As the sun goes down over the horizon, life underwater takes notice. The daily cycle of sunlight and then dark changes and influences animal behavior, and many of these behaviors are important to a species’ survival. Some ocean creatures depend upon the cloaking darkness of night as cover from predators, while others time their reproductive cycles to the rising of the moon. Reproduction, predation, navigation, growth, and communication are often synced to night and day.
In a world where human activity relies on the use of artificial light, light pollution can take a toll on ocean life. A 2010 study found that 22 percent of the world’s coastlines were lit up at night. Many behaviors that are regulated by the darkness of night are now being disrupted by the addition of this intense artificial light. Below, we’ve outlined three key behaviors that rely on the dark for their survival, and how artificial light is challenging their way of life.
Diel Vertical Migration
The largest migration on the planet happens every day, right beneath the ocean surface. As the sun sets, fishes, squids, shrimps and zooplankton make massive migrations from the dark ocean depths upward to near the ocean’s surface. Despite the small size of some (no bigger than a mosquito), these creatures can travel hundreds of meters in just a few hours. Under the protection of darkness, they feast on phytoplankton that grew during the day at the surface and on other animals eating the phytoplankton. Then, when the sun comes out and there is again enough light for predators to see them, the migrators return to the deep darkness. Generally, this repeats every single day.
The vertical migrators are so sensitive to changes in light that even the presence of one lit research vessel can alter their migration pattern. In many parts of the world fishing vessels use bright lights to lure their targeted catch to the surface, like the jig fishery for squids in many oceanic areas. Disruptions of the daily migration likely have large implications for the ocean ecosystem and potentially the entire world.
Coral Spawning
The annual spawning of many coral species takes place by the light of the moon. For these species, every single individual coral times the release of eggs and sperm down to the exact same moment. The synchronized spawning is believed to help maximize egg fertilization due to the sheer number of floating eggs and sperm, ensure genetic diversity through many different parent couplings, and hinder predators from engorging on every single fertilized egg. Once the seawater reaches optimal temperature the corals prepare to spawn. Often, corals spawn during the same month year after year. This is when the moon comes into play. As the monthly lunar cycle continues from new moon to full moon the moon rises progressively later into the day. Early in the lunar cycle the moon rises before the sun sets but by the time of the full moon the moon rises at the same time that the sun sets. In the nights following the full moon there is a period of time after the sun sets before the moon rises. Preliminary research on coral species Dipsastraea speciosa suggests it is this period of darkness that triggers the onset of spawning. Some species spawn a day after the full moon while others spawn a couple days after.
If corals rely on a period of darkness to spur spawning, the increased presence of artificial light poses a threat to their survival. Corals are sensitive to blue light, the wavelengths that are dominant in moonlight, and as LED lights (which produce more blue light) become more prevalent they are beginning to interfere with the corals’ natural cycle. Studies show that the presence of artificial light disrupts the synchronization of spawning, leading corals in the same place to spawn at different times. Some corals only reproduce once a year, so missed cues can be disastrous for the next generation. Illuminated coastlines add to the threats reefs already face from rising temperatures and ocean acidification.
Turtle Nesting
Year after year, mother sea turtles journey to the same beach to lay their eggs. When the eggs hatch, the brand-new turtles use the light of the moon and the reflection on the ocean to find their way to the sea. These beaches were once quiet and free of human development, the perfect place for a mother to lay her eggs in peace. Now condominiums line the shores and tourists flock to the beaches by the thousands. The human development includes the addition of artificial lights from houses and streets, which cause several problems for the nesting turtles. Mother turtles can shy away from bright lights, leading them to lay their eggs at a poor-quality site or in the water. Lights can also lure hatchlings in the wrong direction, which often leads to their death. Far from the shore, the hatchlings become easy prey for predators, can become fatigued, dehydrated, drown in pools, and get run over by cars.
Many beach communities now mandate the use of special lighting to ensure hatchlings don’t mistake artificial lighting for the light of the moon and stars. In Florida, where about 90 percent of the sea turtles that nest in the United States are found, some communities have enacted ordinances requiring red or amber hued LED lights that don’t disorient turtles. In the 1990s most nests in Florida were affected by artificial lighting, which led to tens of thousands of hatchlings becoming disoriented. The Florida Department of Natural Resources estimated about 20,000 turtles became disoriented in 1990. Since then, significant efforts by government agencies and conservation groups have helped educate the public about how to help the turtles. Research shows that the efforts are working—even though the Florida population is growing, coastal light pollution has decreased since 1992. As of 2022, the recorded average number of hatchlings that become disoriented every year in Florida is 2,800 turtles, though the number is likely much larger.
As humans become more aware of how their actions impact the environment, protections and conservation measures follow. Perhaps, in a not-too-distant future humans and animals alike can look up to the stars in the night sky regardless of where they live.