Artificial light pollution has long been recognized as a significant disruptor of terrestrial ecosystems, yet its impact on marine environments remains underexplored. This study addresses that knowledge gap by evaluating how urban light intensity affects melatonin levels in nurse sharks (Ginglymostoma cirratum) and blacktip sharks (Carcharhinus limbatus), species whose activity patterns differ markedly. Nurse sharks, characterized by their relatively sedentary behavior and site fidelity, contrasted with the wide-ranging, highly mobile blacktip sharks, provided an ideal model to investigate behavioral vulnerabilities to ALAN. The research was conducted in the coastal waters off Miami, Florida, a global hotspot of urban illumination.

Melatonin plays a pivotal role in orchestrating circadian rhythms—biological processes aligned to day-night cycles—across a wide array of vertebrates. In sharks, however, the hormone’s physiological roles have remained largely enigmatic until this study. By directly measuring plasma melatonin concentrations extracted during nocturnal sampling expeditions, the researchers revealed that nurse sharks inhabiting brightly illuminated urban waters present a pronounced suppression of melatonin levels relative to individuals captured in darker, less disturbed zones. Conversely, blacktip sharks did not exhibit significant hormonal fluctuations attributable to light exposure, suggesting an intrinsic resilience potentially linked to their broad spatial movements.

The methodology employed was meticulously designed to minimize sampling bias and animal stress. Utilizing short-duration research drumlines, sharks were captured at night under low-impact red lighting conditions, which do not interfere with normal melatonin rhythms. Immediate blood sample collection allowed for precise baseline hormone quantification. In parallel, the study accounted for relevant environmental variables such as ambient light intensity, water temperature, and depth to robustly establish causative links between ALAN and endocrine changes. This rigorous experimental approach represents a significant advancement in field-based marine physiological research.

Urban coastal environments, typified by Miami’s metropolis, emanate intense artificial light from street lamps, buildings, and marine vessel activities. This anthropogenic illumination extends far beyond terrestrial boundaries, penetrating shallow marine habitats where sharks and many other organisms conduct critical life cycle activities. Suppression of melatonin in resident nurse sharks implies that light pollution might disrupt essential behaviors such as foraging, reproduction, and predator avoidance that are temporally regulated by natural light-dark cycles. Such hormonal imbalances could therefore cascade into altered population dynamics and ecosystem functions.

The study’s findings also invoke broader ecological concerns. Sharks are apex predators that maintain the structure and stability of marine food webs. Hormonal disruption in these keystone species could precipitate shifts in species interactions and trophic cascades with far-reaching consequences. Physiological stress induced by ALAN might impair immunity, metabolism, and growth, further jeopardizing shark health and their ecological effectiveness. The evidence underscores the urgency of incorporating artificial light pollution as a recognized environmental stressor alongside chemical pollutants and habitat degradation within marine conservation frameworks.

Interestingly, the differential effects observed between nurse and blacktip sharks hinge on behavioral ecologies. The nurse shark’s relatively fixed home ranges within illuminated areas make them more vulnerable to prolonged exposure, whereas blacktip sharks, through their mobile lifestyle, likely experience episodic or limited light exposure, potentially allowing physiological recovery. This species-specific vulnerability highlights the necessity for conservation strategies tailored to life history traits and spatial ecology, rather than generalized management paradigms.

From an evolutionary perspective, the suppression of melatonin due to light pollution in sharks—ancient creatures with origins dating back over 400 million years—illustrates the deeply conserved nature of light-sensitive hormonal processes across vertebrates. Such findings not only enhance understanding of marine chronobiology but also evoke parallels with terrestrial species, including humans, where circadian disruption has been increasingly implicated in health disorders. The study thus bridges disciplines, suggesting that mitigating light pollution benefits biodiversity and human well-being alike.

Moreover, exploring the molecular and receptor-level mechanisms underlying melatonin regulation in sharks could unlock novel biomedical insights. Sharks possess unique physiological adaptations that have long fascinated researchers; deciphering their melatonin receptor pathways may inform new therapeutic approaches for circadian-related diseases in humans. Consequently, this research opens new frontiers in comparative endocrinology and translational medicine.

