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Light Pollution Stops Coral Spawning

Coral exposed to light pollution are unable to spawn, a new study has found

Israeli and Australian scientists have found that artificial light prevents coral from spawning. 

An impact of light pollution on the health of coral reefs was first suspected in 2007 when researchers discovered that the mass release of eggs and sperm is triggered by moonlight.

The new study led by Bar-Ilan University, in collaboration with the University of Queensland, confirms the effect after a first round of field trials was conducted during a spawning event. 

The researchers were also able to identify genes that are active during mass spawning events, giving new insight into how outside factors influence the reproduction cycle and the synchronisation of the release of spawn. 

'Sexual reproduction is one of the most important processes for the persistence of coral reefs,' says lead researcher Oren Levy from Bar-Ilan University.

Coral only reproduce once a year and monitor external cues in order to increase their chances of success. Moonlight, sea state, salinity, food availability and the colour of twilight all impact spawning events.

For the study, the researchers collected 20 colonies of Acropora millepora coral eight days before they were due to spawn in the southern Great Barrier Reef. Four were left in the field and the remaining 16 were transferred to a research facility where they were exposed to sunlight and moonlight.

Colonies in the aquaria system were divided into three further groups. Six were exposed to natural day and night cycles. Five were exposed to natural light in the day and to artificial light in the evening - from 6.15pm just after sunset, until midnight, from when they were left in the dark. The remaining five were exposed to natural daylight and then left in the dark between 6.15pm and sunrise.

While the colonies exposed to natural conditions spawned at around the same time as those on the reef, the other two groups showed no sign of spawning behaviour. The activity of their genes showed to be disrupted and neither of the groups released sperm or eggs.

The experiment also showed that coral are much more susceptible to light pollution than previously thought. Researchers had assumed that corals would take months to adjust to changing light conditions, however, the current study suggests that disruption can occur within a week of being exposed to changes in nocturnal light.

'If the process is disrupted, we assume reefs might work round it for a few years using the mechanisms they have to cope with a variable world, but we just don’t know what the impact could be on their long-term health,' says first author Paulina Kaniewska from the University of Queensland.

While there is only very little light pollution on the Great Barrier Reef, the northern tip of the Gulf of Aqaba is subject artificial light from the Jordanian and Israeli coastlines.

Study co-author Oren Levy says more research is necessary and other coral species need to be examined but suggests to create a coral reef reserve in the less industrialised parts of the Gulf.

'Protecting the reef in a reserve could help maintain it for the marine life it supports and for sustaining the interest and income from generations of tourists to come.'

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