Coral Hybrids : Restoration Revolution or Conservation Catastrophe?

The Good, The Bad and, The Unknown of Hybrids on Caribbean Reefs

Spawning staghorn coral. Spawing is a positive indicator of reef recovery, and an important anual event for coral conservationists. Source

What image does the word hybrid conjure up? Mythological beings like Griffins and Centaurs? Or maybe Ligers and Labradoodles, splices of the worlds most beloved creatures? Either way, you probably don’t think of corals, in their silent underwater “cities”. Whilst coral hybrids may not seem quite as intriguing, in a world where reefs are under constant attack, there’s an important debate over coral hybrids. Are they the future of these invaluable underwater ecosystems? Or are they simply hastening their demise?

Rise of the Hybrids

The onslaught of climate change, over-fishing and, pollution has decimated Caribbean reefs – particularly its two most iconic corals, staghorn (Acropora cervicornis) and elkhorn (Acropora palmata) (1, 2). They are now at the centre of conservation efforts. But there is another coral lurking amongst them, Acropora prolifera. The offspring of staghorn and elkhorn (2). A hybrid. And as its name suggests, it is proliferating rapidly across the Caribbean (2).

For years, scientists believed prolifera hybrids were a recent phenomenon. But, new fossil records place them in small numbers across the Caribbean as long as 120,000 years ago (3). So, what’s changed?

Elkhorn, staghorn and prolifera corals on Caribbean reefs (photo credit: elkhorn: The Positive Reef Initiative, staghorn: NOAA Fisheries, prolifera: Ernesto Weil). Their branching, 3D structures provide essential habitat and protection for all manner of marine life.

Before their decline, these two corals formed dense thickets at different depths (3). During mass spawning, when trillions of eggs and sperm are released in unison, eggs were swamped in their own species’ sperm and the two species rarely mixed (3). But nowadays, as colonies have gotten fewer and farther between (1), unfertilised eggs float further from their parents, increasing their chances of meeting another species sperm. Prolifera are the result of these “accidental fertilizations”. These young hybrids build new colonies, which can also reproduce. As coral cover has declined, more reef real estate has opened up, ripe for colonisation by these not-so-new kids on the block.

Acropora prolifera spawning in the Caribbean (Source, edited by Author). There is debate amongst scientists over the sexual reproduction capabilties of this hybrid species. Note: this coral was mis-identified in the orginal source as staghorn coral.

The Good

Many scientists are firmly in the “prolifera” camp. They see it as a perfect example of hybrid vigour – where hybrid offspring perform better that their parent species (1). The fast spread of prolifera gives this some credence. Like it’s parents, prolifera can clone itself (a coral “magic trick”) to reproduce (1). And its vigour means it can devote all its energy to growth, doing so more rapidly than either of its parents (Fig. 1) (4).

Fig 1. Growth of staghorn (A. cervicornis), elkhorn (A. palmata) and prolifera fragments from a coral nursery in the Bahamas (4). Prolifera far out performed its parent species in this case study.

Staghorn and elkhorn have been devasted by disease but prolifera is noticeably less affected, indicating natural genetic resilience (3). Some conservationists also believe that if disease-resistant prolifera reproduce with their parent species, this may pass back to staghorn and elkhorn populations (1). If this is the case, we may be looking at a reef full of more resilient corals in the future. And this could only be a good thing.

Roatan Marine Park (RMP) is one organisation utilizing prolifera in their coral nurseries. Research co-ordinator Grace Horberry, said this of prolifera in restoration projects:

“Hybrid vigour makes prolifera more successful in coral restoration projects. A higher level of resilience to heat and disease means it can thrive on the reef, where other species can’t. It is able to reproduce rapidly via indefinite cloning. And its ability to sexually reproduce with its parent species, means it could increase the genetic diversity of Acropora coals”

RMP scientists hope that out-planted prolifera will provide the essential habitats that staghorn and elkhorn traditionally provided.

