The Mystery of the Coral Triangle

The Enigma

Coral reefs are prime examples of thriving diversity, none more than the Coral Triangle, which is located in the ocean surrounding the archipelagos of the Philippines and other islands above Australia, pictured in the orange area of Figure 1. For hundreds of years, the Coral Triangle has fascinated scientists. This area contains more species of corals than any other area in the world, but scientists do not know how so many corals came to occupy the Coral Triangle. Recently, a group of scientists led by Danwei Huang came together to investigate four hypotheses on diversity in hopes to find which one is most accurate with regard to the corals of the region.

A map of the area north of Australia where the Coral Triangle is located, along with ecologically relevant and similar areas.
Figure 1. A map depicting the Coral Triangle in orange  and ecologically similar regions surrounding it in blue (Figure 1 of Huang et al. 2018).

What’s in a hypothesis?

In science, a hypothesis is a proposed explanation for how the world works based on initial observations. In the Coral Triangle, scientists do not know what contributes to the richness of species, but have proposed several models for what might be occurring.

One hypothesis, called the center of origin hypothesis, proposes that new species evolve as corals spread out from an initial starting point (Huang et al. 2018). This would mean that youngest species of corals would be found on the center of the Coral Triangle, while the oldest species would be at the edges.

The second hypothesis, the center of survival hypothesis, focuses on extinction rates. It says that species within the Coral Triangle simply do not go extinct as often as species outside the triangle- this would result in an opposite pattern compared to the center of origin hypothesis (Huang et al. 2018). In this case, species in the center of the Coral Triangle would be older than the species surrounding it because they would be surviving for longer.

Another idea is called the center of overlap hypothesis; it states that the diversity of the Coral Triangle is due to the overlap of many species which already exist (Huang et al. 2018). The last hypothesis is the central accumulation hypothesis, which proposes that species expand into the Coral Triangle, and then experience low extinction rates once established.

The Investigation

Huang and his team started by analyzing the genes of the corals. By looking into the genetic histories of different species of corals, they could see how corals were related to each other. Using molecular clocks, they could also determine how old a species was, which was crucial to the center of origin and the center of survival hypotheses. They investigated in two ways; the first was a complete genetic ‘roadmap’ of all 1547 corals found in the Coral Triangle, while the second only looked at one species to observe whether it changed from one area to another.

In addition to looking at the genetic differences between corals, scientists also looked at the geography of the Coral Triangle. This has been well-studied, so researchers were able to look at how the different species of coral were arranged within the Coral Triangle, and used this information in conjunction with their genetic discoveries (Huang et al. 2018 & Hughes et al. 2013). The Coral Triangle is unique in that it is subdivided into 141 different ecosystems, each one with hundreds of coral species (Veron et al. 2009).

And the Winner is:

After conducting all their research, the researchers uncovered a surprising pattern. In the Coral Triangle, species go extinct more quickly than in other areas. Not only this, but there are very few species unique to the Coral triangle. Instead, the variation seen in the Coral Triangle is due to other corals expanding into the region at a rapid rate. This means that none of the hypotheses previously proposed were completely supported, although the center of overlap hypothesis, which states expansion of preexisting species makes the Coral Triangle diverse, is the closest. The center of overlap theory is consistent with the idea of range expansion into the Coral Triangle, but since the theory inherently lacks predictions regarding species variation or extinction rates, the theory is not fully supported.

The scientists proposed that the reason for higher extinction rates is how the area is subdivided into distinct habitats. These habitats can be thought of like islands; each one unique. Because of this, if something bad were to impact one of the habitats where a species was living, it would likely go extinct, since corals are stationary as adults and cannot move to a better environment. This also explains why new species do not appear very often; if corals are constantly spreading in, but dying out quickly, there is no chance for new species to appear.

To Be Continued…

The researchers agree that there is likely something else influencing the species richness of the Coral Triangle, but right now, they do not know what this might be. Huang’s research team proposed that instead of focusing on origins radiating from a central point, it might be more accurate to look at how corals spread, lived, and died during different periods of history. Past events of warming, cooling, and geographic shifts likely influenced the Coral Triangle and surrounding regions, but scientists do not yet know how.

Even beyond this, there is still much to investigate. Does the pattern found in this reef apply outside of the Coral Triangle? How do different species of corals react to life in the Coral Triangle, especially since their life cycles can vary so much? How do the corals spread so easily? There is a very high diversity of other animals in the Coral Triangle; is that diversity related to the diversity of corals, or is something else influencing them? Huang and his team uncovered evidence that the global climate affects how corals can spread–what would climate change mean for the fate of the Coral Triangle (2018)?

Other studies have observed distinct genetic differences between invertebrates along oceanic regions within the Coral Triangle, especially along a region known as Wallace’s line, but do these other animals also experience the high rate of extinction and other patterns observed in corals (Barber et al. 2000)? For example, a team of researchers looked into the diversity of mantis shrimp and discovered that the genetic diversity of the shrimp was remarkably high despite little movement by the shrimp over time (Barber et al. 2000).

I believe the next step in uncovering this mystery is to investigate the alternate proposal given by the scientists, and look at the history of coral expansions. This might also give insight into how the corals affect other organisms within the ecosystem.

One practical application for this knowledge is in marine conservation efforts. Understanding the necessity of outside corals for maintained diversity of species and the high rate of extinction in the region may have impact on the structure of conservation policies. All corals of the region, not just those within the Coral Triangle, would need protection in order to maintain the diversity currently observed. This means that the ecosystems surrounding the Coral Triangle require attention and monitoring as well, even if they do not display the same level of richness.

Additional Learning

For further investigation, please consider looking into the following resources:

A video on coral which includes footage of coral spawning, which is how coral spreads here.

An earlier paper by Paul Barber (2009) on the biodiversity of the Coral Triangle here.

An earlier paper by Danwei Huang (2013) investigating extinction in reef corals here.

A paper on conservation efforts and emphasis on the importance of the Coral Triangle here.

A paper investigating center of origin models in regards to the Coral Triangle here.


References:

Barber, P.H., Palumbi, S.R., Erdmann, M.V., Moosa, M.K. (2000). Biogeography: A marine Wallace’s line?. Nature. 406: 692-693. doi:10.1038/35021135

Huang, D., Goldberg, E. E., Chou, L. M. and Roy, K. (2018). The origin and evolution of coral species richness in a marine biodiversity hotspot. Evolution. 72: 288–302. doi:10.1111/evo.13402

Hughes, T. P., S. R. Connolly, and S. A. Keith. (2013). Geographic ranges of reef corals (Cnidaria: Anthozoa: Scleractinia) in the Indo‐Pacific. Ecology. 94: 1659. doi:10.1890/13-0361.1

Veron, J. E. N., L. M. DeVantier, E. Turak, A. L. Green, S. Kininmonth, M. G. Stafford‐Smith, and N. Peterson. (2009). Delineating the Coral Triangle. Galaxea. 11: 91–100. doi:10.3755/galaxea.11.91