Obura-Journal_of_Biogeography

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D. O. Obura currently known only from the Red Sea and Gulf of Aden (Arrigoni et al., 2014). S. margaritifera had erroneously been classified in Symphyllia and Cynarina, and its congener S. maxima in Acanthastrea. As with the Coscinaraeids and Euphylliids mentioned above, genetic studies were necessary to overcome the problems associated with morphological classification. The genus Siderastrea is more species rich in the Atlantic than the Indo-Pacific, with until recently four nominal species in the Atlantic and a single species in the Indo-Pacific (Veron, 2000). The Indo-Pacific species (S. savignyana) is now differentiated into two allopatric species, one in the WIO (Oman and Kenya), the other in the West Pacific (Taiwan and Australia) (Chuang, 2006). Divergence between the Indo-Pacific species preceded divergence of the Atlantic and WIO species (Y. Chuang, unpublised data), indicating isolation first across the Eastern Indian Ocean, then by closure of the Tethys Sea. This is corroborated by morphology, where the Indian Ocean species shows characteristics that are more similar to certain Atlantic species than to the Pacific species (Obura et al., 2007). Finally, the genus Stylophora is undergoing heavy revision (Fig. 2i,l). Three branching morphs were distinguished phylogenetically by Stefani et al. (2011) and four by Keshavmurthy et al. (2013), who dated branch divergence to the Eocene (50 Ma) and late Oligocene (30–35 Ma). These studies and Flot et al. (2011) found the greatest species and genetic diversity in the W&NIO, and the ancestral taxa in the group are found in the Red Sea. Moreover two encrusting species, S. wellsi and S. mammilata, are endemic to the Red Sea (Veron, 2000). DISCUSSION An Indian Ocean coral fauna The most recent global biogeographical analysis of corals (Veron et al., 2015) shows a distinct fauna in the Indian Ocean west of 90° E, on the basis of 2–3% of Indo-Pacific corals being restricted to the Indian Ocean (inferring < 20 species). In a more restricted analysis based on the IUCN Red List (2011), Obura (2012a) found a similar distinct Indian Ocean fauna: in a field data set from the W&NIO of 369 species, he found 10% of species to be restricted to the region. Reef fish show a similar pattern of a dominant widespread Indo-Pacific fauna, though with higher levels of regional differentiation; approximately 25% of reef fish species in the Indian Ocean are endemic (Allen, 2008; Briggs & Bowen, 2012). The most recent review of Indo-Pacific fish biogeography (Kulbicki et al., 2013) concurs with Obura’s (2012a) and Veron et al.’s (2015) division between the Indian Ocean and Indo-Pacific faunae, at the large gap in coral reef habitats in the Bay of Bengal, at about 90° E (Fig. 4). This division lies between those proposed in the Marine Ecoregions of the World (Spalding et al., 2007), in which the eastern Indian Ocean is grouped with the W&NIO, and Briggs & Bowen 6 (2012, 2013), in which South Asia and Chagos are grouped with the Central Indo-Pacific (Fig. 4). Focusing on ‘Indian Ocean’ species west of the 90 o E division, two sets of coral species are apparent – those with older origins in the Palaeogene, and those with younger origins in the Neogene (Table 2, Figs 2 & 3). Palaeogene origins The evidence for Palaeogene origins of a Tethyan fauna is provided by seven coral genera (Acropora, Anomastrea, Coscinaraea, Craterastrea, Ctenella, Gyrosmilia and Horastrea; Table 2, Fig. 2). It is based on tectonic, palaeoceanographic and phylogenetic grounds, and is consistent with the theory of tectonic drivers of major biodiversity hotspots in shallow seas (Renema et al., 2008). The mono-specific genera that are endemic to the W&NIO show characteristics that are consistent with expectations for relict species and late stages in the ‘taxon cycle’ (Ricklefs, 2011), where species persist in isolated pockets of once-larger distributions. These include species with highly restricted distributions (e.g. Ctenella chagius, Craterastrea levis) and species with distributions that are wide-ranging but rare (e.g. Gyrosmilia interrupta, Horastrea