Excerpt:
While ocean warming and coral bleaching events make the headlines around the globe, there’s a team of pioneering marine researchers quietly working on new methods to rebuild coral populations more resilient to rising temperatures through groundbreaking means. And it’s all happening in Cuba.
It’s the middle of the night. The sky is dusty white with stars, so many that it’s difficult to distinguish one from another. The full moon hangs low above the ocean’s inky surface and creatures scuttle across the seafloor. Stripy lionfish dance their poisonous dance, fins fanned in dazzling display. And throughout the intricate passageways of an extensive reef, corals get ready to spawn.
Each August, under the cover of darkness, a species of coral at Playa el Coral – off the northern coast of Cuba – releases a cloud of eggs and sperm. The underwater world comes to mirror the sky above, the sea sprinkled with millions of microscopic particles.
Coral species reproduce either through “brooding” or “broadcasting.” The former release fully fertilised juveniles; the others, called broadcast spawners, release sperm and eggs separately. If all goes well, somewhere in the vast water column, a tiny sperm and egg will find each other.
If by a moon dance miracle, the two gametes do connect, they become a planula, or coral larvae. They are carried by currents or settle on the reef below, trying to beat the odds: only 1% of corals survive their first year of life.
Along the two square kilometres of Playa el Coral, hundreds of species reproduce this way: the vibrant purple fan coral, the branching orange elkhorn, and the boulder star coral that encrusts rocks in tiny green polka dots. The scientists and divers who know this spot well all agree: it is one of the healthiest reefs in the Caribbean, if not the world.
Many narratives about coral reefs are centred on bleaching, death, and extinction. Which is, for the most part, accurate. According to the World Economic Forum, 14% of reefs have been lost since 2009. In Australia, over 70% of the Great Barrier Reef has bleached. In Florida, 90% of the reefs – stretching some 350 miles – have disappeared in just the past 40 years.
Many things contribute to reef loss: overfishing and trawling, destructive tourism practices, and pathogens like stony coral tissue loss disease. The biggest threat facing corals, however, is climate change-induced marine heat waves. This type of oceanic warming causes corals to bleach, a process that can end in fatality.
“We have a serious extinction risk for many of these corals,” said Margaret Miller, who works for SECORE International, a non-profit that specialises in coral restoration.
Most of the planet’s over 100,000 square miles of reef are concentrated around the equator, where, for millions of years, corals have historically been happiest. This is mostly because temperature and light conditions were just right for their survival and reproduction. (A few cold water corals are the rare exception.) But the earth’s tropics are growing increasingly inhospitable for the over 6,000 known species of coral.
Corals are animals, communal organisms made up of individuals called polyps, living together in a colony. The foundation of these colonies is the corals’ skeleton, made primarily of calcium carbonate. Covering this limestone skeleton is their tissue, which includes not just coral polyps, but symbiotic algae, roommates of sort, without which the coral could not survive.
These algae, which are called zooxanthellae, provide valuable, life-giving nutrients to the coral. The algae also give coral their colors: without them, the polyps would appear white.
When ocean temperatures get too warm – sometimes by just 1.8 degrees Fahrenheit (1 degree Celsius) – corals become stressed and expel the symbionts that sustain them, exposing the ghost white skeleton beneath through their transparent tissue. Bleached coral doesn’t necessarily mean dead coral, but often, bleaching can be the first step towards mortality.
In 2023, the Caribbean Sea broke records with the average temperature staying above 66 degrees Fahrenheit – 19 degrees Celsius – for nearly two months. This is well above the 20th century average of 57 degrees Fahrenheit (13.9 degrees Celsius). This February, news came from western Australia about a severe bleaching event on the Ningaloo reef, where divers and scientists alike were shocked by water temperatures that skyrocketed above 120 degrees Fahrenheit, roughly 50 degrees Celsius.
In both places, vast stretches of coral bleached and then died. Their limestone foundations became vulnerable to overgrowth by seaweed, a sure sign that a coral is dead. Engulfed in a fuzzy shock of algae, these corals won’t regrow. Instead, they will turn to rubble, piling up in dark mounds on the ocean floor.
Marine biologist Fernando Bretos remembers the first time he saw coral, more than three decades ago, in the Florida Keys. As a teenager, he was awestruck by the underwater world. He recalls how the tissue of pillar coral, a species now facing extinction, was almost fluffy in appearance, jutting up from the seafloor in cylindrical shoots like small buildings, part of a miniature underwater city.
He was smitten. But when Bretos returned to Florida’s Pennekamp State Park in 2018, what he discovered was shocking. “Everything was rubble,” he said. “I could only see their skeletons,” said Bretos of the brain coral that had been thriving there just six months before…
