Holocene sea level change drives different fates of two Asian horseshoe crab species Qian TANG1, Prashant SHINGATE2, Yusli WARDIATNO3, Akbar JOHN4, Boon Hui TAY2, Laura-Marie YAP5, Jasmin LIM5, Hor Yee TONG6, Karenne TUN6, Byrappa VENKATESH2, Frank RHEINDT1 1National University of Singapore, Singapore; 2Institute of Molecular and Cell Biology, Singapore; 3Institute Pertanian Bogor, Indonesia; 4International Islamic University Malaysia, Malaysia; 5Republic Polytechnic, Singapore; 6National Parks Board, Singapore Overlapping distribution range but different habitat preference Different genetic diversity as evolutionary response The coastal horseshoe crab Tachypleus gigas Study area Bintan, Indonesia Changi Beach, Singapore Preferred habitats: Mangroves, mudflats Preferred habitats: Sandy beaches The mangrove horseshoe crab Carcinoscorpius rotundicauda -0.1 0 0.1 Correlation coefficient Spatial autocorrelation analysis (GenAlEx3) within 200km: CR disperses within 35km TG disperses beyond study area PCA plot: Spatial-genetic correlation in CR Panmixia in TG ADMIXTURE4 bar plots: Subdivision along the SS in CR F coefficient: Relatively low genetic diversity in CR, especially ones at south of SS Effective population size (LinkNe5) over Holocene indicates: Sea level rise and maintain at high level disconnection among habitats CR decline Rapid sea level rise coastal area increase TG increase Sea level maintain rapid mangrove formation in coastal area TG decline Conclusions Capability of population genomic analyses at small spatio-temporal scale may redirect conservation measures. Dispersal capability and habitat preference determine species’ evolutionary response to climate change. Further decline in CR with impending sea level rise, whereas TG may increase but subject to habitat dynamics and human disturbance. Methods ~300 individuals across the Singapore Strait (SS) Hundreds of thousand genomic SNPs using double-digest RADseq Dispersal capability was examined using spatial autocorrelation and resistance mapping. Genetic diversity was examined by calculating heterozygosity and effective population size CR hereafter TG hereafter 35km Upper and lower 95% confidence limits of null spatial structure models are illustrated with red dash lines. Mapping of resistance to dispersal (DResD2) indicates depth- limited dispersal of CR No significant resistance to dispersal of TG (data not displayed) References 1. Sekiguchi (1988). Biology of horseshoe crabs. 2. Keis et al. (2013). Journal of Biogeography 40, 915-927. 3. Smouse & Peakall (1999). Heredity 82, 561-573. 4. Giri et al. (2011). Global Ecology and Biogeography 20, 154-159. 5. Hollenbeck et al. (2016). Heredity 117, 207-216. 6. Bird et al. (2010). Geology 38, 803-806. Introduction Two of four extant horseshoe crab species, widely distributed across Southeast Asia1 Ecological & commercial values Conservation status is still under evaluation Sea level fluctuation (dashed line) and carbon isotope composition (dotted line) in the Singapore Strait during the Holocene6. Mangrove distribution4 is illustrated as blackish coastal areas. Acknowledgement This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Marine Science Research and Development programme (Award No. MSRDP-P19). Field work is supported by the Ah Meng Memorial Conservation Fund. CR CR CR TG TG TG Both Different dispersal capability