The coast grows brighter by the day with more human settlements introducing artificial lights near beaches, sometimes into the water. Ships, boats, and trawlers all use bright halogen lamps to help navigation and fishing at night. All this human activity brings artificial light, which might be disrupting the reproductive cycle of certain species of coral. 

The daily cycle of light and darkness is crucial to all living beings. In humans, the circadian rhythm is in tune with the sun and allows our body to trigger certain bodily functions during the day and relax involuntary systems during sleep. This is true  for marine organisms too, who depend on this continuous cycle to regulate various physiological, biological and behavioral processes.

A team of researchers from Bar-Ilan University found out that artificial lights along the coastline and light emitted from the underside oif ships and trawlers for navigation is negatively impacting the reproductive cycles of certain species of coral found off the coast of Israel. 

Artificial Light Pollution at Night (ALAN) is negatively impacting the reproductive cycle of two coral species. Reef-building corals in the Red Sea’s Gulf of Eilat have lost their synchrony, “dramatically reducing” their chances of reproducing, according to this article on the Jerusalem Post.

Findings of Study 

The increase in the number of artificial lights produced by powerful street lamps, billboards, sports and industrial facilities, hotels, and office buildings effectively extends the day for work and leisure activities. These lights  introduce reflections, light pollution, excess glare and over-illumination is a major cause of concern for corals. 

Coral reproduction involves gametes being released into the water. They are externally fertilized and settle in a new location to form a colony. This release of sperm and eggs is triggered by seasonal changes which indicate the optimal time to reproduce. The introduction of more artificial light is hampering the synchronicity corals share with their natural ecosystem. 

According to the research paper published in Current Biology, corals, especially the reef-building species rely on environmental triggers and cues to bring about a controlled release which is known as coral spawning. By tracking the two primary reef building species in the Indo-Pacific Ocean, namely the Acropora millepora and Acropora digitifera, the team relocated these native species to the Bolinao Marine Laboratory in Philippines to a secluded area with minimal to no light pollution. Here they used artificial lights to completely disrupt the day-night cycle to study the effects it had on coral reproduction. 

Method of Study 

The researchers placed these corals in an outdoor tank at a depth of 5 meters and divided the corals into three groups with one being a control group. Each group was composed of 15 colonies from each Acropora species divided randomly into three tanks. The experimental groups were treated with LED lamps possessing both cold (yellowish with less blue light) and warm (white with more blue light) spectra, according to the study. 

For a period of three months, the corals were subjected to LED lights of varying brightness which were switched on after sundown until sunrise. This exposed the test coral to light throughout the day, completely disrupting the natural day-night schedule.  The control groups were exposed to the same conditions as the experimental colonies but they only received natural sunlight during the day and were not subjected to artificial LED lights. The health of the three groups was monitored constantly to try and understand the role of light in coral reproduction. 

Results of the Study

The test groups exposed to light throughout the 3-month period showed delayed gametogenesis and also erratic gamete release that decreased the chances of healthy population expansion. The test showed a severe negative impact on the reproductive ability and quality of offspring in reef building coral species. 

“Both key coral species were affected by ecological light pollution. They exhibited asynchrony in the reproductive state which was reflected in the number of oocytes per polyp, gametogenesis, and gamete maturation,” said the study’s lead author Prof. Oren Levy, of the Mina and Everard Goodman Faculty of Life Sciences at Bar-Ilan University in a ScienceDirect article

“This was further reflected at the population level where only corals exposed to natural light cycles succeeded in spawning synchronization. Light treatment with both cold and warm LED’s had a similar impact on the gametogenesis cycle,” added Levy. Levy led the Bar-Ilan study with the participation of the Interuniversity Institute for Marine Sciences in Eilat and Tel Aviv University team members Inbal Ayalon and Dr. Yaeli Rosenberg, and in collaboration with team leader Patrick Cabaitan, from The Marine Science Institute at the University of the Philippines, and light pollution specialists Dr. Christopher Kyba and Dr. Helga Kuechly from the German Research Centre for Geosciences GFZ.

Impact of Findings

The team were able to pinpoint areas with higher than average light pollution where corals might be susceptible to these negative effects. Through the findings, the team can show how even indirect human interaction can have catastrophic effects on coral health and reproduction. The team ascertained that areas including the Caribbean Sea, the Pacific and Indian Oceans could be subject to higher than average ALAN levels. Even the Gulf of Aqaba/Eilat in the northern Red Sea was quoted as an example of how dense human settlements near the coast can cause light pollution that can completely disrupt the natural reproductive cycles of corals. Here, the least affected area is 47% brighter than a natural night sky, and this rises to a maximum of 60 times brighter, meaning the corals in the region are subjected to light 60 times brighter than natural moonlight. For organisms that rely heavily on ces from their surroundings, this is a monumental change. 

The team also identified LED lights to be the most harmful as they have higher amount of light that falls under the blue light spectrum which throws off circadian rhythms in humans too. Smartphones and computer screens produce blue light which impacts our sleep and vision negatively. Near shorelines, LEd lights can trigger massive changes in coral populations. 

This is an important discovery for coral conservation efforts as it allows teams to find areas with minimal light pollution. Coral repopulation efforts require careful planning and execution and this discovery will allow researchers to better help corals flourish in previously areen areas. Also a switch to less severe lighting at night, especially near the coastline could help minimize light pollution which could help protect corals.