Corals trade growth for survival as oceans warm

Coral reefs are under heavy strain. Heatwaves, pollution, and overfishing chip away at their health year after year. Some hardy corals still hang on, but there’s a catch: survival isn’t the same as thriving.

A new study tracked how a heat-tolerant Red Sea coral copes when warm water sticks around for months. The short answer: it survives. The longer answer: it pays for that survival with slower growth and higher metabolic costs.

Months of heat exposure

The research focused on a northern Red Sea species, Stylophora pistillata – often seen as resilient during hot spells. The team tested how this coral handles elevated temperatures not for days, but for 26 weeks, then watched what happened after a month back at 77°F.

Ann Marie Hulver is the lead author of the study and a former graduate student and postdoctoral scholar in Earth sciences at The Ohio State University.

“In theory, if corals in the wild at these temperatures are smaller, reefs might not be as diverse and may not be able to support as much marine life,” she said. “This could have adverse effects on people that depend on the reef for tourism, fishing, or food.”

Measuring coral survival costs

Corals lived at three steady temperatures: 77°F (baseline), 81.5°F, and 86°F – levels expected in many tropical seas by 2050 and 2100.

The team looked at surface area growth, metabolism, energy reserves, feeding, and the makeup of their algal partners over six months.

Stylophora pistillata survived at both warmer settings, but shrank compared to controls. At 81.5°F, corals were 30 percent smaller.

At 86°F, they were 70 percent smaller. Growth slowed, respiration rose, and energy budgets tightened, even though overall biomass and reserves were maintained.

Survival over reef growth

For the first 11 weeks, warm-water stress barely registered, but by week 26 at 86°F, corals showed higher metabolic demand and reduced growth.

The symbiotic algae community remained stable, indicating the stress wasn’t caused by swapping partners. Encouragingly, recovery was possible – after four weeks back at 77°F, both metabolic demand and growth rebounded, though previously heated corals stayed darker in color than those that had never been warmed.

The findings suggest a clear threshold: corals can endure extended, sub-bleaching heat, but they sacrifice skeletal accretion in favor of tissue maintenance to survive.

“Survival is certainly the number one important thing for coral, but when they’re physiologically compromised, they can’t do that forever,” said Andrea Grottoli, study co-author and professor of Earth sciences at Ohio State.

“So there’s a limit to how long these resilient corals can cope with an ever-increasing warming ocean.”

This trade-off carries real consequences. Reefs support fisheries, tourism, and coasts, but slower coral growth risks flattening structures, disrupting ecosystems, and local economies.

Global reefs under threat

The challenges these corals face aren’t isolated – they reflect a global pattern. As the planet warms, oceans absorb most of the excess heat, driving more frequent and intense marine heatwaves. What used to be temporary stress events are becoming persistent conditions.

This shift raises the stakes for coral reefs. Long-term exposure to warmer water – even below bleaching thresholds – can weaken reef growth, shrink habitat complexity, and reduce biodiversity.

Without serious cuts to greenhouse gas emissions, many reefs may survive in name only, stripped of the structure and life they once supported.

Conserving coral strongholds

“Conservation efforts could focus on areas where resilient coral are present and create protected sanctuaries so that there are some ecosystems that grow as high-probability-success reefs for the future,” Grottoli said.

That matches the study’s takeaway: target protection toward proven strongholds while acknowledging the costs corals pay to endure chronic heat.

Ocean temperatures are expected to climb about 5°F by 2100. The study’s six-month exposure shows that even tough corals survive but do not thrive under elevated temperatures, with growth taking the hit.

The research also highlights a vital lifeline: a cool-down period can restore metabolic balance and restart growth. For reefs, time at cooler temperatures may make all the difference.

The full study was published in the journal Science of The Total Environment.

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