Joost André Maria Raeymaekers

Urban environments impose strong selective pressures through biotic and abiotic factors, driving changes in behavior, physiology, and morphology. Yet, responses vary across taxa and cities, and it remains unclear which traits respond consistently and what factors moderate this variation. We addressed these questions using the widespread European garden spider ( Araneus diadematus ) as a model, measuring size, color, and web-building traits along urban–rural transects in 22 cities across its distribution range. Using a meta-analytic framework, we assessed how city-specific characteristics influenced trait variation. Urbanization consistently reduced relative abdomen surface area, a proxy for body condition. Exploratory meta-regressions suggest that web-building response was predicted by temperature: compared to their non-urban surroundings, urban webs are larger in colder regions and smaller in warmer regions. In contrast, body size and abdomen brightness varied among cities without clear environmental predictors. These findings show that urbanization effects are trait- and context-dependent, likely influenced by local factors such as heat island intensity, microclimate, or prey availability. Linking within- and between-city variation will improve understanding of species phenotypic responses to urban environments.

Riverine fish conservation in eco‐hydraulics is often based on restoring or regulating river habitat to a semi‐natural state. For example, ecohydraulic projects support fish populations by increasing river connectivity through fishways, and by maintaining or restoring natural flow variance to enhance habitat quality and availability. However, the application of these concepts is challenging due to climate change and human perturbations that can modify the riverine environment in unpredictable ways. To address these challenges, we propose broadening eco‐hydraulics to eco‐evo‐hydraulics by incorporating the evolutionary dynamics of fish populations into eco‐hydraulics. We argue that this may improve the resilience and resistance of fish populations to future climate and human perturbation. Moreover, the evolutionary response of fishes to perturbations also alters their capability to “zoogeomorphically engineer” their physical environment, with associated feedbacks such as changes in sediment mobilization. Thus, understanding such hydro‐evolutionary feedbacks can improve the long‐term prediction of river hydraulics. In this position paper, we explore the interactions between hydraulics, hydrology, and rapid evolution based on a literature review to identify gaps and opportunities that need joint efforts from water resources engineers and evolutionary biologists. We propose the concept of eco‐evo‐hydraulics with hydro‐evolutionary feedbacks to encapsulate the interactions between the three disciplines, and suggest research avenues for the future. Finally, we suggest potential managerial strategies based on the eco‐evo‐hydraulic concept for local and basin‐wide fish conservation. We aim to fill the gap between water resources management and evolutionary biology to provide a new theory for fish conservation.

Introduction: Environmental micropollutants and other anthropogenic xenobiotics are potential drivers behind compositional shifts and functional dysregulation of gut microbial communities. Mercury and many of its compounds are highly toxic and ubiquitous environmental pollutants that pose a risk for aquatic biota and humans. Here we compared the gut microbial communities of natural three-spined stickleback ( Gasterosteus aculeatus Linnaeus, 1758) populations in Flanders, Belgium, with contrasting muscle mercury concentrations. We hypothesized that exposure to a high mercury load selects for gut flora species with the capacity to tolerate or adapt to this stressor and, thus, leads to a change in the composition of the gut microbiota. Methods: The gut microbiota of 128 host individuals from four populations with low levels of accumulated mercury and four populations with high mercury levels were characterized using 16S rRNA amplicon sequencing. Gut microbial communities were compared across host muscle mercury content levels, host populations and sexes to consider the contribution of these factors in the observed differences in gut microbial diversity and composition. Results: Microbial community composition varied significantly between males and females, as well as between host populations with high and low muscle mercury content. While the abundance of 22 amplicon sequence variants (ASVs) was associated with the host’s muscle mercury content, we detected no specific indicator species for high mercury. Conclusion: Overall, our results suggest that local factors specific to a host population, potentially including mercury accumulation and sex-specific factors, differentiate the microbial communities inhabiting the gastrointestinal tracts of the three-spined stickleback.

