Degree and Pattern of Gene Flow in Several Scleractinian Corals in the Ryukyu Archipelago, Southern Japan

Dispersal distance of planktonic larvae of coral reef organisms is influenced by their ecological characteristics and environmental factors such as current flow and physical structure of reefs. This study reviews the degree and pattern of genetic differentiation in scleractinian corals in the Ryukyu Archipelago, compared with other regions. Small-scale genetic heterogeneity, but broadscale homogeneity, was detected in some species, including brooders and spawners in the Ryukyus. Comparison with other regions indicated that limited gene flow on a small spatial scale (i.e., self-recruitment) seemed to occur in many regions. However, the degree of gene flow over larger distances was complex and species-dependent. With an implication for conservation in the Ryukyus, the larval source hypothesis, which states that coral larvae were recruited from the Kerama Islands to the Okinawa Islands, was consistent with results illustrating high gene flow in some species. Thus, conservation of corals in the Kerama Islands is high priority. Detection of genetic breaks between the southern and central Ryukyus was not common among species. The genetic structure observed in corals is highly variable and depends on both species and spatial scale in the Ryukyus. In addition, the complex genetic structures of corals may be related to coral-specific destructive events, such as bleaching, outbreaks of crown-of-thorns starfish, and disease. Further studies will provide new insights and a more detailed view of the genetic structure of corals by using different markers (e.g., microsatellites) and approaches (assignment tests and clustering analysis), which will provide useful information for coral reef conservation. Connectivity among reefs is important in the conservation and management of coral reefs. The dispersal distance of planktonic larvae of coral reef organisms is influenced by ecological features of the larval species, such as reproductive modes and larval duration periods (Doherty et al. 1995, Ayre et al. 1997, Nishikawa et al. 2003), and by environmental factors, such as current flow (Hohenlohe 2004, Baums et al. 2006) and the physical structure of the reefs (e.g., shoreline or isolated archipelago). For example, some aspects of long-distance migrations can occur via a series of relatively short steps (e.g., between reef patches within a reef and between adjacent reefs) along a continental shoreline, such as the Great Barrier Reef in Australia, but it is difficult for such short-step dispersal patterns to occur in isolated island systems surrounded by oceans, such as Pacific reefs (e.g., Hawai‘i, Marshall Islands, Society Islands). The Ryukyu Archipelago has unique geographic, hydrodynamic, and historical features. The Ryukyu Archipelago is isolated, being located approximately 500 km from the Eurasian continent, and comprises many islands, which consists of short and long steps of island connections (from several kilometers to over 200 km apart) between Taiwan and the Japanese mainland (Figure 1). Pacific Science (2008), vol. 62, no. 3:413–421 : 2008 by University of Hawai‘i Press All rights reserved 1 This study was supported by a grant from the 21st Century Center of Excellence (COE) Program to the University of the Ryukyus. Manuscript accepted 26 August 2007. 2 Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan (e-mail: akira_nishikawa27@