indica, Anomastrea irregularis). All of these species also have long clade branch lengths. This contrasts with expectations for new species, which should show a contiguous restricted range (Bellwood & Meyer, 2009), and short branch lengths between sister species. These genera also failed to disperse to or survive in the IAA in the late Oligocene/Miocene. Thus, they apparently ‘stalled’ in the EAAP in the Oligocene, failing to take advantage of new evolutionary opportunities available in the tectonically active IAA. Instead, these genera were preserved in the tectonically inactive W&NIO, supporting a hypothesis that they are remnants from the Tethys/EAAP with once broader distributions, having dispersed southwards into the W&NIO due to restricted circulation up to and including in the Oligocene (Table 1). The question arises whether supporting evidence for Palaeogene origins in Indian Ocean taxa is found in other taxonomic groups. The clam subfamily Tridacninae was represented by 5 genera and 15 species in the WT in the Eocene (Harzhauser et al., 2008). Progressive extinction of tridacnines first in the WT in the Eocene, and then in the EAAP in the Oligocene, was associated with closure of the Tethys Sea (R€ogl, 1998), followed by migration to the IAA in the Miocene and Quaternary (Harzhauser et al., 2008). However, tridacnines did not subsequently diversify in the IAA, remaining with two genera and nine species in the Indo- West Pacific (Newman & Gomez, 2000). Among soft corals (Octocorallia), three taxa show some support. The family Melitheidae shows a deep division between W&NIO versus Central Indo-Pacific taxa (Reijnen et al., 2014), and within the former, between western and northern Indian Ocean species (Fig. 4). The order Helioporaceaea has only five species in three genera (Heliopora, Epiphaxum, Nanipora), with origins dating back to the Journal of Biogeography ª 2015 John Wiley & Sons Ltd
Indian Ocean centre of origin Cretaceaous, and little diversification. One species of Epiphaxum was first found in the WT in the Eocene, and the genus shows evidence of speciation prior to closure of the Tethys Sea, with two extant species in the Caribbean and two in the Indo-Pacific (Lozouet & Molodtsova, 2008). However, as with scleractinian corals, traditional taxonomy of soft corals based on macro-morphological characters does not match, and in fact did much to hide, phylogenetic and biogeographical pattern, and the gaps in phylogenetic and palaeo-geographical knowledge of soft corals (McFadden et al., 2010) are still too large for strong support (or rejection) of this hypothesis. Among reef fish, deeper evolutionary relationships from the Eocene and Oligocene are evident in some families (Cowman & Bellwood, 2011), though in contrast with corals, they do not show evidence of higher diversity in the Tethys Sea than in the Indo-Australian region of the time. The continental shorelines of the WIO have an unusually stable tectonic history of some 150 Myr, and though some latitudinal shift has occurred, the coastlines of what is now the Mozambique Channel have changed little over this time (Yoder & Nowak, 2006). The continental crust shorelines (including that of Madagascar) are steep, minimizing the effect of sea level fluctuations on habitat migration, connectivity and speciation/extinction processes (see Potts, 1985). There are no large carbonate platforms in the WIO from the Cenozoic, providing further evidence of the absence of extensive shallow seas and complex habitats associated with active tectonic margins (Wilson & Rosen, 1998; Renema et al., 2008; Cowman & Bellwood, 2011). Finally, the Africa/Madagascar plate has migrated northwards some 15° during the Cenozoic, potentially tracking the narrowing tropical belt during Oligocene cooling (Brass et al., 1982). Consequently, the African and Madagascan coasts, and the Mozambique Channel in particular, may have had an unusually stable climate for much of the Cenozoic, presenting a refuge for species throughout the era. The WIO continental slopes may thus have acted as a museum preserving relict species of the once-dominant Tethyan fauna from the Palaeogene. Neogene origins The