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As rising atmospheric carbon dioxide (CO₂) acidifies the world’s oceans, corals—vital architects of marine ecosystems—are among the most threatened. Increased acidity reduces the availability of carbonate ions, the essential building blocks of coral skeletons, while corrosive waters can also dissolve existing reefs. Together, these processes endanger reefs that support a quarter of all marine species.

Yet in Mexico’s Gulf of California, the coral Porites panamensis seems to defy the odds. Thriving in spots naturally enriched with CO₂ from volcanic vents, this modest, honey-brown encrusting coral survives where most others would perish.

"There are some extreme areas where the water is saturated with COâ‚‚, with very low pH—conditions lethal to most other corals," says marine biologist Michael Hellberg of º£½ÇÉçÇø. "They shouldn’t be here, but they are."

This extraordinary stretch of ocean is a true natural laboratory for coral survival and adaptation. Tectonic CO₂ seeps, upwelling currents, and limited water circulation create uniquely challenging conditions. Backed by a new National Science Foundation (NSF) grant, Hellberg and his colleagues—including David Paz-García at CIBNOR, a research center in La Paz, Mexico—are leveraging this environment to discover how the corals persist. Their research focuses on evolutionary differences between males and females, investigating whether sex-specific physiological and genetic responses help corals adapt to environmental stress and perhaps provide an edge as ocean acidification intensifies.

The region’s extremes range from "merely bad" acidity at the Gulf’s mouth to catastrophic conditions farther north, with pH levels so low they can dissolve shells and skeletons. Yet, even here, the corals endure: they look healthy, their symbiotic bacteria remain stable, and their gene expression shifts more with the seasons than with acid levels.

"At first they seem to be taking it all in stride," Hellberg says. "But the data show they’re growing much more slowly." Calcification rates in these acidic populations of P. panamensis can be half those found elsewhere, a slowdown that severely impacts females. In these high-stress conditions, males now far outnumber females, suggesting sex plays a role in tolerance to low pH.

The Gender Divide Under Stress

The reason, Hellberg explains, may lie in energy economics. Female corals are a gonochoric brooder species, meaning they invest in brooding large, yolky eggs inside their bodies—a costly enterprise in a world where calcium carbonate, the raw material of coral skeletons, is increasingly scarce. "Females are already spending more just to reproduce," he says. "When acidification adds another stress, they have less to throw at the problem. Males can just pour energy into it."

That energetic imbalance creates striking population patterns: near the Gulf’s mouth, colonies show a ratio of roughly 1.5 males to every female. In the highly acidified north, it widens to 3 to 1. Whether this skew arises because females die faster—or because larvae "decide" to develop as males in stressful environments—is still unknown.

"We are still working to determine how sex is decided in these corals," Hellberg notes. Scientists are generating whole genomes to test whether sex is set by chromosomes (like the XY system in humans), or if the sex ratio is being skewed by differential mortality or a non-genetic change based on the environment or age (plastic sex determination). Identifying the correct mechanism of sex determination will ultimately help them figure out what influences the ratio.

If true, the result would be evolutionary triage in real time: as oceans acidify, coral populations might adapt through shifts in sex ratios—an unexpected, gendered form of resilience.

Genes, Energy, and the Architecture of Survival

The team’s genomic work probes these survival strategies at the molecular level. Among the genes most active in corals near acid vents are those involved in calcium binding and transport —the machinery that lets corals deposit skeletons. "Those are the genes that are fighting to keep the coral a coral," Hellberg says.

Every coral carries the same set of genes, but not every gene behaves the same in males and females. Sometimes, a gene that benefits males can harm females, or vice versa—a phenomenon called sexual antagonism.

Though it sounds negative, sexual antagonism can be advantageous in evolutionary terms. When different versions of a gene benefit each sex, both tend to persist in the population, maintaining genetic diversity and giving the species more flexibility to adapt to future changes.

The researchers aim to discover whether sexually antagonistic selection is helping corals survive acidification. If males and females have evolved distinct genetic strategies for coping with acid stress, it could enhance resilience at the species level.

To explore this, the team combines field samples from across the Gulf with controlled aquarium experiments in La Paz, where acidity can be adjusted by bubbling COâ‚‚ into seawater. Using gene expression analysis, they compare how males and females activate or suppress different genes under identical stress.

Teaching Machines to Read Coral Skeletons

Identifying male and female colonies remains one of the biggest obstacles. Traditionally, scientists must collect coral fragments, dissolve away the tissue, and spend weeks slicing and staining samples under a microscope. "It’s a huge investment just to say, ‘This one’s male,’" Hellberg says.

To break that bottleneck, the team is pioneering AI-based coral sexing. They’re using computer vision to detect minute structural differences in coral skeletons—subtleties invisible to human eyes. The goal: train an algorithm to recognize those patterns and one day identify sex from fossilized corals, revealing how populations shifted through time. The team anticipates the algorithm will accurately distinguish sexes with over 90% accuracy.

The project’s AI effort will double as a Course-based Undergraduate Research Experience (CURE), involving º£½ÇÉçÇø students in training neural networks with real coral images.

Science with Broader Impact

Beyond the lab benches and microscopes, the team’s work is finding new life in the community. Partnering with local researchers, educators, and artist Ulises Martínez, they are creating a bilingual exhibit and mural that bring the invisible science of ocean acidification to the surface. Hosted at the Museo de la Ballena y Ciencias del Mar, the project transforms complex chemistry into color and story—showing how shifts in the ocean’s balance ripple through coral reefs and the lives they support.

In Baja California Sur, where fisheries and tourism depend on healthy marine ecosystems, understanding how corals adapt to changing ocean chemistry is vital. By connecting ongoing research with community outreach, the project aims to deepen public understanding of ocean acidification and support local efforts to conserve reef ecosystems and the livelihoods they sustain.