Thriving populations with low genetic diversity in giant clam species, Tridacna maxima and Tridacna noae, at Dongsha Atoll, South China Sea

Giant clams provide valuable functions to the coral reefs and traditional fisheries in the South China Sea (SCS), but many populations face escalating threats of overexploitation and habitat loss. To provide critical biodiversity data for development of protection and sustainable utilisation strategies, we examine the status of wild tridacnine population at Dongsha Atoll, the largest northernmost atoll of the SCS. This study also examines the genetic patterns of Tridacna maxima and Tridacna noae, based on partial mitochondrial COI gene sequences. We found four species at Dongsha Atoll with an overall density of 3.14 per 100 m2. Over-harvesting may have depleted populations of T. squamosa and Hippopus hippopus, and they may no longer be reproductively viable. Populations of T. maxima and T. noae appear to be thriving and replenished by recruits, but species showed low levels of mitochondrial genetic diversity that could reduce its adaptability and may become further impacted by exploitation and global warming. Haplotype networks for T. maxima and T. noae showed strong significant population structuring consistent with contemporary geographic boundaries: Dongsha Atoll, Taiwan and Philippines-likely established by the intrusion of the Kuroshio Current. Given the contrasting species status and genetic diversity, conservation strategies need to be tailored to increase success of these efforts.