Once a year, on cue, corals across a reef release their eggs and sperm into the sea in one of nature's most spectacular synchronized events—but scientists have long wondered what was orchestrating that invisible countdown. Now, researchers tracking reproductive hormones in Red Sea corals over three consecutive years have uncovered a hidden biological rhythm that may reshape how we understand reef resilience and protect them as climate change closes in.
The discovery emerged from a meticulous three-year study conducted in the Gulf of Aqaba, where Ph.D. student Chen Azulay and Professor Maoz Fine of the Hebrew University and the Interuniversity Institute for Marine Sciences in Eilat, alongside Dr. Karine Kleinhaus of Stony Brook University, analyzed the reproductive cycles of Acropora eurystoma, a reef-building coral species. Published in iScience, their findings reveal that corals rely on hormone cycles strikingly similar to those in humans and other animals—a biological echo across evolutionary distance that surprised even the team. "Corals reproduce with incredible timing. We know about the environmental cues involved," Azulay explained. "But we wanted to understand the internal signals within the coral that keep that timing on track."
The researchers' first major finding upended decades of scientific assumptions. Scientists had expected estrogen-like hormones to spike just before spawning, but instead the team discovered 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. The patterns mirror reproductive hormone dynamics in far more familiar animals, yet coral and humans split evolutionarily hundreds of millions of years ago.
Even more striking was what drove these hormonal shifts. In an era when ocean warming dominates reef conservation discussions, sunlight emerged as the dominant driver of hormone levels across all three years of the study. Day length and ultraviolet radiation were stronger predictors of estrogen than seawater temperature—a finding that suggests the internal clocks governing coral reproduction may operate by mechanisms different from what warming-focused research has long assumed. As Dr. Kleinhaus noted, "For decades, researchers have focused primarily on the timing of spawning itself. 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."
Within individual coral colonies, hormone levels were distributed relatively evenly, yet the central portions of corals were far more likely to contain developing eggs than the growing outer edges. This hints that local conditions within a colony—age, energy reserves, developmental stage—may determine which polyps respond to reproductive signals, adding another layer of sophistication to coral biology.
The implications extend beyond fundamental science into conservation strategy. Reef recovery after storms, bleaching events, or other disturbances depends entirely on successful reproduction. The hormone patterns documented here provide a baseline of what healthy reproductive cycles look like—a reference point future studies can use to detect reproductive failure before it becomes visible in the reef itself. As ocean conditions shift, scientists may soon be able to monitor coral reproductive stress through hormone markers rather than waiting for spawning failure to signal that reefs are in trouble. The research team plans to investigate whether similar hormone cycles occur in other coral species worldwide, potentially unlocking a new frontier in reef protection.