Tropical Marine Ecology. Daniel M. Alongi

Some faunal differentiation but no clear high‐diversity loci. Paleogene (65–23 Ma)‐continuity of the tropical Tethyan Ocean Largely homogenous tropical fauna. Major pulse in coral reef development at the end of Oligocene (23 Ma); marked similarities between western Tethys (Mediterranean) and Caribbean/ Gulf of Mexico. Early Miocene (20 Ma)‐closure of Tethyan Seaway by northward movement of Africa/Arabia landmass Westerly flowing tropical current drastically curtailed. Mediterranean Sea excluded from reef belt. Caribbean and eastern Pacific regions become progressively isolated marking beginning of the distinction between IWP and Atlantic Caribbean–East Pacific (ACEP) foci. Further development through the Neogene (<20 Ma) sees relative impoverishment of ACEP and enrichment of IWP. Collision of Australia/New Guinea with Southeast Asia Beginning of Cenozoic era (65 Ma)‐Australia/New Guinea separated from SE Asia by deep‐water gateway Single tropical Tethyan ocean. No differentiation between Indian and Pacific Oceans. Paleogene (65–23 Ma)‐progressive closure of Indo‐Pacific gateway; northward subduction of Indian–Australian lithosphere beneath the Sunda–Java–Sulawesi arcs Mid‐Oligocene (30 Ma)‐gap narrowed but still a clear deep‐water passage formed by oceanic crust Latest Oligocene (25 Ma)‐New Guinea collides with leading edge of eastern Philippines–Halmahara–New Guinea arc system Early Miocene (20 Ma)‐deep water passage between Indian and Pacific Oceans closes Major reorganisation of tropical current systems; new shallow‐water habitats appear in Indonesian region. Early‐late Miocene (20–10 Ma)‐continued northward movement of Australia/New Guinea. Rotation of several plate boundaries and formation of tectonic provinces that are recognizable today Widespread growth of coral reefs in IWP region; huge rise in numbers of reef and reef‐associated taxa; many modern genera and species evolve. Later Neogene (10 Ma–present)‐continued northern movement of Australia/New Guinea; in Early Pliocene (4 Ma)‐a critical point when close contact is made with the island of Halmahara Warm Pacific waters deflected eastward at the Halmahera eddy to form Northern Equatorial Counter Current. Warm waters in ITF are replaced by relatively cold ones from the North Pacific. These changes affect heat balance between East and West Pacific and help promote onset of Northern Hemisphere glaciation. Uplift of Central American Isthmus (CAI) Mid Miocene (15–13 Ma)‐sedimentary evidence of earliest phases of shallowing Still a deep‐water connection at CAI. Latest Miocene–early Pliocene (6–4 Ma)‐continued shallowing to <100 m depth Major effect on oceanic circulation. Gulf Stream begins to deflect warm shallow waters northward to eventually initiate the major “conveyor belt” of deep ocean circulation. Mid‐Pliocene (3.6 Ma)‐further closure of CAI North Atlantic thermohaline circulation system intensifies; Arctic Ocean isolated from warm Atlantic leading eventually to onset of Northern Hemisphere glaciation at 2.5 Ma. Late Pliocene (3 Ma)‐complete closure of CAI Final separation of shallow water Atlantic–Caribbean and Eastern Pacific Provinces. Reverse flow of water through the Bering Strait; this influences Pliocene–Pleistocene patterns of thermohaline circulation in Arctic and North Atlantic Oceans.

There are two basic versions of the centre of origin hypothesis. The first is what Bellwood et al. (2012) have called the ‘spreading dye model’ in which species originating within the Coral Triangle expanded their geographic range by chance. The second version states that new species with superior competitive abilities displaced older, inferior competitors towards the periphery of the region’s boundaries. Thus, in theory, newer species will be at the centre of the boundary and that there would be a “peripheral halo” of predominantly older species that will also have greater geographic range.

      The second hypothesis is the ‘Centre of Overlap’ (or Vicariance) model. This idea states that the centre of biodiversity consists of overlapping distribution ranges that extend into either the Pacific or the Indian Ocean resulting from either larval dispersal or ancient plate tectonics. These biogeographic boundaries are well recognised, but any subsequent expansion or movement of ranges would lead to an overlap of geographic ranges and a localised increase in species richness in the overlap zone (Santini and Winterbottom 2002). The inherent complexity of the physical environment within the Coral Triangle makes it highly likely that this area exhibited many such divisions that were susceptible to frequent changes through time. It may thus be considered a ‘dynamic mosaic’ (Bellwood et al. 2012) of constantly changing distributions driven by continual climatic, geologic, and oceanographic processes. This hypothesis has found some support from reef fish (Woodland 1986), corals (Wallace 2002), crustaceans (Fransen 2007), and gastropods (Reid et al. 2006).

      The third hypothesis is the ‘Centre of Accumulation’ model that is the opposite of the centre of origin hypothesis. It proposes that species arose in peripheral locations, around, or at some distance from the margin of the centre and that they subsequently moved into the centre. New species can be from anywhere outside the centre. It does not require overlap with related taxa. There are two distinct versions of this hypothesis. First, the centre of accumulation by individuals places the emphasis on isolation on peripheral oceanic islands in underpinning the speciation process. Second, the centre of accumulation by faunas reflects the accumulation of entire faunas on moving land masses. In this version, species richness may be enhanced by the merging of entire faunas via the merging of island arcs on the north coast of New Guinea with the numerous land fragments from Asia, Gondwana, and Australia.

      The fourth hypothesis is the ‘Centre of Survival’ idea that is a composite hypothesis emphasising persistence and survival in an area rather than on origin of the species in question. The key aspect of this hypothesis is regional variation in the relative rates of extinction, and it makes no assumptions about the rates or location of origin of species. Speciation may thus have occurred anywhere. The centre of biodiversity is just an area of survival with species extinctions outside the centre’s boundaries. The maintenance of habitat diversity and the availability of sufficient abundance for each species is an important condition for this hypothesis.

      The fifth hypothesis is the ‘Centre of Mid‐Domain Overlaps’ idea that is a variation on the second hypothesis. In this version, a maximum in species richness in the middle of a geographical area has formed by overlying randomised distributions of the locations of individual geographic ranges within species groups. The maximum is predicted to occur where the probability of maximum of species range overlaps is highest, that is, if the ranges are randomly placed within the Indo‐Pacific, the resultant pattern of species richness forms a peak in the middle. The hotspot is thus the result of random placement of species’ geographic ranges (Bellwood et al. 2012).

The sixth hypothesis is ‘Reticulate Evolution’ that is arguably the main mechanism of evolutionary change in most marine taxa (Veron et al. 2009). It recognises “(a) that currents are both genetic barriers (as in vicariance) and paths of genetic connectivity, (b) that species fuse as well as divide over time and space, (c) that species are generally not isolated units