As noted earlier, the species records are most complete for the stony corals and it is with this group that the Coral Triangle is best delineated (Veron et al. 2009, 2015). The global patterns of species richness for zooxanthellate corals show peak biodiversity within the ecoregions of the Coral Triangle. Sixteen regions of the world have greater than 500 species, and, in total, the Coral Triangle has 605 zooxanthellate coral species of which 66% are common to all ecoregions (Figure 5.3). This diversity amounts to 76% of the world’s total species. More than 80% of all Coral Triangle species are found in at least 12 of the 16 Coral Triangle ecoregions (Figure 5.3). Ninety‐five percent of Coral Triangle species are found in one or more adjacent ecoregions, notably other parts of Southeast Asia, Japan, Micronesia, the Great Barrier Reef, Vanuatu, New Caledonia, and Fiji.
Mushroom corals (Scleractinia, Fungiidae), based predominantly on a taxonomic revision, have a concentration of species in the Coral Triangle, especially in the area comprising Indonesia, the Philippines, and New Guinea (Hoeksema 2007). The shape of this centre overlaps with the position of the continental margin as during low sea‐level stands this margin represented part of the continental coastline where coral reefs occurred. The centre of origin in the Coral Triangle was completely established only about 10 Ma, but its predecessor had existed in the Tethys Sea between Africa and Eurasia since the early Cretaceous (Briggs 2006).
The Coral Triangle, like most of the marine tropics, is threatened by human influence. Pollution, overfishing, habitat destruction, ocean acidification, and climate change are having a severe impact on this heavily populated region (Hoegh‐Guldberg et al. 2009; Peñaflor et al. 2009; Mcleod et al. 2010; McManus et al. 2020). Over the past two decades, SSTs have risen by 0.4 °C (Peñaflor et al. 2009), and much of the area is highly susceptible to rising sea levels (Mcleod et al. 2010), although impacts may differ between subregions (McManus et al. 2020).
FIGURE 5.3 Ecoregions of zooxanthellate corals delineated based on known faunal and/or environmental uniformity. Numbers of species in each ecoregion can be found in Veron et al. (2009).
Source: www.coralsoftheworld.org (accessed 5 January 2021). © Japanese Coral Reef Society.
It is not possible to understand the development of mangroves, seagrass, coral reefs, and the Coral Triangle without understanding the development of the Tethys Sea and its subsequent history (Table 5.1). By about 132 Ma, Eastern and Western Mediterranean sub‐provinces could be distinguished from what was an Indo‐Mediterranean Province. A separate Caribbean Province was formed about 124 Ma, and the endemism that distinguished it reflects the increasing distance between the Old and New World as the Atlantic Ocean became wider (Briggs 2006). During the early Palaeogene (65–45 Ma), many extant families and genera, including the earliest coral reef fish assemblages, evolved in the Indo‐Mediterranean Province. As the climate grew colder, the tropical biota trapped in the Mediterranean was gradually eliminated. This happened after the early Miocene collision between Africa and Eurasia eliminated the Tethys Sea and formed the Mediterranean.
The Tethyan fauna that had become isolated to form the Caribbean Province became divided into the West Central American and Antillean Provinces; the late Cretaceous subdivision of the Caribbean Province may have been a response to the formation of a Central American archipelago, and an early Central American isthmus may have formed at that time. The modern, high‐diversity fauna of the Southern Caribbean was largely derived from the Caribbean Province of the Tethys Sea, although as Briggs (2006) points out, some of it came from the Western Pacific across the East Pacific Barrier before the formation of the isthmus. The East Indies fauna was rich and inherited from the Indo‐Mediterranean Province via the Indian Ocean.
The origin of the great majority of our present species probably took place in the Pliocene and Pleistocene with a large proportion of species richness originating within the two tropical centres. What this means in practical terms is that more than 75% of the Indo‐Pacific reef fish and about 450 species of hermatypic corals were also present in the Coral Triangle.
5.3 Origins Explained
At least six main hypotheses have been put forward to explain the centre of biodiversity. The ‘Centre of Origin’ hypothesis considers the centre of biodiversity to be the centre of origin, with speciation occurring inside the centre with successive outward dispersal to adjacent areas (Briggs 1999). Successive periods of glaciations and low sea‐level stands caused the emergence of barriers and the isolation of populations in deep sea basins in between the island groups of Indonesia and the Philippines, although this idea does not explain the high biodiversity along northern New Guinea. As the hypothesis indicates, however, it is the species that are successful that have given rise to phyletic lines leading to new genera and families. It is relatively easy for species of probable origins within the Coral Triangle to penetrate other tropical regions. Indeed, many species have achieved circumtropical distributions. There is some support based on molecular phylogenetic and biogeographic data to support the centre of origin hypothesis. Molecular data for reef dwarf gobies (Gobiidae: Eviota) indicate that two species complexes contain multiple genetically distinct, geographically restricted, colour morphs indicative of recently diverged species originating in the Coral Triangle (Tornabene et al. 2015). These data also suggest that regional isolation due to sea‐level fluctuations may explain some speciation, but other species show no evidence of physical isolation, thus both allopatric and sympatric speciation may have taken place within the region.
TABLE 5.1 Key tectonic events of the Cenozoic and their effects on the oceans and paleocurrents.
Source: Briggs (1974, 2006) and Lomolino et al. (2016). © John Wiley & Sons.
| Event | Effects |
|---|---|
| Isolation of Antarctica | |
| Early Eocene (50 Ma)‐full deep‐water separation of South Tasman Rise | First indications of global cooling at 50–40 Ma and significant 2°C temperature drops in both the late–middle Eocene and middle–late Eocene boundary. Further isolation of Antarctic marine fauna. |
| Eocene–Oligocene boundary (37 Ma) | Major cooling of both surface and bottom waters by 5°C. Onset of widespread Antarctic glaciation. |
| Opening of Drake Passage (36–23 Ma) | Almost complete isolation of Antarctic marine fauna. |
| Mid‐Miocene (15 Ma)‐full establishment of Antarctic Circumpolar Current | Latitudinal temperature gradient like that of today. Development of Polar Frontal Zone. |
| Closure of Tethyan Seaway | |
|
End of Cretaceous period (75–65 Ma)‐vast circumpolar‐equatorial
|