five sites along the mainstem Mississippi River, from Minnesota to Louisiana, United States. Microplastics were found in all fish species and at all sites; however, microplastics increased in occurrence in the Lower Mississippi River. Fragments were the most common morphologies and polypropylene was the most common polymer detected. We also examined the hypothesis that microplastic loads in fishes increased downstream, but found support for this hypothesis only when examining Largemouth Bass; Flathead Catfish, Shortnose Gar, and Bluegill were all found to have similar microplastic loads along the mainstem Mississippi River. It is clear that microplastics are heterogeneously distributed throughout ecosystems, and further understanding of microplastic distributional patterns and varying species burdens are needed to fully understand threats that microplastics present.

An invasive species is defined as a biological species introduced, most often through human action, that causes direct harm, such as reducing or driving native species to extinction. This introduction can result in long-term changes to ecosystem structures and ultimately contribute to a decline in biodiversity within areas where it was not previously present. Thus, continuous monitoring and management are essential. This study examined the impact of two exotic species, largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus), on fish diversity in rivers across South Korea. Data from the “Stream Ecosystem Survey and Health Assessment” project, which was conducted along five major river basins between 2013 and 2021, were analyzed. The data were divided into three periods: 2013–2015, 2016–2018, and 2019–2021, covering a total of 960 sites. The results revealed that largemouth bass and bluegill were present at 360 sites (37.5%) from 2013 to 2015, 325 sites (33.9%) from 2016 to 2018, and 356 sites (37.1%) from 2019 to 2021. Additionally, when comparing sites where exotic fish were present with those where they were absent, the average number of native species was slightly higher in areas where these invasive species were present, yet the average Fish Assessment Index (FAI) value was lower. Statistically significant differences were observed in fish diversity, especially in trophic and tolerance traits. This study provides valuable insight into the impact of invasive species on South Korean river ecosystems and offers important information for developing future conservation and management strategies for biological resources.

Passage of fishes through hydropower turbines and water pumping stations may cause mortal injury as the result of exposure to blade strike impact. Laboratory trials of simulated blade strike on two morphologically distinct fishes, American eel (Anguilla rostrata) and bluegill sunfish (Lepomis macrochirus) were undertaken to assess injury and mortality rates. We hypothesized that bluegill would have comparable rates of injury and mortality to other laterally compressed fishes while anguilliform American eel would be more resistant to injury. American eel had low observed mortality rates at the highest velocity tested (13.6 m/s), but many fish were observed with vertebral fractures which we categorized as functionally dead individuals. Bluegill were more susceptible to blade strike with high rates of mortality regardless of blade thickness, velocity, or impact conditions (location, angle, or fish orientation). These data have broadened our understanding of the range of responses among entrained fishes exposed to blade strike and represent species with low (American eel) and high (bluegill) susceptibility to injury and mortality. Our blade strike data can help inform safer turbine designs or prioritization of pumps that minimize traumatic injury and mortality of fishes during non-volitional passage through hydropower turbines or water pumping stations.

Ecological theory has long recognised the importance of positive and negative species interactions as drivers of food web structure, yet many studies have only focused on competition. Because competitive and facilitative mechanisms operate simultaneously, but through different food web pathways, the balance of their combined effects can produce complex and variable responses. We used a response‐surface experimental design to assess the roles of negative (e.g. intra‐, interspecific competition) and positive (e.g. facilitation) interactions between native and invasive juvenile fishes. We tested whether these interactions alter the densities of planktonic and benthic invertebrates to evaluate the magnitude and mechanism(s) influencing the acceptance or resistance of biological invaders. Interactions between bighead carp (Hypophthalmichthys nobilis) and bluegill (Lepomis macrochirus) or common carp (Cyprinus carpio) were evaluated in mesocosms. Intraspecific interactions were 1.5–2.4 times stronger than interspecific interactions between carp species. The only instance of interspecific competition resulted in bighead carp reducing the daily growth of bluegill, whereas the reciprocal interaction resulted in facilitation. Facilitation occurred when bluegill increased the daily growth of low density bighead carp treatments, despite increased numbers of fishes. Bighead carp also increased densities of benthic Chironomidae larvae, which were subsequently consumed by bluegill, but did not result in enhanced bluegill growth. These suites of interactions were not observed between common and bighead carp. Our response‐surface design proved useful for comparing the relative magnitude of intra‐ vs. interspecific competition, identifying facilitation among species, and tracing attendant effects on invertebrate communities. By accounting for the directionality of interactions within our experimental framework and tracking responses of prey at lower trophic levels, we provide a clearer understanding of how competitive effects and stressed consumers alter prey communities and influence facilitation. A plain language summary is available for this article. Plain Language Summary

Microsporidia are diverse opportunistic parasites abundant in aquatic organisms with some species hyperparasitic in digenean parasites. In the current study, we describe a unique microsporidian parasite, Ovipleistophora diplostomuri n. sp. that has a tropism for both the bluegill sunfish Lepomis macrochirus, and its digenean parasite Posthodiplostomum minimum. Though the microsporidium first infects a fish, the subsequent infection causes hypertrophy of the metacercarial wall and degeneration of the P. minimum metacercariae within the fish tissue. Genetic analysis placed this species within Ovipleistophora and ultrastructural characteristics were consistent with the genus, including the presence of dimorphic spores within sporophorous vesicles. Meronts did not have a surface coat of dense material, which has been previously reported for the genus. This is the first Ovipleistophora species described that does not have a tropism for ovary. Genetics demonstrated that O. diplostomuri n. sp. groups closely within fish microsporidia and not other species known to be hyperparasitic in digeneans, suggesting that it evolved from fish-infecting microsporidians and developed a secondary tropism for a common and widespread digenean parasite. The high genetic identity to Ovipleistophora species demonstrates the close relationship of this unique microsporidian with other microsporidia that infect ovary.