BLUE BAY, Mauritius — Every October, Blue Bay in southeastern Mauritius, named for its enticing waters, takes on a pinkish hue. The cause: corals releasing millions of eggs and sperm.

This type of annual synchronized spawning is crucial to the survival of these corals. Now, scientists are pinning their hopes of saving corals battered by climate change on this natural phenomenon. A team of researchers from Mauritius and abroad is preparing to collect millions of larvae from Blue Bay this October.

It’s part of one of the largest projects in the Western Indian Ocean, aimed at restoring corals through sexual propagation.

“The project really starts from scratch,” says Gaëlle Quéré, marine scientist and project lead at Secore International, one of the partners for the Mauritius project launched in 2024. “The main objectives in the first year are to understand which species are spawning and when they’re spawning.”

Secore, short for SExual COral REproduction, is a U.S.-based research nonprofit. The project itself is spearheaded by the Mauritius-based Odysseo Oceanarium, an expansive aquarium showcasing marine species. The three-year undertaking also brings together experts from the University of Mauritius and the Mauritian NGO Eco-Sud.

The initiative aligns with a recent policy push by the Mauritian government to promote coral restoration through sexual propagation. The decision came on the heels of an audit that reported disappointing results for the survival of corals restored through asexual propagation — the most popular approach in coral restoration currently.

Coral species can reproduce both sexually and asexually. Scientists trying to revive coral colonies can choose either approach to boost their numbers and also actively promote species known to be more tolerant of heat.

In March, a bleaching episode affected around 80% of the corals in Mauritian waters, as ocean temperatures reached record highs. Across the planet, corals subjected to intense heat stress suffered bleaching. Oceanic heat waves occur naturally, but in recent decades, human-caused climate change has driven an increase in these events’ frequency and intensity.

The loss of nearly all coral reefs, a threat that could be realized in this century, would shrivel oceans in unthinkable ways. Around a quarter of all marine life congregates on coral reefs for refuge, to feed, or give birth.

Corals, like humans, don’t just exist; they conjure entire habitats around them. Reef-building corals pull calcium and carbon from seawater to build exoskeletons, a kind of limestone platform. Millions of individual coral polyps can grow on these platforms. Within their transparent tissue, they host tiny, colorful algae called zooxanthellae. The algae photosynthesize, nourishing the polyp. But heat stress hampers the algae’s ability to photosynthesize, and they may produce toxic substances in the process. So, during extreme heat the polyps flush out the algae, turning entire reefs ghostly white. If another bout of bleaching hits before the colony can recover, the corals may starve to death.

Ocean acidification — a result of the sheer volume of carbon dioxide the oceans absorb from the atmosphere even as we continue to emit — also limits the availability of raw materials for reef building, specifically carbonate ions. This erodes coral survivability, but the more dramatic and visible damage is from the overheating of their ocean homes.

In recent years, as coral restoration efforts have come under greater scrutiny, some experts have suggested sexual propagation may be the way forward. Hitting on an effective restoration strategy often centers around three questions: Can it be done at scale? Can it be done at a reasonable cost? And does it stand a chance in the face of mounting threats from climate change?

In the Caribbean Sea, another hotspot for coral diversity, Secore tested sexual propagation-led restoration techniques. It reported that the corals it cultivated in nurseries and transplanted back at sea withstood heat stress better than their wild counterparts during a 2023 mass bleaching event.

“The difference with the Caribbean is that in the Western Indian Ocean, there’s very little known about coral reproduction,” Quéré said.

So, for now, the team in Mauritius is focused on knowledge building to help choose donor sites. The October extravaganza in Blue Bay is one of the most promising. The team says it also hopes to uncover other areas where mass spawning occurs, so it can develop a spawning calendar and work in multiple sites.

“When the corals reproduce, they release millions of gametes [eggs and sperm cells] that could lead to millions of embryos and millions of coral babies that you can out-plant,” Quéré said. “This allows us to scale up.”

But the biggest benefit touted by champions of assisted sexual propagation is that the reproduced corals are more genetically diverse compared to those derived from asexual propagation.

In the latter method, a fragment from the mother colony grows into a genetically identical copy of the parent coral. When human beings are involved, they gather or remove these fragments and farm them in nurseries (located on land or at sea).