evidence for Neogene origins of Indian Ocean corals is shown by four coral genera (Acropora, Coscinaraea, Siderastrea and Stylophora; Table 2, Fig. 3), with Sclerophyllia a possible fifth. It is based on phylogenetic and biogeographical grounds, and is consistent with general drivers of speciation on coral reefs (Cowman & Bellwood, 2011; Bowen et al., 2013). However, the evidence for Neogene origins in Indian Ocean corals is weaker than it is for Palaeogene origins, and will require considerably more systematic and phylogenetic work (e.g. Arrigoni et al., 2012). By contrast, reef fish illustrate these patterns more strongly, and research on reef fish has tended to focus on Neogene speciation, particularly in the Coral Triangle and in hotspots of endemism where these processes are strongest (e.g. Potts, 1985; Carpenter et al., Journal of Biogeography ª 2015 John Wiley & Sons Ltd 2011; Briggs & Bowen, 2012; Bowen et al., 2013; Kulbicki et al., 2013). Within the Indian Ocean the Mascarene Islands and Red Sea have historically been the focus of reef fish biogeographical studies, due to having the highest levels of endemism (Allen, 2008; Briggs & Bowen, 2012), and have been identified as centres of diversity (DiBattista et al., 2013; Postaire et al., 2014), a situation also reflected among corals (Sheppard, 1987; Veron et al., 2015). Not coincidentally, these are the two main regions of geological activity in the Indian Ocean during the Neogene. The Northern Indian Ocean (NIO), and particularly its peripheral seas, the Red Sea, Gulf of Aden and Arabian Sea have been tectonically active during the Neogene (Bosworth et al., 2005). The Red Sea and Gulf of Aden formed by rifting processes beginning some 25–31 Ma, though marine habitats that support coral reefs likely did not appear in the Red Sea until much later, in the Pleistocene (DiBattista et al., 2013). Thus the fauna of the Red Sea and Arabian Sea is likely derived from the broader W&NIO species pool with more recent and smaller scale processes resulting in differentiation from this pool. The origination of species in this northern region is suggested by phylogenetic patterns for younger clades of Acropora (e.g. A. maryae), maximal genetic and species diversity in Stylophora, and in recent restoration of the coral genus Sclerophyllia (Arrigoni et al., 2014; Table 2), and is abundantly supported in phylogeographical patterns of fish (DiBattista et al., 2013). Many species initially described as Red Sea endemics due to the focus of work there (e.g. the corals Pleasiastrea devantieri, Acropora maryae; Veron, 2000) have with further surveys been found to be more widespread in the W&NIO (Obura, 2012a). Vectors for dispersal between the Red Sea/Arabian Sea and the broader Indian Ocean are the northern Indian Oean gyre and the reversing Somali Current, with ‘northern’ species being recorded in locations such as northern Kenya and the northern Seychelles islands (Obura, 2012a), and in the Chagos Archipelago (see Sheppard et al., 2012). The Mascarene Ridge is strung north–south across the middle of the WIO over 20° of latitude, creating a string of shallow banks and emergent islands of varying sizes and configurations over the last 40 Myr (Fig. 1). During the Eocene and Oligocene, with no source fauna in what is now the IAA, the main effect of the island chain may have been to isolate the WIO from open ocean currents from the east, thus enhancing connectivity with the Tethyan hotspots to the north. During the Miocene, however, with an increasingly ‘leaky’ barrier resulting from production of smaller islands by the hotspot, island chain subsidence, northward crustal migration, intensification of westerly equatorial currents, and an actively diversifying source fauna in the IAA, transport of genetic material from east to west would have increased. This may have been enhanced through a steppingstone effect of the Mascarene islands (Postaire et al., 2014) mirroring their role for terrestrial fauna dispersing from Asia to Madagascar (e.g. Warren et al., 2010; Strijk et al., 2012). Nevertheless, this effect may be relatively minor due to the 7
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