Breakdown in coral spawning places species at risk of extinction

Coral reefs are among the most diverse and productive ecosystems on our planet. But due to climate change and other human stressors, reef-building corals that reproduce by means of broadcast-spawning -- the simultaneous release of eggs and sperm into open water -- may now be under threat of extinction.

In-situ spawning of the studied species. (A) At ~30 minutes prior to spawning, Acropora eurystoma colonies can be
observed in a 'setting' mode, in which egg-sperm bundles are already visible at the mouth-opening of each polyp.
(B) Synchronized whole-colony spawning of A. eurystoma. (C) Partial spawning of Platygyra lamellina without
the formation of egg-sperm bundles, in contrast to (D) Synchronized whole-colony spawning of egg-sperm
bundles by P. lamellina. (E) Acanthastrea echinata spawning and (F) a male Galaxea fascicularis colony.
Scale bars indicate ~1cm [Credit: Tel Aviv University]

A new Tel Aviv University study finds that the highly synchronized, iconic spawning events of certain reef-building corals in the Gulf of Eilat/Aqaba, Red Sea, have completely changed over time and lost their vital synchrony, dramatically reducing chances of successful fertilization.

According to the research, led by Prof. Yossi Loya and PhD candidate Tom Shlesinger of TAU's School of Zoology and published in Science, the breakdown in coral spawning synchrony has led to a dearth of new recruits and stagnant aging populations, creating circumstances for extinction.

"Coral spawning, often described as 'the greatest orgy in the world,' is one of the greatest examples of synchronized phenomena in nature," explains Prof. Loya.

"Once a year, thousands of corals along hundreds of kilometers of a coral reef release their eggs and sperm simultaneously into the open water, where fertilization will later take place. Since both the eggs and the sperm of corals can persist only a few hours in the water, the timing of this event is critical."

Successful fertilization, which can take place only within this narrow time window, has led to the evolution of a precise spawning synchrony. Such synchronicity relies on environmental cues: sea temperature, solar irradiance, wind, the phase of the moon and the time of sunset.

In 2015, the researchers initiated a long-term monitoring of coral spawning in the Gulf of Eilat/Aqaba. Over four years, they performed 225 night field surveys lasting three to six hours each during the annual coral reproductive season from June to September and recorded the number of spawning individuals of each coral species.