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

The study, published in the journal Ecosphere, employs an observational methodology that rigorously tracks monarch butterfly presence, breeding activity, and parasite loads across multiple seasons within urban neighborhoods of the East Bay. Researchers conducted systematic monthly surveys along fifteen designated three-mile routes, meticulously cataloging both native and non-native milkweed species and monitoring monarch life stages on these plants. Adult butterflies were captured temporarily for parasite screening, providing critical data that links butterfly health with their ecological contexts. By integrating these detailed field observations, the research team offers crucial insights into whether non-migratory monarchs interbreed or otherwise interact substantially with the traditional migratory population.

Historically, western monarch populations undertook annual migrations from interior breeding grounds that spanned various states, including California, Arizona, Nevada, Oregon, Washington, Idaho, and Utah, with coastal California serving as their sanctuary for the winter. Over the last decade, however, the migratory population has plummeted dramatically, raising alarms within conservation circles about the species’ long-term viability. This decline coincides temporally with the rising incidence of year-round, resident monarch populations localized in urban settings—most notably in the Bay Area—where monarchs exploit the winter-persistent, non-native evergreen milkweeds introduced in ornamental horticulture. These milkweeds, such as tropical milkweed (Asclepias curassavica), provide crucial larval host plants and nectar sources even when native milkweeds have long senesced.

Contrary to some initial concerns, the UC Davis team found that these urban, resident monarch populations do not appear synonymous or integrally connected to the migratory monarch circuit. Genetic and observational data suggest that non-migratory monarchs effectively constitute a distinct demographic unit, somewhat isolated from the migratory cohort. This dissociation implies that resident monarchs neither serve as a source population to replenish migratory numbers nor act as a detrimental “trap” exacerbating population declines through parasite transmission or other ecological liabilities. Infection rates with Ophryocystis elektroscirrha (OE), a common protozoan parasite linked to milkweed availability and monarch health, were closely monitored and found to follow seasonal trends reflective of resident population dynamics independent of migratory input.

An ongoing debate in conservation biology hinges on the role of non-native milkweeds in monarch health, given the potential for these plants to disrupt migratory triggers and foster year-round breeding that could increase parasite loads. In response, some jurisdictions have enacted policies banning the planting of tropical milkweed with the intent of protecting migratory monarchs. The new findings suggest, however, that blanket removal of non-native milkweeds from urban landscapes may be misplaced or premature. Instead, the study advocates for a nuanced approach that balances supporting monarch habitat with understanding metapopulation structure and disease ecology.

Urban ecosystems, once considered marginal or even hostile habitats for wildlife, are increasingly recognized as critical contributors to regional biodiversity and conservation. The vibrant presence of resident monarchs in the Bay Area demonstrates how urban gardens can sustain ecologically significant populations, providing continuous resources that facilitate breeding and survival independent of larger migratory cycles. Beyond monarchs themselves, the cultivation of native or non-native milkweeds fosters broader pollinator communities, enhancing urban ecological resilience and connecting people with nature in their daily lives.

One of the compelling implications of this research is how it reframes the role of urban residents and gardeners as active participants in conservation. By planting milkweeds, cultivating nectar sources, and engaging in butterfly stewardship, city dwellers contribute tangibly to sustaining resident monarch populations. This proximity fosters heightened public awareness, educational opportunities, and a grassroots constituency dedicated to pollinator conservation. As postdoctoral researcher Emily Erickson notes, seeing monarch butterflies in everyday settings can galvanize community support for broader environmental initiatives—a vital component in an era of accelerating biodiversity loss.

The study’s observations revealed striking seasonal fluctuations in monarch abundance, with adults peaking during summer months and declining in winter, correspondingly reflecting availability of milkweed plants. Both native and non-native milkweeds supported monarch reproduction, evident from the consistent presence of eggs and caterpillars throughout the seasons. Parasite burden assessments aligned with these population dynamics, showing decreased infection rates during winter and increases in summer and fall. Importantly, these infection patterns did not indicate spillover effects from migrating populations, reinforcing the notion of relative isolation between urban resident and migratory monarchs.

Funding for this innovative research was provided by Google, which has been actively investing in monarch butterfly habitat restoration and outreach since 2021, capitalizing on corporate environmental responsibility to bolster conservation impact. Elizabeth Crone, the study’s senior author and a professor of Evolution and Ecology, expressed enthusiasm for this partnership, underscoring the rare and invaluable opportunity to leverage private-sector resources toward butterfly conservation—an alliance not previously witnessed in her nearly three decades of academic experience.

The findings by Crone and colleagues also underscore the need for adaptive conservation strategies that fully acknowledge the complexity of monarch butterfly population structures and urban-rural interactions. While the resident monarchs of the Bay Area do not currently threaten migrating populations, future environmental changes or shifts in disease dynamics could alter this balance. Consequently, monitoring must continue, paired with research into migratory physiology, landscape connectivity, and climate-mediated variability to safeguard the species throughout its North American range.

In conclusion, the emergence of non-migratory monarch populations in urban gardens represents a fascinating, multifaceted phenomenon at the intersection of ecology, conservation biology, and human-altered environments. Far from being a detrimental anomaly, these resident populations demonstrate the potential for urbanization to create refuges for wildlife despite broader environmental challenges. By fostering informed, evidence-based management practices and nurturing community stewardship, scientists and citizens alike can work towards enhancing monarch conservation in both traditional migratory habitats and the novel urban ecosystems where these butterflies now thrive year-round.

Subject of Research: Animals

Article Title: Neither source nor trap: Urban gardens as habitat for nonmigratory monarch butterflies in Northern California

News Publication Date: 10-May-2025

Web References:

• https://doi.org/10.1002/ecs2.70259
• https://biology.ucdavis.edu/people/elizabeth-crone
• https://www.secondnatureeco.com/team
• https://blog.google/outreach-initiatives/sustainability/monarch-butterflies-california/

References:
Crone, E.E., Erickson, E.R., Schultz, C.B. (2025). Neither source nor trap: Urban gardens as habitat for nonmigratory monarch butterflies in Northern California. Ecosphere. DOI: 10.1002/ecs2.70259

Image Credits: Sylvie Finn