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This immense biodiversity is supported by scleractinian reef-building corals that create the primary reef structure. Corals have an essential symbiotic partnership with millions of microscopic algae that use sunlight to photosynthesise, and this supplies much of the energy needed for coral polyps to rapidly build their hard skeletons. This symbiosis also provides healthy corals with enough energy to invest in production of eggs and sperm, enabling sexual reproduction and evolution.

SO, WHEN AND HOW DO CORALS REPRODUCE? Until 1980, most corals were assumed to be brooders with eggs fertilized within each polyp, and embryos developing into small planula larvae that are brooded until they are released and settle on the reef. However, in the early 1980s I was part of a team of young researchers that made an unexpected discovery which transformed global understanding of sexual reproduction in corals. We found that many coral species on the Great Barrier Reef (GBR) were not brooding larvae, but instead broadcast spawned eggs and sperm into the sea in highly synchronous mass spawning events on a few nights after full moon periods in late spring and early summer (link to Harrison et al. 1984 Science paper). These annual mass coral spawning events not only involved many colonies from each species, but also many different species from diverse families of corals including branching, plate and massive corals (OG 14 Global Spawning, Harrison).

That discovery catalysed a renaissance in coral reproduction studies in many reef regions, and we now know that most of the more than 450 coral species studied around the world are broadcast spawners, with relatively few additional brooding species discovered. Large synchronous spawning events involving many coral species have now been recorded in other Indo-Pacific reef regions. In contrast, in some regions such as Kenya and the Caribbean, spawning is synchronous within species but few species spawn together. Most coral species studied so far are hermaphrodites that develop both eggs and sperm within each polyp and colony. Other corals have separate sexes and are either female or male, although some mushroom corals are able to change sex between breeding seasons!

 

MASS SPAWNING EVENTS CREATE AN UNDERWATER ‘BLIZZARD’ of trillions of tiny eggs and microscopic sperm, most of which rise to the sea surface where cross-fertilization of eggs and sperm from different colonies occurs. Mass spawning leads to increased genetic diversity and sometimes natural hybridization between coral species. Under calm conditions, vast coral spawn slicks can develop, which can stretch for kilometres and contain billions of developing embryos and larvae. Following our discovery, while swimming through coral spawn slicks at night, the larval restoration concept came to me: by collecting some of these developing embryos we could grow millions of larvae that could be settled back onto damaged reef areas to catalyse the recovery of coral communities.

Coral larvae are usually less than 0.5-1 mm long and most develop at sea for about five to seven days until they are ready to settle. During this period, many larvae drift away from their natal reef and usually only a very small fraction remains alive to settle. A few of the surviving larvae evade the mouths of predators as they drift back onto reefs and find somewhere to settle, and their dispersal enables populations on separate reefs to become genetically interconnected. In some cases, a small number of larvae remain alive for a few months, leading to long distance dispersal over hundreds or potentially thousands of kilometres.

Coral larvae initially attach to suitable areas of dead coral skeletons, often near certain types of encrusting coralline algae that may attract larvae and enhance survival. They then metamorphose into small polyps that grow tentacles and start to calcify and grow their skeletons. Most of these microscopic polyps die within the first weeks and months after settlement. However, once recruits grow into small but visible sized juveniles after about six to nine months, their chances of survival are high. If suitable environmental conditions persist and they are not eaten by predators or succumb to diseases or stress, the coral polyps within each colony continue to grow and asexually bud new genetically identical polyps as the colony expands. When colonies grow large enough, they divert some of their energy into developing eggs and sperm, and the sexual reproductive life cycle is completed.

Under normal conditions, successful sexual reproduction and recruitment enables damaged coral communities to recover from natural disturbances, a process often requiring many years. However, increasing human disturbances such as destructive fishing, pollution and climate change are degrading reefs faster than natural recovery allows, leading to loss of foundation corals and reef function. As a result, reefs are increasingly being transformed from beautiful coral-dominated systems with highly diverse fish and other communities, into seaweed-dominated systems with low biodiversity and greatly reduced ecological and human values.