This investigative effort was supported by The Batchelor Foundation Inc. and Canon Solutions USA, emphasizing the growing recognition by funding bodies of the environmental ramifications of urbanization on marine health. The integration of marine biology, physiology, environmental science, and conservation in this study exemplifies the multidisciplinary approach essential for addressing complex anthropogenic challenges.

Looking ahead, the baseline melatonin concentrations established provide a critical reference point for continued monitoring as coastal urbanization intensifies globally. Future studies should expand to include additional shark species and varied geographic locales, advancing comprehensive understanding of ALAN’s ecological impact. Such research will be indispensable for informing policy decisions, urban planning, and marine protected area design that minimize light pollution and safeguard oceanic biodiversity.

In conclusion, the study presents a compelling narrative that the glow of coastal cities does not end at the shoreline but infiltrates marine ecosystems, physiologically perturbing top predators like sharks. Recognizing artificial light pollution as a significant environmental stressor necessitates urgent action to develop mitigation strategies that harmonize urban development with marine ecosystem integrity. As researchers continue to illuminate these hidden interactions, the urgent message is clear: safeguarding the ocean’s rhythmic balance is vital for preserving both marine life and the planetary health upon which human existence depends.

Subject of Research: Animals
Article Title: Sharks at night, exposed to city light: Melatonin concentrations in two shark species differ in response to artificial light at night
News Publication Date: 20-Jan-2026
Web References:
Science of the Total Environment Article
References:
Tinari, A. M., McDonald, M. D., Cooke, S. J., Gallagher, A. J., & Hammerschlag, N. (2026). Sharks at night, exposed to city light: Melatonin concentrations in two shark species differ in response to artificial light at night. Science of the Total Environment.
Image Credits: @Sharktagging.com
Keywords: Marine fishes, Environmental stresses, Stressors

At its core, urbanization constitutes a profound transformation of natural habitats, fragmenting continuous ecosystems into patches steeped in artificial structures, pollution, and altered resource availability. These anthropogenic modifications impose stringent environmental pressures that dictate which organisms can persist. Emerging studies reveal that this initial filter – species presence – hinges on traits such as tolerance to disturbed conditions, mobility, and flexible diets. Urban environments selectively favor generalist species capable of exploiting novel resources, often at the expense of specialists. This shift not only influences the composition of urban biota but also sets the stage for altered ecological networks.

Beyond determining who is present, urbanization impacts the probability of species co-occurring in shared spaces. The spatial configuration of green patches, connectivity of habitats, and microclimatic variations form a second filtering layer governing temporal and spatial overlap among species. For instance, fragmented tree canopies and isolated parks may support only a subset of the native fauna, concentrating species interactions into smaller, more frequent encounters yet with reduced diversity. Conversely, novel assemblages may emerge as exotic species infiltrate and animals adapt to urban niches, creating untested ecological communities with unpredictable dynamics.

Crucially, the relationships themselves – the third filter – are transformed under urban influence. Interactions that define ecosystems, such as predation, competition, mutualism, and parasitism, can be weakened, intensified, or redirected. Urban settings modify species behavior and population densities, which in turn alter the strength and nature of these interactions. For example, predator-prey dynamics may be disrupted by changes in predator hunting efficiency amidst human structures or availability of alternative prey. Similarly, pollination networks might collapse or rewire due to declines in specialist pollinators and proliferation of generalist insects.

This framework, which parses urban ecosystems into discrete filters, offers a powerful lens to ask nuanced questions about eco-evolutionary processes in cities. It encourages integrative studies that transcend traditional species inventories to incorporate interactions and evolutionary trajectories affected by urban stressors. These insights are vital because altered interactions cascade through ecosystems, influencing community stability, functional diversity, and ecosystem services that humans rely upon, from pest control to pollination.

Recent empirical evidence highlights dramatic consequences of urbanization on species interactions across diverse taxa. For example, bird communities in metropolitan areas often exhibit increased nesting competition but decreased predator diversity, disrupting trophic cascades. In insect assemblages, urban warming modifies phenology and interaction timings, potentially leading to mismatches between pollinators and flowering plants. Similarly, mammalian carnivores may shift diets toward human refuse, thereby altering food webs and disease transmission dynamics.