The Bad

So this all sounds great right? An (almost) immortal coral that is more resilient to diseases and stress – the perfect coral-decline solution! Not so fast. There are also some worrying trends, drawing other conservationists into the “nolifera” corner.

Hybrids could push their parents off the reef (3). Higher resilience is allowing prolifera to outcompete staghorn and elkhorn corals for reef real estate (2, 4). They have also been observed growing directly over their parental species, smothering them. If this continues, we may be trading out two ancient and iconic species for their offspring. Of course, if there are no staghorn or elkhorn corals left in the future because of anthropogenic stress, then this becomes a moot point. But if prolifera are part of the problem, then perhaps we should be more wary of their use in conservation.

Whilst hybrids are usually infertile, genetic tests indicate prolifera is fertile – at least in some form. First-generation (F1) hybrids can reproduce with both parents – the back-hybrid features depending on the parent species (Fig. 2) (2). Recent research also suggests that these Caribbean hybrids could self-fertilize (2), as found in some Pacific Acropora hybrids (5). But these second-generation (F2) hybrids may have lower hybrid vigour. This creates a double coral catch-22. The better the F1 hybrids do, the more their genes clutter spawning events, leading to more hybridisation and fewer parent species (1). Then, if they can self-fertilize and there is a rise in F2 prolifera, all the benefits of hybrid resilience may be lost. Along with the species we have been trying to save.

Fig 2. Diagram of the potential fertilisations and offspring that could occur within staghorn- elkhorn – prolifera populations. F1 and F2 hybrids are shown. There are still many unknowns regarding prolifera reproduction and if F2 hybridisation is even possible. (Source: Author, created in Canva. Photo credit: elkhorn: The Positive Reef Initiative, staghorn: NOAA Fisheries, prolifera:Ernesto Weil)

The Unknown

So, revolution or catastrophe?

Well, we don’t really know. As humans continue to cause massive environmental shifts, animals will keep evolving. But like so many evolutionary processes, the outcome of hybridisation remains a mystery. We don’t much about prolifera –  as reef builder, coral competitor or a reproductive partner. Let alone what the resilience and ecosystem value of future generations might look like.

For Caribbean Acropora they could be the saving grace. A new, more resilient species, ensuring the continuation of these beautiful and invaluable ecosystems. Or it could be disastrous, wiping out biodiversity and long-term resilience. And we might be helping it on its way.

The truth is – only time, and more research, will tell.

If conservation measures work, we could preserve these beautiful ecosystems for years to come (photo credit: Patric Lengacher).

References

  1. H. Nylander-Asplin, R. Hill, J. Doerr, L. Greer, N. Fogarty, Population dynamics and genotypic richness of threatened Acropora species and their hybrid in the U.S. Virgin Islands. Coral Reefs 40, 1-7 (2021).. H. Nylander-Asplin, R. Hill, J. Doerr, L. Greer, N. Fogarty, Population dynamics and genotypic richness of threatened Acropora species and their hybrid in the U.S. Virgin Islands. Coral Reefs 40, 1-7 (2021).
  2. A. Aguilar-Perera, R. C. Hernández-Landa, Occurrence of large thickets of Acropora prolifera (Scleractinia: Acroporidae) in the southern Gulf of Mexico. Marine Biodiversity 48, 2203-2205 (2018).
  3. W. Precht, S. Vollmer, A. Modys, L. Kaufman, Fossil Acropora prolifera (Lamarck, 1816) reveals coral hybridization is not only a recent phenomenon. Proceedings of the Biological Society of Washington 132, 40 (2019).
  4. C. VanWynen, M. Hightshoe, N. Fogarty, C. Dahlgren, D. Gilliam, Should Hybrids Be Used in Coral Nurseries? A Case Study Comparing Caribbean Acropora spp. and Their Hybrid in the Bahamas. Frontiers in Marine Science 8, 669966 (2021).
  5. S. Kitanobo, K. Iwao, H. Fukami, N. Isomura, M. Morita, First evidence for backcrossing of F1 hybrids in Acropora corals under sperm competition. Scientific Reports 12, 5356 (2022).
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