The origin and evolution of coral species richness in a marine biodiversity hotspot*

Abstract

The Coral Triangle (CT) region of the Indo-Pacific realm harbors an extraordinary number of species, with richness decreasing away from this biodiversity hotspot. Despite multiple competing hypotheses, the dynamics underlying this regional diversity pattern remain poorly understood. Here, we use a time-calibrated evolutionary tree of living reef coral species, their current geographic ranges, and model-based estimates of regional rates of speciation, extinction, and geographic range shifts to show that origination rates within the CT are lower than in surrounding regions, a result inconsistent with the long-standing center of origin hypothesis. Furthermore, endemism of coral species in the CT is low, and the CT endemics are older than relatives found outside this region. Overall, our model results suggest that the high diversity of reef corals in the CT is largely due to range expansions into this region of species that evolved elsewhere. These findings strongly support the notion that geographic range shifts play a critical role in generating species diversity gradients. They also show that preserving the processes that gave rise to the striking diversity of corals in the CT requires protecting not just reefs within the hotspot, but also those in the surrounding areas.

Uncovering hidden coral diversity: a new cryptic lobophylliid scleractinian from the Indian Ocean

“…The description of Paraechinophyllia increases the number of coral genera endemic to the northern and/or western Indian Ocean, including Anomastraea, Coscinaraea, Craterastrea, Ctenella, Gyrosmilia, Horastrea, and Sclerophyllia (Veron, 2000;Benzoni et al, 2012a;Arrigoni et al, 2015). This result confirms the evolutionary distinctiveness of corals living in this region (Obura 2012) and strengthens the hypothesis of a second biodiversity hotspot for scleractinian corals besidesafter the Coral Triangle and the adjacent South China Sea (Hoeksema, 2007;Huang et al, 2015Huang et al, , 2017. In particular, two distinct centres of diversity are supposed to occur in the Indian Ocean, the northern Mozambique Channel and the Red Sea (Obura, 2012;Veron et al, 2015).…”

Molecular operational taxonomic units reveal restricted geographic ranges and regional endemism in the Indo‐Pacific octocoral family Xeniidae

“…Whether or not the MOTUs we have defined here represent species, they are at least genetically differentiated populations from which we can begin to understand the biogeographic distribution of genetic variation and the processes that generate biodiversity. Although our sampling of locations across the Indo‐Pacific is relatively sparse so far, the biogeographic pattern that is emerging for xeniids is in general agreement with that shown for scleractinian corals (Huang et al., ; Hughes et al., ), namely that although species richness is highest in the Coral Triangle (represented here by Sulawesi), endemicity peaks in isolated, peripheral regions such as the Red Sea (e.g. DiBattista et al., , ), Madagascar and Western Australia (Roberts et al., ) (Table ).…”

“…DiBattista et al., , ), Madagascar and Western Australia (Roberts et al., ) (Table ). Analyses based on the estimated ages of scleractinian lineages suggest that this pattern can be explained by a combination of higher speciation rates and lower extinction rates in peripheral populations, coupled with range expansion into the centre of diversity (Huang et al., ). In the absence of any fossil record or estimates of lineage ages for the group, whether or not these same dynamics might explain the observed distribution of xeniid biodiversity cannot currently be tested.…”

Digest: Drivers of coral diversification in a major marine biodiversity hotspot*

“…In this issue, Huang et al. () utilize a solid phylogenetic framework based on two sets of trees of coral species: one that uses earlier supertree studies incorporating published molecular data (seven mitochondrial DNA markers) and morphological trees, and one that uses an expanded molecular dataset (seven mitochondrial plus two nuclear DNA regions). In both cases, fossil calibrations were used to date the trees.…”

“…Lastly, Huang et al. () highlight the importance of such evolutionary studies for conservation efforts, as the regions surrounding biodiversity hotspots may be as important to conserve as the hotspots themselves.…”