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The terrestrial fauna and flora of South-East Asia has been a source of inspiration for biogeography since the field was founded following Alfred Russell Wallace's voyage through the Malay Archipelago in the 19th century (Wallace, Reference Wallace1869). Many marine organisms, including corals and reef fish, also have their global centre of diversity in South-East Asia (Veron, Reference Veron1995; Roberts et al., Reference Roberts, McClean, Veron, Hawkins, Allen and McAllister2002). More recent compilations of the global scleractinian and reef fish fauna have revealed a hotspot within South-East Asia now called the Coral Triangle (Allen, Reference Allen2008; Veron et al., Reference Veron, DeVantier, Turak, Green, Kinnmonth, Stafford-Smith and Peterson2009), bounded by the Philippines to the north, the Solomon Islands to the east and Bali to the west (Fig. 1). Veron et al. (Reference Veron, DeVantier, Turak, Green, Kinnmonth, Stafford-Smith and Peterson2009) further divided the Coral Triangle into 16 ecoregions, each with > 500 scleractinian species. The Coral Triangle contains a total of 605 scleractinian species, 76% of the world's total, including c. 15 (2.5%) endemic to the area (Veron et al., Reference Veron, DeVantier, Turak, Green, Kinnmonth, Stafford-Smith and Peterson2009). The reefs of the Coral Triangle are also amongst those most threatened globally by human activities such as destructive fishing and pollution (Todd et al., Reference Todd, Ong and Chou2010; Fisher et al., Reference Fisher, Radford, Knowlton, Brainard, Michaelis and Caley2011). Consequently, the protection of this hotspot of biodiversity is one of the major goals of marine conservation (Clifton, Reference Clifton2009).

Fig. 1 Indonesia and Papua New Guinea, showing the Coral Triangle and the location of the nine regions used to compare the species richness of Acropora.

The Coral Triangle Initiative is a collaborative conservation project that includes the governments of Indonesia, Malaysia, the Philippines, Papua New Guinea, Timor Leste and the Solomon Islands (Clifton, Reference Clifton2009). Areas outside the Coral Triangle, however, are important given the regional variation of the scleractinian fauna within Indonesia (e.g. Wallace, Reference Wallace1999) but may be overlooked with such intense focus on the Coral Triangle (Fisher et al., Reference Fisher, Radford, Knowlton, Brainard, Michaelis and Caley2011). Sumatra, although part of Indonesia, is not considered part of the Coral Triangle and is therefore not a focus of the Initiative. In particular, the province of Aceh, in northern Sumatra, is likely to be of high biogeographical significance because of its position at the junction of the Andaman Sea, the Indian Ocean and the Straits of Malacca (Brown, Reference Brown2007). Because of political instability over most of the last 150 years Aceh remains almost completely unexplored scientifically (Brown, Reference Brown2007). Neither Veron (Reference Veron2000) nor Wallace (Reference Wallace1999) examined material from Aceh and to the best of our knowledge the area has not be visited by a coral taxonomist since the Dutch colonial period. Consequently, we have a poor understanding of the scleractinian fauna of this region. Data deficient areas, such as Aceh, the Andaman and Nicobar Islands, and the Mergui Archipelago in Myanmar, require biodiversity surveys to facilitate assessment of regional conservation priorities (Brown, Reference Brown2007; Fisher et al., Reference Fisher, Radford, Knowlton, Brainard, Michaelis and Caley2011).

The aim of this study is to describe the shallow water Scleractinia of Pulau Weh, Aceh, to facilitate an assessment of its conservation significance. A species list for Pulau Weh was compiled from 4–8 10-m line intercept transects at 11 sites in 2005 (Baird et al., Reference Baird, Campbell, Anggoro, Ardiwijaya, Fadli and Herdiana2005), six of which were resurveyed in November 2009. The list was supplemented with species encountered incidentally during these surveys. We also compare the species richness of the Acropora of Pulau Weh with another eight regions within Indonesia and Papua New Guinea, five of which are within the Coral Triangle. For this comparison the number of Acropora species was estimated in 40-minute haphazard swims at four sites on Pulau Weh in November 2009 and 3–5 sites in each of the further eight regions (Fig. 1). Sites in these eight regions were surveyed by AHB between 2001 and 2007. A one-way ANOVA followed by Tukey's post-hoc test was used to test for difference in the mean number of Acropora species among the regions. All data were collected by snorkelling, restricting the surveys to depths < 4 m. All species identifications where performed by AHB in the field, from images taken in the field, or from small samples of the colony skeleton with reference to Wallace (Reference Wallace1999) and/or Veron (Reference Veron2000). All reference material for identifications is stored at the Centre for Marine and Fisheries Studies, Syiah Kuala University, Banda Aceh, Aceh, Indonesia.

