It's also a problem in the world's underwater gardens: artificial light from coastal settlements can lure corals on nearby reefs to spawn outside of their optimal breeding season, researchers report. Light pollution data combined with observations show that cnidarians released their germ cells prematurely by up to three days in lighted reef areas. Impaired reproductive synchronization could harm coral reefs, particularly along the densely built-up coasts of the Red Sea, the researchers say.
Where once only the moon and stars shone softly, today street lamps and the like turn night into day. As studies in recent years have shown, this so-called light pollution can have a very negative effect on many living beings. Above all, nocturnal animals such as insects, birds or bats are irritated by the artificial light sources. In addition, light pollution can disrupt the internal clocks and "schedules" of creatures based on day lengths or phases of the moon. For a long time, the focus was on land creatures. However, there are now increasing indications that aquatic ecosystems are also disturbed by incident artificial light. Because in the life of amphibians, fish, corals and the like, lighting conditions can also have an important signaling function.
How do corals spawn in the unnatural shimmer?
The results of the researchers led by Thomas Davies from the University of Plymouth are now contributing to this knowledge. The study builds on an earlier investigation in which the team mapped the world's marine areas most affected by light pollution. This revealed that 1.9 million square kilometers of near-shore water surface down to a depth of one meter are affected by biologically relevant amounts of artificial light radiation. For their current study, the researchers combined this data with information from 2135 observations of mass spawning events in coral reefs.
The tiny coral polyps in a region usually release their germ cells into the water at the same time, so that as many fruitful contacts as possible can result in the water. By combining the data on light pollution and the spawning observations, the researchers were now able to clearly show for the first time: In reef areas that are exposed to biologically relevant amounts of artificial light at night, the corals spawn one to three days earlier than in unpolluted regions.
Fake moonlight signals
As the scientists explain, this is because the cnidarians use the lunar cycle as a guide. The results suggest that the artificial light can change the natural brightness conditions in connection with sunset and moonrise in such a way that a premature signal for spawning is triggered. Spawning on different nights then reduces the likelihood that the coral eggs will be fertilized and survive to produce new pioneers for reef regeneration, the researchers explain. “Corals are vital to the health of the world's oceans, but are increasingly being degraded by human activities. This study demonstrates that not only are changes in the ocean affecting corals, but also the continued development of coastal cities,” says Davies.
The coral reefs in the Red Sea and the Persian Gulf are particularly affected, the researchers say. Because in recent years, the shore areas have been greatly expanded there in many places. The settlement structures are often within sight of the coral reefs, which are only flatly covered by water. “The Red Sea and Gulf of Eilat/Aqaba are heavily impacted by artificial light at night due to urbanization and the proximity of reefs to shore. Urgent measures should now be taken there to reduce the impact on the vulnerable marine ecosystems,” says co-author Oren Levy from Bar-Ilan University in Israel.
Davies suggests: “One might try delaying the turn-on of nighttime lights in coastal regions at certain times to ensure that the natural period of darkness between sunset and moonrise, which triggers spawning, is maintained. This would potentially raise a number of economic and safety issues, but is something we may need to consider in order to give coral reefs the best chance of survival," the researcher said.
Source: University of Plymouth, professional article: Nature Communications, doi: 10.1038/s41467-023-38070-y