Tropical Marine Ecology. Daniel M. Alongi

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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.