For projects that rely on asexual propagation, long-term costs add up due to the need to maintain the structures, tend to the young corals, remove harmful algae, and scuba dive to monitor the corals regularly (with the attendant costs of leasing boats, fuel and equipment).

Even with sexual propagation, the costs depend on the configuration. If these efforts rely on ocean nurseries, maintaining those can be costly. Monitoring transplantation sites also come with substantial costs.

Relying on the coral’s natural spawning cycles also has its limitations. The Acropora genus of corals, which are hermaphroditic, are the most common in Mauritius, and they only spawn once or twice a year. An Acropora colony will synchronize the release of bundles of eggs and sperm, which float to the surface and fuse to form embryos. As they mature, they become larvae that then settle onto existing reefs or on the seabed.

By contrast, asexual propagation involves humans gathering coral fragments and planting them in nurseries, something that conservationists can do year-round.

In nature, a large portion of coral propagules don’t survive. They may be eaten, drift off into oblivion without forming a partnership, or fail to settle successfully. This is where human restorers could come in. They can collect the eggs and sperm and provide suitable conditions for them to fertilize and then rear them, either in land-based or sea-based nurseries.

Land-based facilities have the advantage of easy and continuous monitoring at lower costs. However, setting up these labs is expensive.

The Albion Fisheries Research Centre (AFRC), under the Mauritian fisheries ministry, is building a land-based coral nursery that will rely on sexual methods of reproduction under a $10 million U.N. Development Programme initiative.

Secore researchers have found that using sea-based facilities reduces costs. The G20’s Coral Research & Development Accelerator Platform (CORDAP) contributed seed funding — approximately $1.5 million — to launch the Odysseo project.

The nonprofit developed a device called the CRIB, or the Coral Rearing In-Situ Basin, a type of floating nursery placed near where coral gametes are collected.

“You pour the millions of embryos and the gametes you collected into your device … You don’t need to do anything else. They just do their own thing,” Quéré said.

CRIBs boost chances of fertilization and provide a sheltered environment for coral just starting out in life. The nurseries also feature large containers filled with star-shaped seeding units, where the baby corals can attach and grow. They’re designed to prevent algal growth, which is a bane of oceanic nurseries.

However, coral larvae survival rates are significantly lower in CRIBs compared to when corals are reared aboard a sea-based vessel where scientists can create a more controlled environment, according to a study that included researchers from Secore.

The CRIBS are associated with lower costs and easier logistics compared to running a full-fledged lab on a vessel.

Ultimately, however, corals belong in the ocean.

Even with sexual propagation, once embryos develop, the next step is finding the right nook in the sea to place them for good. Secore’s current efforts rely on humans to transplant the seeded units in appropriate sites.

In Mauritius, coral barriers have formed sedate lagoons, which also host fringing corals. It’s easier to place restored corals here, primarily because they’re readily accessible.

But these lagoons are also the center of coastal life, and bear the brunt of human activities. From pollution to sedimentation, there’s a lot that can endanger these corals. Lagoon waters are also likely to get warmer than the open ocean, given that there’s less water flow and the water is shallower. Temperatures can get really high, really quickly.

If temperatures continue rising at the current pace, whether you grow corals asexually or sexually, their ocean homes might still remain as inhospitable.

Banner image: A turkey moray in a coral reef in Mauritius. Image by Johan J.Ingles-Le Nobel via Flickr (CC BY-ND 2.0).

Citations:

Miller, M. W., Mendoza Quiroz, S., Lachs, L., Banaszak, A. T., Chamberland, V. F., Guest, J. R., … Petersen, D. (2024). Assisted sexual coral recruits show high thermal tolerance to the 2023 Caribbean mass bleaching event. PLOS ONE, 19(9). doi:10.1371/journal.pone.0309719

Randall, C. J., Chamberland, V. F., Giuliano, C., Page, C. A., Allen, K., Briggs, N., … Severati, A. (2025). A comparison of in situ and on‐vessel larval rearing for coral seeding. Restoration Ecology, 33(4). doi:10.1111/rec.70001

Cameron, K. A., & Harrison, P. L. (2020). Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration. Scientific Reports, 10(1). doi:10.1038/s41598-020-62366-4

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Jeremy Hance Editor

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BiodiversityCoral ReefsMarineMarine AnimalsMarine BiodiversityMarine ConservationMarine EcosystemsMarine MammalsOceansResearchRewildingWildlifeWildlife ConservationAfricaEast AfricaMauritius

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