Artificial light at night (ALAN) is a pervasive pollutant which can extend into coastal waters. Previous studies have revealed that ALAN can suppress melatonin levels in teleost fishes. However, the effects of ALAN on elasmobranch physiology have yet to be investigated. To address this knowledge gap, we examined the relationship between ALAN exposure and blood melatonin levels in wild nurse (Ginglymostoma cirratum) and blacktip (Carcharhinus limbatus) sharks sampled off Miami, Florida (USA). We hypothesized that sharks sampled at night in metropolitan areas exposed to high ALAN, would exhibit lower blood melatonin concentrations, compared to conspecifics sampled in adjacent more pristine areas with low ALAN. As shark level of mobility would likely influence exposure to ALAN, we further hypothesized species-specific differences in our results as nurse sharks are relatively sedentary, whereas blacktips are highly mobile. We also tested for the potential influence of other biological and environmental factors, along with exposure to ALAN, on species-specific melatonin levels. Consistent with our hypothesis, nurse sharks exposed to higher ALAN exhibited significantly lower melatonin concentrations compared to individuals sampled in areas with lower ALAN. Melatonin concentrations measured in blacktips did not differ between individuals sampled in high versus low areas of ALAN. These results suggest that exposure to ALAN can suppress melatonin levels in wild sharks, and that these effects may be influenced by species-specific mobility; specifically, species that are highly resident to areas of high ALAN may be more prone to this anthropogenic pollutant compared to highly mobile species that readily move between areas of high and low ALAN. The melatonin levels found here for nurse (24.6 to 425.2 pg/mL) and blacktip sharks (27.4 to 628.7 pg/mL) also represent the first assessment of blood melatonin levels reported in sharks, providing baseline information for future monitoring and inter- and intra-species comparisons.
