Tropical Marine Ecology. Daniel M. Alongi

(especially ocean currents) rather than biological mechanisms (e.g., competition). Furthermore, reticulate evolution does not deny the existence of Darwinian evolution which could become uppermost were genetic mixing to weaken sufficiently to create isolated gene pools. This would allow evolution to occur through biological selection. Conditions which promote reticulate evolution are at a maximum in the Coral Triangle (sometimes referred to as an ‘evolutionary cauldron’) because of habitat diversity and the ever‐changing complexity of ocean surface currents.”

      Another hypothesis based on fossil and molecular evidence is the idea of ‘hopping hotspots’ (Renema et al. 2008). There is good evidence for the fact that during the past 50 Ma, there have been at least three marine biodiversity hotspots that have moved across half the globe, with their timing and locations coinciding with major movements of the earth’s plates. Based on generic diversity of large benthic foraminifera, three successive movements of biodiversity hotspots have been identified: (a) in the late Middle Eocene (42–39 Ma), (b) in the early Miocene (23–16 Ma), and (c) in the present day (Renema et al. 2008). These are known as the West Tethys, Arabian, and Indo‐Australian Archipelago biodiversity hotspots. During the Eocene, diversity peaked in SW Europe, NW Africa, and along the eastern shore of the Arabian Peninsula, Pakistan, and West India. The fossil record of mangroves and reef corals suggests maximal global diversity in the West Tethyan hotspot. By the late Eocene, the highest diversity was recorded in the Arabian hotspot that has an overlapping taxonomic composition with the earlier West Tethys and the later Indo‐Australian Archipelago hotspots. The Miocene is the most diverse period in Southeast Asia for both large benthic foraminifera and mangroves. Regional uplift during the Arabia–Eurasia collision resulted in the demise of the Arabian hotspot during the middle to late Miocene. The present hotspot appeared with the disappearance of the Arabian hotspot and has been extant at least since the early Miocene (20 Ma) as shown by fossil records of foraminifera, corals, mangroves, and gastropods (Renema et al. 2008).

      It is unlikely that there is a single explanation to account for the Coral Triangle hotspot, although for corals and most major taxa equatorial temperatures and habitat diversity are highly explanatory. The relative importance of any one hypothesis will change with increases in data and methodology. Some faunal groups, such as the stomatopods and the bryopsidale algae, do not conform to any of the hypotheses. What is clear is that there is a biodiversity hotspot in the Coral Triangle in which species richness patterns differ little from random expectations. Also, there are strong correlations between total species richness and a limited range of key environmental factors; the consensus among taxa is that there is a shared evolutionary and geological history with most taxa in the Coral Triangle being primarily of Miocene origin with ancestors from the Cenozoic hotspots of West Tethys and Arabia (Bellwood et al. 2012).

      With respect to the origins of tropical marine biodiversity, it is clear that (i) physical isolation (allopatric speciation) is not the sole mechanism for speciation, (ii) oceanic archipelagos that were thought to be peripheral habitats for speciation are in fact regions that can export biodiversity, and (iii) opportunities are fewer for allopatric speciation in the oceans leaving greater opportunity for speciation along ecological boundaries (Di Martino et al. 2018). Bowen et al. (2013) have emphasised that areas such as the Coral Triangle and the Caribbean produce and export species but can also accumulate biodiversity produced in marginal habitats. This benefit has been dubbed the “biodiversity feedback” (Bowen et al. 2013).

5.4 Marine Ecoregions and Provinces

Marine realms (Figure 5.4a) and ecoregions (Figure 5.4b) have been identified across the marine biome representing what is an ecological characterisation based on water temperature, depth, and substrate rather than historical distributions based on long‐term climatic patterns (Figure 5.4). Some history, however, is preserved in some benthic taxa with limited dispersal ability and some show a latitudinal pattern in species diversity and composition. These latitudinal patterns still appear to represent modern differences rather than historical ones, thus the boundaries between regions have shifted latitudinally with changing ocean temperature during the Pleistocene, and the composition of local assemblages has changed as species’ ranges have expanded or contracted (Spalding et al. 2007).