Degradation of coral reefs has been occurring for decades, but is now accelerating at an alarming rate, leading to global loss of corals and healthy reef ecosystems. Recent estimates suggest that more than 60% of the world’s coral reefs have been badly damaged or severely degraded, with accelerating human population growth and greenhouse gas emissions now threatening the existence of many of the remaining healthy reef systems. Even highly-protected and massive coral reef systems such as the Great Barrier Reef have suffered severe loss of corals in recent decades, exacerbated by recent marine heatwaves resulting in mass coral bleaching and death of many corals. Consequently, the GBR and most other reefs around the world are now increasingly disturbed mosaics of healthy and damaged reef communities. Patches of coral regrowth are evident in some reef areas, but many reefs are failing to recover naturally, therefore active interventions and new approaches to coral and reef restoration are needed. The loss of so many adult breeding corals also greatly reduces fertilization rates and larval supply, resulting in low or no natural recruitment to enable recovery.

SO, WHAT CAN BE DONE TO HALT THE DECLINE OF CORAL COMMUNITIES and reefs, and enable these immensely valuable ecosystems to recover? The biggest threats to coral reefs and most other ecosystems on Earth, are rapidly increasing human populations and climate change caused by unregulated greenhouse gas emissions resulting in increased global warming and marine heatwaves that stress and kill corals and many other reef organisms. Global action on climate change is clearly inadequate, and needs urgent and effective global coordination to reduce greenhouse gas emissions and transform economies through more sustainable energy production. These actions are potentially achievable but unlikely to occur quickly enough to protect the remaining healthy and recovering reefs from further marine heatwaves. Therefore, while we escalate pressure for meaningful and strong global action on climate change, increasing numbers of researchers, managers and organisations are examining ways to actively restore corals on damaged and degraded reefs. The aim is to ‘buy time’ for coral communities while a coordinated response to climate change is planned.

Early attempts to restore coral populations relied mainly on breaking coral branches and transplanting the fragments onto reefs that had lost most of their previous coral communities. Many of these early projects failed to restore corals due to low survival and relatively high costs of maintenance, and most projects were short-term and small-scale. Further development of this coral gardening approach included a nursery phase where corals are grown in aquaria or on structures at sea, resulting in increased survival and growth. These asexual fragmentation methods have the advantages of being relatively simple techniques for training new volunteers and practitioners, and can increase coral cover over shorter timeframes on small reef patches, including tourism sites. More recent longer-term fragmentation projects use of broken ‘branches of opportunity’ after storms and other disturbances, and some have achieved multi-hectare scales.

However, a key disadvantage of fragmentation is that it relies on asexual production of new colonies, and in some cases recent mass coral bleaching events have decimated restored populations which had low genetic diversity. Further research is underway to increase genetic diversity by selecting more heat-tolerant corals that should have a higher chance of surviving future marine heatwaves and bleaching. Additional research is examining methods to simplify the production and ‘outplanting’ of fragments onto damaged reefs, to reduce the relatively high costs of fragmentation and coral maintenance, and increase the scales of successful restoration.

An alternative approach to coral restoration is to take advantage of the increased genetic diversity, adaptation and evolutionary potential resulting from sexual reproduction. Mass spawning events provide predicable access to billions of eggs and embryos that can be cultured to produce many millions of genetically diverse larvae for settlement on degraded reefs. The challenge is to develop new techniques and equipment to successfully manage capturing coral spawn, culturing larvae until they are ready to settle, and deploying these larvae onto target reef areas to increase settlement and recruitment.

MY LARVAL RESTORATION RESEARCH WORK WITH RESEARCH students and colleagues in the Philippines and on the GBR, has shown that coral spawn slicks can be readily collected to produce many millions of larvae for culturing in tanks or at larger scales in floating larval nursery pools on reefs. This research has been supported by the Australian Centre for International Agricultural Research, Great Barrier Reef Foundation, Great Barrier Reef Marine Park Authority, Paul G. Allen Family Foundation, Australian Government’s Reef Trust Reef Restoration and Adaptation Program, and other philanthropic and agency grants.

At smaller scales, larvae can then be released under fine mesh nets temporarily laid over dead corals to retain them on the reef during settlement. Using these approaches, we have successfully restored breeding coral populations on badly degraded reefs areas in the Philippines after two to three years’ growth in many small experimental plots.

As the scale of larval production increases, we have started releasing millions of larvae as ‘larval clouds’ that drift over damaged reef areas to increase larval supply and potentially catalyse coral recovery over larger reef areas. We have also combined ecological and technological approaches to larval restoration, using GPS controlled robots to release millions of larvae over multi-hectare reef areas. Additional research is examining the effects of supplying different densities of larvae onto reefs, and optimising the timing of larval release during periods of low tidal flow and at peak development stages and periods of the day to increase rapid settlement on reefs. 

