Tropical Marine Ecology. Daniel M. Alongi

with decreasing latitude with the greatest richness occurring in Southeast Asia (Hogarth 2015).

5.2 The Coral Triangle

The greatest concentration of marine biodiversity lies in the Coral Triangle (Veron et al. 2009). The area encompasses the nations of Malaysia, Indonesia, Papua New Guinea, the Philippines, and the Solomon Islands (Figure 5.2) and is composed of 21 ecoregions encompassing the eastern Philippines, Palawan/North Borneo, Sulawesi Sea/Makassar Strait, NE Sulawesi, Halmahera, Papua, Banda Sea, Lesser Sunda, Bismark Sea, Solomon Sea, and the Solomon Archipelago. These ecoregions are delineated into four groups that have their own unique (Figure 5.2, violet, brown, orange, and purple) characteristics regarding species richness and connectivity. The Coral Triangle has 605 zooxanthellate corals including 15 regional endemics, amounting to 76% of the world’s total coral species (Veron et al. 2009). Within the triangle, the area of highest biodiversity is the Bird’s Head Peninsula of West Papua (the Papua ecoregion) which hosts 574 species with individual reefs supporting up to 270 species ha⁻¹. Reef fish have a remarkably similar pattern.

      The plankton of the open ocean contains relatively few species in diverse groups, owing to the scarcity of credible barriers. Pelagic organisms have widespread distributions through dispersal; allopatric speciation seems to be unimportant. They seem to live quite well at the edges of their range. However, very rapid species turnover has been established from the fossil record and is most likely the result of sympatric speciation (Norris 2000). Thus, it is unsurprising that there is no unambiguous centre of biodiversity, although planktonic Foraminifera show some evidence of a large circumtropical belt of diversity (Tokeshi 1999).

FIGURE 5.2 Ecoregions of zooxanthellate corals of the Coral Triangle with dendrogram indicating similarity between ecoregions. Numbers of species in each ecoregion can be found in Veron et al. (2009).

      Source: www.coralsoftheworld.org (accessed 4 January 2021). © Japanese Coral Reef Society.

Although the taxonomy is weak, there are at least 35 centres of pelagic fauna in the sea, without distinct boundaries, and the Coral Triangle is only one of these centres (Spalding et al. 2012). There is a latitudinal increase in pelagic species richness from the polar regions to the tropics, but most studies have concentrated on open ocean rather than coastal waters (Angel 1997). Many tropical species have ocean‐wide or circum‐equatorial ranges (Norris 2000). Pelagic biogeographic zones reflect specific hydrographic features, such as particular current regimes, and large‐scale oceanic circulation patterns (Longhurst 2007).

      Corals are not the only benthic organisms to attain peak species richness in the Coral Triangle. Both mangroves and seagrasses have their highest biodiversity in the IWP with significant overlap with the Coral Triangle. That is, mangroves, seagrasses, and corals all show similar distributions as delineated by the 20 °C winter isotherm.

      The biogeography of seagrasses is complex with nine floras, including an Indo‐Pacific and a West Pacific flora whose ranges overlap with the Coral Triangle where the highest concentration of seagrass species is found (Larkum et al. 2018). There are five areas of high seagrass species diversity in the IWP: insular Southeast Asia, Japan and Korea, SW Australia, East Africa, and Southeast India (Green et al. 2003). Green et al. (2003) contend that the Philippines, New Guinea, and Indonesia constitute the centre of biodiversity of seagrasses. The biogeography of seagrasses suggests that they evolved in the Tethys Sea during the late Cretaceous (Larkum et al. 2018).

With respect to benthic infauna and epifauna, many groups show high biodiversity in or overlapping with the Coral Triangle. Molluscs, for instance, have high concentrations of species in the Indo‐Malayan region or a larger part of the central IWP (Briggs 1999). Members of the genus Strombus attain highest diversity in the Philippines and in eastern Indonesia. For gastropods, the highest species numbers have been found in New Guinea and to a lesser degree in Australia (Wells 1990). Only a few species in most gastropod families show widespread Indo‐Pacific distributions.

      Bivalve molluscs show three large regions of over 500 species in the East Indian‐West Pacific, the southern Caribbean, and the tropical eastern Pacific (Clarke and Crame 1997). In the East Indian‐West Pacific, there are two smaller biodiversity hotspots of over 1000 species in which the Philippines and Indonesia are the richest. Species richness patterns of most benthic invertebrates, such as the bivalve molluscs, are reef‐associated in that their biodiversity patterns mirror those of their coral reef hosts. Mollusc families show highest biodiversity in or near the Coral Triangle: the Cerithiidae (Cerithium, Clypeomorus), the Conidae, the Cypraeidae, the Haliotidae, the Littorinidae (Littoraria), the Muricidae (Murex‐Haustellum), the Olividae (Oliva), and the Strombidae. Diversity patterns may differ between coral reef‐associated molluscs and those associated with mangroves and seagrass meadows; on average, there is greater species richness on coral reefs than in the other two habitats.

      Coral‐associated barnacles, decapod crustaceans, sipunculids, fish, and many symbiotic taxa all show biodiversity patterns that mirror corals (Baeza et al. 2013). The decapods are among the most diverse crustacean taxa associated with the coral reefs of the IWP but are outnumbered by members of the Alpheidae (de Grave 2001). Most range overlap occurs in both the Indo‐Philippine and in the Indo‐Malayan regions where highest biodiversity also occurs. Like many other invertebrates, the decapods show a high level of restricted distribution with their coral reef hosts. Thus, their ranges and diversity patterns depend completely on coral reefs.

      The richest marine fish fauna is found in eastern Indonesia, New Guinea, and the Philippines and is directly attributable to high habitat diversity. Allen (2002) considers the eastern Indonesia–southern Philippines corridor as the region of highest biodiversity. Indonesia is an especially rich region with a high concentration of endemic and rare fish species (Roberts et al. 2002). Based on the 20°C winter isotherm, the northernmost boundary for species‐rich faunas is the region between the Indo‐Malayan and the Sino‐Japanese subtropical zone.

      The highest concentration of large Foraminifera species is found in the Indo‐Malayan area where a hotspot has been identified from southern Japan to the Sahul shelf, which includes the Philippines and most of Indonesia (Langer and Hottinger 2000). Foraminifera at the generic level have a biodiversity centre from Borneo to the northern coast of New Guinea. Hoeksema (2007) indicated that the centre of biodiversity for larger benthic invertebrates not only includes the Philippines, eastern Indonesia, and southern Japan but also northern New Guinea.

There have been five major extinction episodes (Veron 1995) involving coral reefs: (i) at the end of the Ordovician (444 Ma), (ii) the late Devonian (375 Ma), (iii) the end of the Permian (251 Ma), (iv) the end of the Triassic (200 Ma), and (v) the end of the Cretaceous (65 Ma). By the end of the Cretaceous, about 70% of corals had become extinct; 10 families of coral survived from this period and 185 genera evolved recently over the last 8 Ma. When the Tethys Sea closed 16 Ma, relict species survived. Following these mass extinctions, a decline in corals and many other faunas occurred. Some important reef‐building corals suffered during the late Devonian with extinctions of stromatoporoids and tabulate corals. The extinction at the end of the Permian brought huge changes to coral history as well as further evolutionary development. The stony corals of the Palaeozoic became extinct, but several genera of calcareous sponges survived to build reefs in the Permian and in the Triassic.

      Modern reefs show great diversity since their last extinction at the end of the Cretaceous with representatives of all phyla and classes found. Corals belong to the class Anthozoa within the phylum Cnidaria and there are more than 6000 species of anthozoans. There are about 1000 species