Light pollution is threatening the success of annual coral spawning, researchers say. Corals release their eggs and sperm based on photosynthetic cues provided by moonlight. However, when artificial light pollution interferes with natural light, corals fail to spawn. This could make matters worse for corals that are already struggling to survive warming waters and bleaching.

"The introduction of artificial light competes with moonlight and can prevent corals from spawning," Dr. Paulina Kaniewska, one of the study researchers from the University of Queensland's (UQ) Global Change Institute, explained in a news release. "Even though corals don't have a brain, they have a spread-out nervous system that allows them to transmit signals in response to sensing changes in light conditions on a cellular level."

Since corals lack any sort of visual system, egg and sperm release are triggered by the photosynthetic cues of natural moonlight. Corals also only reproduce one a year, and in addition to being regulated by moonlight, they carefully monitor their environment for salinity and food availabilty to ensure reproductive success.  

Coral reefs are comprised of thousands of genetically identical polyps that live together as a colony. When egg and sperm cells are released, they combine and develop into larvae, which then settle back on the reef and grown into new colonies. Normally, many coral species spawn simultaneously in order to improve survival rates. Researchers believe a protein similar to melanopsin – a photosensitive molecule that synchronizes a mammal's circadian rhythm with the daily light-dark cycle – is responsible for triggering the release of sex cells in corals.

For their study, researchers from UQ and Bar-Ilan University examined 20 colonies of Acropora millepora coral – one of the dominant coral species that make up the Great Barrier Reef – eight days before they were expected to spawn. While four of the colonies were left in the field, the others were placed in tanks and exposed to various cycles of both natural and artificial light. Coral samples were taken before, during and after spawning.

Compared to those on that were left on the reef, corals exposed to normal conditions in the lab spawned at the same time, but corals exposed to artificial light or left in the dark did not spawn at all. Neither sperm nor eggs were released because coral genes were disrupted.

"Effects of light on the timing of spawning are so important because sexual reproduction is vital to reef survival," Dr. Kaniewska continued. "This research suggests that urban light pollution from excessive artificial light can be a real threat to coral reproduction."

The findings were recently published in the journal eLife.

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