A three-year study has cracked open the hidden biology behind coral reproduction, revealing hormone cycles that echo those of humans and other animals, and a new way to detect reef distress before it's too late.
Once a year, on cue, corals across a reef release their eggs and sperm into the sea simultaneously. Coral reproduction is one of nature's most spectacular events. For reefs that are increasingly threatened by warming, pollution, and overfishing, getting that timing right is a matter of survival.
But that release of gametes is only the finale. What happens in the months leading up to that moment has remained largely mysterious.
Now, a team of researchers has uncovered evidence that corals may rely on hormone cycles like those used by many animals, including humans, to prepare for reproduction.
In a new study published in iScience, PhD student Chen Azulay and Prof. Maoz Fine of the Hebrew University and the Interuniversity Institute for Marine Sciences in Eilat and Dr. Karine Kleinhaus of Stony Brook University, tracked reproductive hormones in Red Sea corals over three consecutive years. Their findings reveal a hidden biological rhythm that may help explain how corals coordinate reproduction and how scientists might detect reproductive stress before spawning failures become visible.
"Corals reproduce with incredible timing, we know about the environmental cues involved" said Chen Azulay. "But we wanted to understand the internal signals within the coral that keep that timing on track."
The researchers focused on Acropora eurystoma, a reef-building coral found in the Gulf of Aqaba, a region often described as a potential refuge from ocean warming. By sampling corals throughout their reproductive cycle from 2021 to 2023, the team produced the first multi-year record of steroid hormone dynamics during coral gamete development.
The results were not what the team expected.
Scientists had previously suspected that estrogen-like hormones would peak just before corals spawned. Instead, the researchers found that estrogen levels reached their highest point months earlier, during the earliest stages of egg development, before steadily declining as eggs matured. Meanwhile, progesterone remained relatively stable throughout the reproductive season but surged several months after spawning, suggesting it may help initiate the next reproductive cycle.
Equally surprising: sunlight, not heat, emerged as the dominant driver of these hormone levels. Across all three years, day length and ultraviolet radiation were stronger predictors of estrogen than seawater temperature. In an era of relentless focus on ocean warming, that's a finding worth sitting with.
"For decades, researchers have focused primarily on the timing of spawning itself," said Dr. Karine Kleinhaus. "But there's a whole process beforehand, driven by these familiar reproductive hormones which are remarkable to find in corals, animals so evolutionarily distant from us."
The team discovered another surprise inside individual coral colonies. Hormone levels were distributed fairly evenly throughout the colony, yet the central portions of corals were far more likely to contain developing eggs than the growing outer edges. The finding suggests that local conditions within a colony, such as age, energy reserves, or developmental stage, may determine which polyps respond to reproductive signals.
Beyond advancing basic science, the findings could have practical implications for conservation.
As climate change places increasing pressure on coral reefs worldwide, scientists are searching for ways to detect reproductive failure before it becomes visible. The hormone patterns identified here offer a baseline of what a healthy reproductive cycle looks like, a reference point that future studies could use to ask how reproduction responds when conditions change.
"Successful reproduction is what allows reefs to recover after disturbances," said Prof. Maoz Fine. " The more we understand the biology behind it, the better positioned we are to monitor and protect these reefs as conditions change."
The researchers say future studies will investigate whether similar hormone cycles occur in other coral species and reef regions around the world. If confirmed, the work could reshape scientific understanding of coral reproduction and provide a powerful new tool for reef conservation efforts.