High-latitude environments represent climate-related challenges, imposing a limit on species distributions. By elucidating how marine species adjust their reproductive strategy to cope with climatic variation, we can better predict the resilience of populations to environmental change. We addressed this question with a large-scale field study of a marine fish with a central role in the northeast Atlantic coastal ecosystem: the two-spotted goby Pomatoschistus flavescens. We developed a multi-faceted approach to examine how the species’ reproductive strategy and life history change along a climatic gradient. Six populations were surveyed along a gradient of 10° latitude, nearly reaching the northern limit of the species’ range, combining environmental, physiological, phenotypic and demographic data with a genomics investigation of population connectivity. Northern fish experienced colder environments for growth and reproduction than their southern counterparts, occurred at lower adult density and had lower body condition. At high latitudes, a different life history strategy emerged, whereby adults reproduced as 2 yr olds rather than 1 yr olds. Genomics analyses unveiled a clear genetic structure, with a break in connectivity between Skagerrak and the Norwegian coast further north, consistent with findings in other marine species. However, our genomic analyses did not provide clear evidence of adaptation to ecological variables, leaving open questions about the roles of adaptation and plasticity in latitudinal shifts in reproductive strategy. The study highlights the complex life history adaptations to high latitudes in a marine species with high gene flow and underscores the importance of sea temperature on fish reproductive strategy.

Eco-hydraulics traditionally aims at managing riverine systems in a semi-natural state while meeting human demands, assuming aquatic species are static. However, evidence of rapid evolution suggests that ignoring evolutionary dynamics of fish species might limit long-term effectiveness of eco-hydraulics frameworks. It remains unclear how freshwater fish adapt to human perturbation. Why are some fish populations more resilient to human perturbation than others? What are the genetic mechanisms behind it? To answer these questions, we genotyped eleven populations of three-spined stickleback in a regulated river system and collected data on river morphology, connectivity, flow regimes, physico-chemistry and parasite abundance through a combination of field surveys and modelling. Gene-environment association analysis detected strong signals of genetic divergence associated with hydraulic features. Gene ontology analysis revealed evolutionary responses that primarily involve functions in the nervous and sensory systems. These findings demonstrate that fish can evolve in response to river regulation, highlighting the need to transition from eco-hydraulics toward eco-evo-hydraulics.

The role of rapid adaptation during species invasions has historically been minimized with the assumption that introductions consist of few colonists and limited genetic diversity. While overwhelming evidence suggests that rapid adaptation is more prevalent than originally assumed, the demographic and adaptive processes underlying successful invasions remain unresolved. Here we leverage a large whole-genome sequence dataset to investigate the relative roles of colonization history and adaptation during the worldwide invasion of the forage crop, Trifolium repens (Fabaceae). We show that introduced populations encompass high levels of genetic variation with little evidence of bottlenecks. Independent colonization histories on different continents are evident from genome-wide population structure. Five haploblocks—large haplotypes with limited recombination—on three chromosomes exist as standing genetic variation within the native and introduced ranges and exhibit strong signatures of parallel climate-associated adaptation across continents. Field experiments in the native and introduced ranges demonstrate that three of the haploblocks strongly affect fitness and exhibit patterns of selection consistent with local adaptation across each range. Our results provide strong evidence that large-effect structural variants contribute substantially to rapid and parallel adaptation of an introduced species throughout the world.

Rapid adaptation during invasion has historically been considered limited and unpredictable. We leverage whole-genome sequencing of >2600 plants across six continents to investigate the relative roles of colonization history and adaptation during the worldwide invasion of Trifolium repens. Introduced populations contain high levels of genetic variation with independent colonization histories evident on different continents. Five large structural variants on three chromosomes exist as standing genetic variation within the native range, and exhibit strong signatures of parallel climate-associated adaptation across continents. Common gardens in the native and introduced ranges demonstrate that three structural variants exhibit patterns of selection consistent with local adaptation across each range. Our results provide strong evidence that rapid and parallel adaptation during invasion is caused by large-effect structural variants introduced throughout the world. Significance Statement Biological invasions occur over short timescales and introductions are often hypothesized to include limited genetic diversity, making the role of adaptation in invasion success controversial. We demonstrate that the invasion of a human-commensal species, Trifolium repens, likely stems from multiple, diverse introductions with significant evidence of climate-associated adaptation following introduction. The genetic basis of adaptation is most strongly linked to five chromosomal rearrangements that each span hundreds of genes – matching theoretical predictions that large-effect variants are key to the initial stages of adaptation to novel environments. Chromosomal rearrangements have remarkably parallel signatures of adaptation across different introductions despite initial colonization from different areas of Europe. Our study highlights the impact of globalization and rapid adaptation for the invasion success of human commensal species.