ONE OF THE KEY CHALLENGES FOR LARVAL RESTORATION is to overcome the natural bottleneck of low rates of survival of the polyps and juvenile colonies after settlement. So, we are exploring innovative methods to enhance the survival of juvenile corals through new mass larval culture methods using spawn collected from corals that survived recent extreme marine heatwaves and mass bleaching events. This should increase the chances of restored coral colonies surviving predicted increases in sea temperatures and other environmental changes while they grow to sexually reproductive size. We are also researching new ways to increase the quality of coral larvae using larval feeding experiments, and by supplying larvae with relatively heat-tolerant types of symbiotic microalgae, and initial results from experiments are promising. Some research groups are also exploring innovative 3D devices to increase initial larval settlement rates, before deploying settled juvenile colonies onto reefs. We are also examining the efficacy of larval restoration in combination with algal removal from inshore reef areas that are now dominated by algae, and working with tourism operators and citizen scientists to simplify methods for restoring corals on badly degraded reefs.

Although a range of different sexual and asexual coral restoration methods are proving successful at smaller scales, we must rapidly expand the scale of operations and increase the cost-effectiveness of restoration to match the much larger scales of coral loss and reef destruction that threaten the GBR and many other reef systems. This problem is time-critical, because each summer season brings with it increased chances of more severe marine heatwaves and mass coral bleaching. Therefore, improving restoration methods and building communities of expert practitioners, traditional owners, reef managers and other stakeholders are essential.

SO, CAN SEXUAL REPRODUCTION HELP SAVE CORAL REEFS?

The answer is yes. Sexual healing can help restore coral reefs and is essential for the maintenance and recovery of coral populations and communities that build reefs. However, we need to ensure that there is even more ‘sex on reefs’ in future, to rapidly restore resilient breeding populations before climate change and other human disturbances overwhelm the capacity of these extraordinary ecosystem engineers to adapt and evolve to rapidly changing reef environments. The future of coral reefs will depend on the success of global action on climate change and managing other human impacts while we develop large-scale restoration to rescue threatened coral communities.

Suggested reading

Peter Harrison and Carden Wallace (2010) Global spawning: Birth of the Reef. Ocean Geographic Issue 14: pages 19-30.

Harrison, P.L., Babcock, R.C., Bull, G.D., Oliver, J.K., Wallace, C.C. and Willis, B.L. (1984). Mass spawning in tropical reef corals. Science, 223: 1186-1189.

Harrison, P.L. (2011). Sexual reproduction of scleractinian corals. In: Z. Dubinsky and N. Stambler (Editors), Coral Reefs: An Ecosystem in Transition Part 3, 59-85, Springer Publishers. ISBN 978-94-007-0113-7.

 

Harrison, P.L., Villanueva, R. and dela Cruz, D. (2016). Coral Reef Restoration using Mass Coral Larval Reseeding. Final Report to Australian Centre for International Agricultural Research, Project SRA FIS/2011/031, June 2016, 60 pages. ISBN 978-1-925436-51-8.

dela Cruz, D. and Harrison, P.L. (2017). Enhanced larval supply and recruitment can replenish reef corals on degraded reefs. Scientific Reports 7: 13985 https://www.nature.com/articles/s41598-017-14546-y

Cameron, K. and Harrison, P.L. (2020). Density of coral larvae can influence settlement, post-settlement colony abundance and coral cover in larval restoration. Scientific Reports 10: 5488. https://www.nature.com/articles/s41598-020-62366-4

Carly J. Randall, Andrew P. Negri, Kate M. Quigley, Taryn Foster, Gerard F. Ricardo, Nicole S. Webster, Line K. Bay, Peter L. Harrison, Russ C. Babcock, Andrew J. Heyward (2020). Sexual production of corals for reef restoration in the Anthropocene. Marine Ecology Progress Series 635: 203-232 https://doi.org/10.3354/meps13206

Boström-Einarsson, L., Babcock, R.C., Bayraktarov, E., Ceccarelli, D., Cook, N., Ferse, S.C.S, Hancock, B., Harrison, P., Hein, M., Shaver, E., Smith, A., Suggett, D., Stewart-Sinclair, P.J., Vardi, T., and McLeod, I.M. (2020). Coral restoration – A systematic review of current methods, successes, failures and future directions. PLoSONE 15 (1), e0226631. https://doi.org/10.1371/journal.pone.0226631

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