First-ever measurements of melatonin in wild sharks shows artificial light can disrupt nighttime hormone levels
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Artificial light from major coastal cities can disrupt the nighttime biology of sharks, according to new research.
The study, published in Science of the Total Environment, found that sharks living in brightly lit coastal waters near large urban areas had altered melatonin levels at night compared to sharks living in darker, less developed environments. The findings reveal artificial light at night – an often-overlooked form of pollution – can influence marine predators and may have broader implications for ocean ecosystems.
Artificial light at night – also known as ALAN – is one of the most pervasive environmental effects of urbanisation. While previous studies have shown that ALAN can suppress melatonin in bony fish, its effects on sharks had not yet been examined until now.
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To address this gap, researchers at the Shark Research and Conservation Program at the University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science conducted nighttime fieldwork off Miami, Florida – one of the most brightly lit coastal metropolitan regions in the United States – and compared sharks sampled in urban waters with individuals from nearby, darker coastal areas.
The team studied two shark species: nurse sharks, which are less mobile and remain in the same areas for long periods, and blacktip sharks, which are highly mobile and move regularly across broad coastal regions.
Results showed a clear species-specific response. Nurse sharks exposed to higher levels of artificial light at night had significantly lower melatonin concentrations than nurse sharks sampled in darker environments. In contrast, melatonin levels in blacktip sharks did not differ between brightly lit and darker areas.
‘These findings suggest that exposure to artificial light at night can suppress melatonin levels in wild sharks, but vulnerability depends on behaviour,’ said Abigail Tinari, the lead author of the study, who conducted the research for her master’s thesis at the Rosenstiel School. ‘Species that are highly resident in light-polluted areas appear more susceptible than species that regularly move between illuminated and darker habitats.’
Melatonin plays an important role in regulating daily biological rhythms and is linked to overall health and physiological functioning across many animals. Disruptions to this hormone have been associated with sleep and metabolic disturbances in humans and terrestrial wildlife, but its role in sharks has remained largely unexplored.
‘Sharks play a key role in maintaining balanced marine ecosystems, and physiological changes in top predators could have cascading effects throughout the food web,’ added Neil Hammerschlag, senior author of the study.
‘Our findings highlight light pollution as a meaningful environmental stressor that warrants consideration alongside more widely recognised threats such as habitat loss and chemical pollution.’
