Sara E. Cannon, PhD

This blog post originally appeared on ReefBites, the student blog of the International Society for Reef Studies.

Some corals in the Gulf of Aqaba (also known as the Gulf of Eilat) in the Red Sea are spawning out of sync, says a new paper published in Science by scientists Tom Shlesinger and Yossi Loya from Tel-Aviv University. The corals are also spawning at different time periods than in the past. The authors say that their results should serve as a “wake-up call to start considering these subtler changes in coral survival.”

Corals in the Gulf of Aqaba have never bleached, despite rising sea surface temperatures and acidification. They are often seen as a source of hope for the future of coral reefs, by providing an example of a reef that may be able to survive (and even thrive) under climate change. Unfortunately, this new study suggests that these reefs are affected by rising ocean temperatures after all. It also shows that common metrics of reef health are likely missing important signs of stress on coral reefs.

Most reef-building corals are broadcast spawners, meaning they release their gametes (sperm and oocytes, or egg cells) into the water column, where the sperm and oocytes meet and fertilization occurs. This process usually occurs simultaneously for all corals of a single species on a given reef, which is integral for the gametes to have the best chance of finding each other and fertilizing; once released into the water column, they are only viable for a few hours. The corals rely on a number of environmental indicators to time the release of gametes, including the water temperature, solar irradiance, and wind (cueing the month of spawning), lunar cycles (cueing the night that spawning occurs), and the time the sun sets (cueing the exact hour the corals release their reproductive materials).

Drs. Shlesinger and Loya made these findings after four field seasons (June – September, spawning season for corals in this location, throughout 2015 to 2018) of nightly field observations. They also collected samples from the five most abundant coral species in the Red Sea, in order to analyze the temporal patterns of reproductive traits using different levels: the cell level, the single coral polyp level, the colony level, and the population level.

The authors found that in some coral species, multiple spawning events had occurred each spawning season over the study period, and that these events did not seem to be linked to the lunar phase. This was true across the study site as well as within some single coral colonies. The amount of spawning within each event was also inconsistent, with some colonies spawning completely, others spawning only partially, and some colonies not spawning at all.

The reef is already showing the impacts of this loss of synchronicity. Surveys showed that the corals with the most recruits were those that spawn by brooding (when fertilization occurs internally) and not the broadcast spawners. The broadcast spawners that were releasing their gametes out of sync had less reproductive success: the study sites had less juvenile corals and were instead dominated by older coral colonies.Comparisons of surveys over the past few years showed that individuals of the species that were spawning out of sync had declined over time.

Shlesinger and Loya were able to rule out reef degradation from local human populations as a potential driver of changing reproductive patterns. Instead, they found that corals were spawning out of sync at both highly degraded and more pristine sites. They also found that light pollution was not a factor, as the same patterns of spawning out-of-sync were observed in corals at sites with both high and low levels of light pollution.

Instead, the authors suggest that temperature may be to blame for changing reproductive patterns on these reefs. A number of past studies documented the role that temperature plays on coral reproduction cycles, even suggesting that high temperatures may result in asynchronicity of spawning. Meanwhile, sea surface temperatures in the Gulf of Aqaba are rising fast (at a rate of 0.31°C per decade).

It is also possible that endocrine-disrupting pollutants may be to blame for the out-of-sync spawning; these pollutants can spread over a wide area, which would include the more pristine areas of the Gulf, and can accumulate in marine environments as a result of herbicides, pesticides, plastics, sewage, and other contaminants.

We need more research from other places in order to fully untangle what could be causing asynchronicity in coral spawning. But regardless of the cause, one thing is clear: physiological responses to environmental stressors could be happening unseen, and reliance on typical abundance data from reef surveys could give misleading results. Because corals have long lifespans, surveys that only consider the percent cover of live corals may provide a skewed picture of reef health. Changes in coral reproduction that are caused by environmental degradation, including climate change, may not manifest in the percent cover of reefs for decades, if at all — if brooding corals replaced broadcast spawners, for example, the percent cover of live coral on a reef might remain high.

 As Shlesinger and Loya put it, “without proper assessment of what constitutes reproductive success, we could easily overlook the possibility that species that appear to be abundant may actually be nearing extinction through reproductive failure.”