Photic induction of locomotor activity is correlated with photic habitat in Anolis lizards

Light Pollution, Circadian Photoreception, and Melatonin in Vertebrates

Artificial light at night (ALAN) is increasing exponentially worldwide, accelerated by the transition to new efficient lighting technologies. However, ALAN and resulting light pollution can cause unintended physiological consequences. In vertebrates, production of melatonin—the “hormone of darkness” and a key player in circadian regulation—can be suppressed by ALAN. In this paper, we provide an overview of research on melatonin and ALAN in vertebrates. We discuss how ALAN disrupts natural photic environments, its effect on melatonin and circadian rhythms, and different photoreceptor systems across vertebrate taxa. We then present the results of a systematic review in which we identified studies on melatonin under typical light-polluted conditions in fishes, amphibians, reptiles, birds, and mammals, including humans. Melatonin is suppressed by extremely low light intensities in many vertebrates, ranging from 0.01–0.03 lx for fishes and rodents to 6 lx for sensitive humans. Even lower, wavelength-dependent intensities are implied by some studies and require rigorous testing in ecological contexts. In many studies, melatonin suppression occurs at the minimum light levels tested, and, in better-studied groups, melatonin suppression is reported to occur at lower light levels. We identify major research gaps and conclude that, for most groups, crucial information is lacking. No studies were identified for amphibians and reptiles and long-term impacts of low-level ALAN exposure are unknown. Given the high sensitivity of vertebrate melatonin production to ALAN and the paucity of available information, it is crucial to research impacts of ALAN further in order to inform effective mitigation strategies for human health and the wellbeing and fitness of vertebrates in natural ecosystems.

The effect of photoperiod regimes on daily RNA:DNA ratio rhythms in Chinese soft-shelled turtles (Pelodiscus sinensis)

The ratio of RNA to DNA is widely used to reflect instantaneous animal growth; however, little is known about its daily variation. Photoperiod can modify expression of internal clocks, providing animals with the flexibility to adapt to variable environments. This study focused on the influence of photoperiod regimes on the daily variation of RNA:DNA ratio in Pelodiscus sinensis. We randomly divided 260 turtles into four groups: constant dark (0L), 8 h light with 16 h dark (8L), 12 h light with 12 h dark (12L), and 16 h light with 8 h dark (16L). Turtles were housed under specific photoperiods for 15 days (fed for first 10 days then starved for 5 days), thereafter we sampled the tissues every 2 h for 24 h. We dissected forelimb muscles and measured the concentration of isolated RNA and DNA. There were rhythmic variations in the RNA:DNA ratio, even in turtles under continuous darkness, indicating that P. sinensis has circadian RNA:DNA ratio rhythms, and the rhythms were likely controlled by internal clocks. Additionally, the acrophase was advanced by two hours in constant darkness in contrast to the other three photoperiods, indicating that the photoperiod considerably modified the rhythm set by the internal clocks. Notably, the RNA:DNA ratio differed between photoperiod regimes, with 0L > 16L > 8L ≈ 12L, indicating the photoperiod may be a seasonal indicator for turtles to synchronize their physiological processes with environmental variations.