Light pollution at night severely disrupts coral reproductive cycle

Last month, researchers at Tel Aviv University found that certain reef-building corals in the Red Sea’s Gulf of Eilat have lost their synchrony, “dramatically reducing” their chances of reproducing.

Bundles release post spawning of the coral Acropora millepora (control corals, no light pollution) (photo credit: CHARLON LIGSON)
Bundles release post spawning of the coral Acropora millepora (control corals, no light pollution)
(photo credit: CHARLON LIGSON)
A study led by researchers from Bar-Ilan University demonstrates how artificial light pollution at night (ALAN) is negatively impacting the reproductive cycle of two coral species.
The study, which was published on Thursday in the journal Current Biology, found that ALAN causes delays and desynchronizes a process which is vital to the successful reproduction of corals.
With the global transition towards LED lighting, which tends to have higher emissions in the blue light spectrum, more near-shore coral reefs could be affected by artificial light, as blue light penetrates deeper into the water. This spectral shift is expected to be amplified by the current rapid population growth in coastal regions. 
The study's results highlight the importance or artificial light being considered in conservation plans for coral reefs near areas of human activity, suggesting that light pollution impact assessment can help incorporate an important variable in coral reef conservation planning.
To shed light on how relevant their findings are on a worldwide scale, the researchers created a first-of-its-kind global map, which highlights areas most threatened by ALAN including the Caribbean Sea and the Pacific and Indian Oceans.
One striking example is the Gulf of Aqaba/Eilat in the northern Red Sea, where considerable urban lighting is present close to shore. There, the least affected area is 47% brighter than a natural night sky, and rises to a maximum of 60 times brighter.
The coral reproductive cycle is controlled by a biological rhythm which is only operational for a few months during every year. At the conclusion, gametes (sperm and eggs) are released into the water for external fertilization.
The mechanism that leads to synchronized spawning is thought to be controlled by both environmental and biological factors. According to the researchers, "successful gamete production and fertilization, development of viable offspring and survival of new coral recruits are possibly the most important processes for replenishing degraded reefs."
LAST MONTH, researchers at Tel Aviv University found that highly synchronized spawning events of certain reef-building corals in the Red Sea’s Gulf of Eilat have completely changed and lost their vital synchrony, “dramatically reducing” their chances of successful fertilization.
In the study, the team of researchers collected and tracked two coral species from the Indo-Pacific Ocean, Acropora millepora and Acropora digitifera.  Ninety colonies were transferred to the Bolinao Marine Laboratory, located in an area in the Philippines with no light pollution. The colonies were placed in outdoor tanks, exposed to natural sunlight, moonlight, and seawater. 
Coral colonies were divided into three groups: two experimental and one control. Each group consisted 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.
For three months the LED lamps were activated every day from sundown until sunrise. The control groups were exposed to the same conditions as the experimental colonies (natural solar light, moonlight phases) but without supplemental light at night.
"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," says the study's lead author Prof. Oren Levy of the Mina and Everard Goodman Faculty of Life Sciences at Bar-Ilan University.
"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," he said.  
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, 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.
Levy hopes in follow-up research to determine whether there are corals more adapted to light pollution and, if so, what mechanism underlies their resilience.