      In the Tropical Atlantic Realm, there are six ecoregions: Tropical NW Atlantic, North Brazil Shelf, Tropical SW Atlantic, St. Helena and Ascension Islands, the West African Transition, and the Gulf of Guinea. The Western Indo‐Pacific Realm consists of seven ecoregions: Red Sea and Gulf of Aden, Somali/Arabian, Western Indian Ocean, West and South India Shelf, Central Indian Ocean Islands, Bay of Bengal, and the Andaman Sea. The Central Indo‐Pacific Realm has several tropical ecoregions: South China Sea, The Sunda Shelf, Java Transitional, Tropical NW Pacific, Western Coral Triangle, Eastern Coral Triangle, Sahul Shelf, NE Australian Shelf, NW Australian Shelf, and the Tropical SW Pacific. The Eastern Indo‐Pacific Realm consists of six island ecoregions: Hawaii, Marshall, Gilbert and Ellis Islands, Central Polynesia, Cook Islands, and Southeast Polynesia, Marquesas and Easter Island. The Tropical East Pacific Realm consists of both the Tropical East Pacific and Galapagos ecoregions.

      There is a level of provincialism that does reflect the influence of tectonic and oceanographic history on the distribution and origin of lineages. Tropical oceans have been a barrier to the distribution of cold‐water organisms, but there has also obviously been some level of distinction among tropical provinces, at least for some taxa. For instance, there has been a historical subdivision between some Pacific and Indian Ocean species of sea horses (Hippocampus; Lourie et al. 2005) and the mantis shrimp (Haptosquilla pulchella; Barber et al. 2000). Thus, there are true biogeographic divisions across the tropical marine biome that may not necessarily reflect history but certainly reflect current or recent environmental cues and are separated by zones of rapidly changing species composition.

5.5 The Latitudinal Diversity Gradient

Tropical ecosystems hold more than three‐quarters of all species on earth. One of the most pervasive features of life on earth is the increase in species diversity from the poles to the tropics. Prime examples of this gradient in the marine biosphere are the diversity gradients of marine prosobranch gastropods in the eastern Pacific and western Atlantic (Figure 5.5) and zooxanthellate (Veron et al. 2015). The gastropod example (Roy et al. 1998) is based on an analysis of the geographic ranges of 3916 species of gastropods living on the shelves of the western Atlantic and eastern Pacific from the tropics to the Arctic Ocean; western Atlantic and eastern Pacific diversities are similar despite many important historical and physical differences between the oceans. This diversity pattern cannot be explained simply by latitudinal differences in species range‐length, species‐area effects, or recent geologic histories. As pointed out by Roy et al. (1998), one parameter that does correlate significantly with diversity in both oceans is solar energy input, that is, the diversity pattern may be linked to sea surface temperatures; warmer temperatures result in a faster pace of evolution, including speciation.

FIGURE 5.4 Marine realms (a) with ecoregions (b) across the global ocean. 1 = Arctic, 2 = Northern European Seas, 3 = Lusitanian, 4 = Mediterranean Sea, 5 = Cold‐Temperate Northwest Atlantic, 6 = Warm‐Temperate Northwest Atlantic, 7 = Black Sea, 8 = Cold‐Temperate Northwest Pacific, 9 = Warm‐Temperate Northwest Pacific, 10 = Cold‐Temperate Northeast Pacific, 11 = Warm‐Temperate Northeast Pacific, 12 = Tropical Northwestern Atlantic, 13 = North Brazil Shelf, 14 = Tropical Southwestern Atlantic, 15 = St. Helena and Ascension Islands, 16 = West African Transition, 17 = Gulf of Guinea, 18 = Red Sea and Gulf of Aden, 19 = Somali/Arabian, 20 = Western Indian Ocean, 21 = West and South Indian Shelf, 22 = Central Indian Ocean