Mercury can be very toxic at low environmental concentrations by impairing immunological, neurological, and other vital pathways in humans and animals. Aquatic ecosystems are heavily impacted by mercury pollution, with evidence of biomagnification through the food web. We examined the effect of mercury toxicity on the spleen, one of the primary immune organs in fish, in natural populations of the three-spined stickleback ( Gasterosteus aculeatus Linnaeus, 1758). Our aim was to better understand adaptation to high mercury environments by investigating transcriptomic changes in the spleen. Three stickleback populations with mean Hg muscle concentrations above and three populations with mean Hg muscle concentrations below the European Biota Quality Standard of 20 ng/g wet weight were selected from the Scheldt and Meuse basin in Belgium. We then conducted RNA sequencing of the spleen tissue of 22 females from these populations. We identified 136 differentially expressed genes between individuals from populations with high and low mean mercury content. The 129 genes that were upregulated were related to the neurological system, immunological activity, hormonal regulation, and inorganic cation transporter activity. Seven genes were downregulated and were all involved in pre-mRNA splicing. The results are indicative of our ability to detect molecular alterations in natural populations that exceed an important environmental quality standard. This allows us to assess the biological relevance of such standards, offering an opportunity to better describe and manage mercury-associated environmental health risks in aquatic populations.

SUMMARY: The analysis of growth parameters in fish stocks, such as the asymptotic length (L _∞ ) and the curvature parameter (K), is crucial for the estimation of production rates and total mortality rates in fisheries management. Here, we estimated the growth parameters of the clupeids Limnothrissa miodon and Stolothrissa tanganicae in the northern part of Lake Tanganyika (Bujumbura sub-basin). Both species are important targets of pelagic fisheries, but the available estimates are based on scarce size frequency data, and rarely on size as a function of age data. We provide new estimates of L _∞ and K based on size as a function of age data, with age being inferred from otolith weights. Furthermore, we reanalyze several existing length frequency datasets using advanced statistical procedures to test if growth parameters estimated by length frequency analysis are consistent with those obtained from size as a function of age. We found that length frequency analysis consistently underestimates L _∞ and overestimates K as compared to length-at-age data. We recommend that length-at-age data are collected at a much broader spatial and temporal scale than currently available to advance our understanding of the growth dynamics of these two sardine species. This will improve the assessment of stock productivity of Lake Tanganyika’s pelagic fisheries and the conservation of these economically important fish species.

Genetic monitoring of populations currently attracts interest in the context of the Convention on Biological Diversity but needs long-term planning and investments. However, genetic diversity has been largely neglected in biodiversity monitoring, and when addressed, it is treated separately, detached from other conservation issues, such as habitat alteration due to climate change. We report an accounting of efforts to monitor population genetic diversity in Europe (genetic monitoring effort, GME), the evaluation of which can help guide future capacity building and collaboration towards areas most in need of expanded monitoring. Overlaying GME with areas where the ranges of selected species of conservation interest approach current and future climate niche limits helps identify whether GME coincides with anticipated climate change effects on biodiversity. Our analysis suggests that country area, financial resources and conservation policy influence GME, high values of which only partially match species’ joint patterns of limits to suitable climatic conditions. Populations at trailing climatic niche margins probably hold genetic diversity that is important for adaptation to changing climate. Our results illuminate the need in Europe for expanded investment in genetic monitoring across climate gradients occupied by focal species, a need arguably greatest in southeastern European countries. This need could be met in part by expanding the European Union’s Birds and Habitats Directives to fully address the conservation and monitoring of genetic diversity.