Moreover, urban environments create evolutionary pressures that drive rapid adaptation in some species, further reshaping interactions. Certain urban-adapted rodents and birds exhibit behavioral plasticity or genetic changes enhancing boldness, dietary range, or reproductive strategies. These evolutionary responses feed back into ecological networks, influencing which interactions are maintained or lost. Recognizing this dynamic interplay between ecology and evolution in the context of urbanization is essential for predicting future biodiversity patterns.

Despite increasing awareness, major knowledge gaps remain. The complexity of urban ecosystems, coupled with methodological challenges in measuring interactions, limits comprehensive understanding. Many studies focus on presence-absence data or population trends without integrating interaction networks or evolutionary parameters. Additionally, the heterogeneity of urban landscapes—varying with culture, geography, and urban planning—complicates generalizations and demands tailored investigations.

Addressing these gaps requires a multidisciplinary approach blending field observations, experimental manipulations, and advanced modeling techniques. For instance, network analysis can elucidate how urban filters reshape interaction webs, while genomic tools may uncover evolutionary shifts linked to urban stressors. Citizen science initiatives could also contribute valuable large-scale data across urban gradients, capturing temporal and spatial variation in species interactions.

From a practical perspective, this framework has profound implications for urban conservation and management strategies. By identifying critical filters and their effects, urban planners can design green spaces that enhance species co-occurrence and facilitate beneficial interactions, such as pollination or pest regulation. Maintaining connectivity between habitat patches may mitigate fragmentation’s negative effects, enabling species to persist and interact more naturally. Furthermore, managing anthropogenic stressors, including pollution and invasive species, can help preserve native interaction networks.

Incorporating eco-evolutionary principles into urban policy promotes resilient urban ecosystems that support both biodiversity and human well-being. Urban environments can transform from ecological deserts into thriving habitats providing multiple ecosystem services. For example, sustaining pollinator networks enhances urban agriculture and green space vitality, while preserving predator-prey balances helps control disease vectors and crop pests. Recognizing these interdependencies elevates the importance of urban biodiversity beyond aesthetic or recreational value to a cornerstone of sustainable city living.

Interdisciplinary collaborations between ecologists, evolutionary biologists, urban planners, and social scientists can foster integrative solutions. Awareness campaigns highlighting the hidden intricacies of urban ecosystems may engage citizens in stewardship, driving community-based conservation actions. Ultimately, cities represent critical arenas where human development and nature converge, demanding innovative frameworks to understand and harmonize their coexistence.

The urban eco-evolutionary filters framework marks a paradigm shift in conceptualizing urban biodiversity as a dynamic, multi-layered phenomenon shaped by presence, co-occurrence, and interaction filters. This perspective guides future research to move beyond static species lists and unravel the mechanisms shaping biodiversity patterns and ecosystem functions in rapidly transforming cityscapes. By unveiling the complexity of species relationships in urban settings, it sets the foundation for evidence-based practices enabling cities to nurture rich, resilient ecological networks.

As humanity’s urban footprint expands, deciphering the ecological and evolutionary consequences of urbanization transcends academic curiosity – it becomes imperative for sustaining life courses intertwined with urban nature. The cascading effects on species interactions underscore the urgency to rethink biodiversity conservation in cities, integrating ecological intricacies into urban design. Harnessing this knowledge offers hope for cities to become not only centers of human ingenuity but also bastions of coexistence with the natural world.

In conclusion, the interplay between urbanization and species interactions unveils a multifaceted challenge and opportunity. Conceptualizing cities through eco-evolutionary filters enhances scientific understanding while informing pragmatic interventions. This holistic framework paves the way for urban environments that do more than accommodate human needs – they actively foster ecological richness and evolutionary potential, embracing biodiversity as a pillar of sustainable futures.

Subject of Research: The impact of urbanization on species interactions through eco-evolutionary filters encompassing species presence, co-occurrence, and ecological relationships.

Article Title: The effects of urbanization on species interactions.

Article References:

Moreno-García, P., Savage, A., Salgado, A.L. et al. The effects of urbanization on species interactions.
Nat Cities (2025). https://doi.org/10.1038/s44284-025-00288-w

Image Credits: AI Generated