Species composition and genetic diversity of habitat-forming oysters in Hong Kong’s intertidal mudflats

Oysters are one of the most important foundation species in intertidal habitats, yet decay of oyster-related ecosystems was observed globally. To facilitate efforts in oyster conservation, it is imperative to gain solid baseline knowledge on the species composition of oysters in populations of conservation interest. In this study, we present a thorough investigation of the species diversity of oysters in ten of Hong Kong’s intertidal mudflats, together with information on their demographics and population genetics. We also examined the species composition of oysters that were farmed commercially in Hong Kong to identify if aquaculture could be a source of larval recruitment for restoration efforts. All oyster habitats surveyed were comprised of multiple oyster species, and a total of nine species of oysters from the genus Magallana and Saccostrea were encountered. Magallana sikamea was the most prominent oyster species in Hong Kong’s western waters, while M. angulata and M. bilineata were both commonly found in the eastern waters of Hong Kong. To our surprise, M. hongkongensis , which is the dominant species in the aquaculture population (and is farmed in diploid), was not found in the natural intertidal habitats. In nine out of ten sites surveyed, only one cohort of oysters was present, indicating high mortality of the oysters. Importantly, this lack of large, older individuals in natural habitats could undermine both the role of oysters as ecosystem engineers and limit natural recruitment as a source of juveniles for restoration efforts.