The scleractinian fauna of Pulau Weh is unique, being a composite of widespread Indo-Pacific species (e.g. Acropora hyacinthus, Acropora humilis, Acropora gemmifera, Acropora digitifera), Indian Ocean species (e.g. Acropora branchi, Acropora roseni), Pacific Ocean species (e.g. Acropora globiceps, Acropora lutkeni, Scapophyllia cylindrica) plus locally abundant species found mainly in Indonesia (e.g. Acropora pinguis, Acropora bifurcata). A total of 133 hermatypic scleractinian species were identified, including three new records for the region (A. branchi, A. roseni, Montastrea serageldini) and five taxa potentially new to science (Appendix 1). Pulau Weh is the western boundary of 13 Pacific Ocean species and the eastern boundary of four Indian Ocean species (Appendix 1). Eight species are found only within the Indonesian archipelago (Appendix 1).

This species composition supports Wallace's (Reference Wallace1999) description of a distinct Sumatran Acropora fauna but the presence of a number of Indian Ocean species and potential endemics suggest that the fauna of Aceh may be even more distinct. The high proportion of potential new species also supports Allen & Adrim's (Reference Allen and Adrim2003) suggestion that the area is a distinct province in the Indo-Pacific. Both Veron (Reference Veron2000) and Wallace (Reference Wallace1999) suggest there are many more species to be found in the area and Wallace & Muir (Reference Wallace and Muir2005) described two endemic species at nearby sites in the Andaman Sea. Similarly, the genetic structure of the giant clam Tridacna crocea indicates that the western Sumatran population is distinct from other regions within Indonesia (DeBoer et al., Reference DeBoer, Subia, Ambariyanto, Kovitvongsa and Barber2008). Furthermore, the relative abundance of different strains of the coral symbiotic algae Symbiodinium in the Andaman Sea is suggestive of a distinct environmental and/ or evolutionary history (LaJeunesse et al., Reference LaJeunesse, Pettay, Sampayo, Phongsuwan, Brown and Obura2010).

The diversity of the Acropora of Aceh is comparable with regions in the Coral Triangle. The mean number of Acropora species per transect at Pulau Weh was similar to five sites in the Coral Triangle and significantly higher than two other sites around Sumatra (Fig. 2). Surveys in Halmahera, in the heart of the Coral Triangle, with a larger geographical scale but similar sampling intensity, recorded 130 scleractinian species (Ardiwijaya et al., Reference Ardiwijaya, Kartawijaya, Setiawan, Prasetia, Yulianto and Herdiana2008), similar to the total of 133 recorded at Pulau Weh.

Fig. 2 Mean number of Acropora species per 10 m line intercept transect at nine regions in Indonesia and Papua New Guinea (Fig. 1). Sites within the Coral Triangle are in grey; sites outside the Coral Triangle are in white. An ANOVA indicated there were significant differences in the mean species richness among sites (F _{(8, 30)}=3.52, P=0.005). Letters indicate groups for which means are not significantly different, as determined by Tukey's post-hoc test.

Hotspots of terrestrial biodiversity generally arise from aggregations of endemic species with narrow ranges. By prioritizing the conservation of such areas both biodiversity and genetic novelty are protected (Roberts et al., Reference Roberts, McClean, Veron, Hawkins, Allen and McAllister2002). In the marine realm, however, areas of high species richness, such as the Coral Triangle, arise from the overlap of species with wide geographical ranges (Connolly et al., Reference Connolly, Hughes, Bellwood and Karlson2005; Veron et al., Reference Veron, DeVantier, Turak, Green, Kinnmonth, Stafford-Smith and Peterson2009). These species-rich areas often contain few endemics. Endemic species in the marine realm are generally more common in isolated regions, such as Hawaii and the Eastern Pacific (Bellwood & Hughes, Reference Bellwood and Hughes2001; Veron et al., Reference Veron, DeVantier, Turak, Green, Kinnmonth, Stafford-Smith and Peterson2009). Consequently, a focus only on areas of high species richness, such as the Coral Triangle, may not be an appropriate approach for the conservation of marine biodiversity, particularly if genetic novelty is also to be preserved (Baird et al., Reference Baird, Bellwood, Connell, Cornell, Hughes, Karlson and Rosen2002). In the marine realm it will be necessary to conserve hotspots and areas with high numbers of endemics, such as Pulau Weh.

In conclusion, the scleractinian fauna of Pulau Weh is unique. It is distinct from other regions in Indonesia and probably has the closest affinity with reefs in the Andaman Sea. The high species richness plus the high proportion of endemics (c. 5%) indicates that the region is of high conservation value. If Pulau Weh is representative of the Andaman Sea an effort similar to that of the Coral Triangle Initiative will be required to conserve the reefs of this area, which include the territorial waters of India, Indonesia, Myanmar, Malaysia and Thailand.