Although they cover a mere 0.1% of the ocean floor, coral reefs support about 25% of all marine species (Reaka-Kudla, 1997). Known as the rainforests of the sea, coral reefs have fascinated divers and scientists alike for centuries, beloved for their fabulously complex structures and striking colors. But now a different scene often befalls divers as they dip below the surface: white coral as far as the eye can see, or remnants of a coral reef that once was. This phenomenon in which corals turn white is known as coral bleaching, and is a stress response to water temperatures that get too warm for too long. In fact, thermally induced bleaching is one of the most extensively studied topics in coral reef ecology.

Bleaching can result in widespread coral mortality and loss of coral cover, but not always. Scientists are interested in the different paths corals can take to survive bleaching events and what consequences those strategies may have for the corals’ reproduction (after all, colonies that survive stressful conditions will populate the next generation of corals). The reproductive costs associated with coral bleaching can be dramatic, including pronounced reductions in oocyte (egg cell) size and development (Szmant and Gassman, 1990), raising concerns about the reproductive state of reefs even after thermal stress has subsided. New research, that I and several colleagues published in the journal Scientific Reports, sought to investigate these questions further.  

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In 2019, a massive thermal anomaly struck Mo’orea, French Polynesia, a small island in the southern Pacific Ocean, triggering the most intense bleaching event recorded in the last 30 years. Mother Nature had set up a prime opportunity to investigate the reproductive impacts of bleaching. Our team of scientists tagged and tracked individual Acropora hyacinthus colonies over time, during and after the bleaching event, recording which colonies were resistant to bleaching and which colonies bleached but later recovered. Five months after the bleaching event, we collected samples from all the tagged colonies. Notably, the colonies’ overall appearances were indistinguishable: all colonies were darkly pigmented, suggesting that, perhaps, both the recovered and resistant colonies were healthy at the time of collection. A closer inspection revealed otherwise.

We probed into the coral tissues using reproductive histological techniques, which involves slicing coral tissues very thinly (about the width of a human hair) and examining them under a microscope for signs of potential reproductive output, such as mature oocytes and spermatocytes (sperm cells). We found that colonies that bleached and later recovered harbored four times fewer oocytes compared to their unbleached neighbors. It is well-documented that bleaching can reduce the number of oocytes a colony produces, so this pattern was not wholly unexpected. However, we also observed the same trend in spermatocytes, which are seldom studied in the context of reproduction after bleaching because they require much less resource input on the coral’s part compared to oocytes.

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What is the culprit behind these drastic differences in reproduction between the two observed heat stress responses? Our team believes it is linked to the energetic state of the coral host. We found that previously bleached colonies displayed significantly depleted energy reserves compared to resistant colonies. During bleaching, corals expel the tiny symbiotic microalgae that live in their tissues. These algal symbionts can provide corals with up to 95% of their daily energy requirements by capturing energy from the sun through photosynthesis (Muscatine et al., 1981). Corals that bleach no longer have access to these miniscule food factories and must instead rely on any stored energy reserves, including proteins, carbohydrates, and lipids, to satisfy their metabolic needs (Schoepf et al., 2015). The previously bleached colonies we sampled likely consumed much of their energy reserves to survive through the bleaching event, leaving little behind to provision into developing gametes. 

Our research contributes to the growing body of literature demonstrating the adverse consequences of bleaching on coral physiology and reproduction, but there is still a whole ocean of questions to explore. For example, we still don’t know how long bleaching-induced reproductive impacts persist in Acropora hyacinthus, or most species for that matter. Previous research on a different tropical coral species, Pocillopora meandrina, revealed that suppressed gamete production can last for multiple spawning seasons (Johnston et al., 2020), forecasting a dreary prognosis for future coral reefs. With fewer coral recruits being produced, it may be difficult for reefs to recover from disturbances with high mortality, such as mass bleaching events. On a more positive note, perhaps the heat resistant corals will pass on their genes and seed the reefs with heat tolerant progeny that can effectively cope with environmental stress. The interplay between climate change and coral reproductive success is complex, and scientists are only just beginning to scathe the surface of these intricacies. Further complicating matters is the fact that the costs of bleaching can vary by species, highlighting the need for studies examining reproductive output after bleaching across many species and locations.

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Understanding the extent to which ecological disturbances like mass bleaching perturb coral reproduction is crucial for projecting population and community dynamics. These predictions can then be used to inform effective conservation and management strategies. Our research emphasizes the importance of considering the invisible, sublethal effects of bleaching when considering coral recovery. Although all colonies we surveyed looked healthy, those with a history of bleaching had reduced energy reserves and gamete production, which would have gone undetected in a traditional ecological survey. Only by looking deeper can we hope to discern what climate change may have in store for us in the future. As temperatures soar due to global climate change, mass bleaching events are projected to increase in both frequency and severity (Hughes et al., 2017). It is a race against time to uncover as much as we can and harness that knowledge to protect our precious coral reef ecosystems. 

Read more about the research behind this article at: doi.org/10.1038/s41598-021-02807-w

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