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Sumatra has some of the most diverse coral reefs in Western Indonesia where marine fauna, and in particular reef fish, share a unique mix of species from the Indian and Pacific Oceans. Of the 2057 reef fish recorded in Indonesia (Allen and Adrim 2003), 60 % have been found in Sumatra. Limitations to genetic exchange between Sumatra and the rest of the Indonesian archipelago are thought to have contributed to the unique assemblages of marine fauna today. Sea level falls of 120 metres during the Pleistocene vicariance caused the Sunda Shelf to emerge into a terrestrial setting (Voris 2000), impeding genetic exchange of marine fauna and flora between west and eastern Indonesia for considerable time periods (A). Marine populations west of the Sunda Shelf were established from the Andaman Sea and Indian Ocean, while east of the Sunda Shelf they were sourced from the Indonesian Throughflow derived from the Pacific Ocean (Hirst and Godfrey 1993, Song et al. 2004, Sathiamurty and Voris 2006) (B). The inundation of the area over time with sea level rise meant that populations could be sourced from larvae to the north, west and east of Sumatra. About 10,000 years ago the passage between Sumatra and Borneo again was opened allowing Pacific Ocean influence to return (C).
Sources of larvae for fish and corals in western Sumatra have been extremely limited by historic sea level falls and poor connectivity with “source” reefs in the Indian Ocean. The consequence for current day marine fauna is that the low survival of coral and reef fish larvae in the past have isolated reefs in Sumatra from genetic mixing and dispersal that limits population growth. In a detailed study of the genetics of the red-belly yellowtail fusilier (Caesio cuning) populations, I have found that the effective population size of this common species in Medan (a region in north east Sumatra) was half that of most of the other Indonesian populations. The findings indicate the low levels of gene flow and limited access of larval dispersal to this area. Her work suggests that this population might be slow to recover from any damage, particularly given that a wide region of the Eastern Indian Ocean was impacted by the tsunami in 2004. The uniqueness of the area is highlighted by new range extensions of coral reef fish in Aceh marine waters with the large mobile reef fish Chlororus rhakoura from the Indian Ocean and the small wrasse Pomacentrus burroughi endemic to the Pacific Ocean recently found by WCS surveys. Based on surveys in northern Aceh from 2005 to 2007, 15% of species found in Aceh are Pacific species, not found in Indian Ocean, and 9% of species are Indian Ocean species not found in the Pacific. It is possible that other range extensions have yet to be recorded in this region, and given the paucity of surveys that new species could be found within the East Indian Ocean Seascape that fringes Sumatra, an area rich in diversity and unique assemblages of marine species. (Pictures by paul barber) References 1.Gerald R. Allen and Mohammed Adrim (2003) Coral Reef Fishes of Indonesia. Zoological Studies 42(1): 1-72. 2.Hirst AC, Godfrey JS 1993 The role of Indonesian Throughflow in a Global Ocean GCM. J. Phys. Oceanogr. 23.:1057–10863.Sathiamurthy E, Voris H 2006 Maps of Holocene Sea Level Transgression and Submerged Lakes on the Sunda Shelf. The Natural History Journal of Chulalongkorn University Supplement 2:1-434.Song Q, Gordon AL, Visbeck M 2004 Spreading of the Indonesian Throughflow in the Indian Ocean. J. Phys. Oceanogr. 34:772-792 5.Voris HK 2000 Maps of Pleistocene sea levels in Southeast Asia: shorelines, river systems and time durations. Journal of Biogeography 27:1153-1167 |
