Population genomics on octocorals in marginal environments: resilient but vulnerable refugia for corals under the Anthropocene

Octocorals are an important component contributing to habitat forming that enhances the three-dimensional structural complexity and facilitates benthic–pelagic coupling for marine benthic ecosystems. Studies on octocorals in marginal environments could provide insight to researchers and conservationists in the face of the deteriorating seascape under the Anthropocene. The octocoral communities in Hony -30g Kong persist under marginal living conditions created by freshwater influx from the western Pearl River Estuary, transitioning from the turbid, hyposaline conditions of the western waters to the relatively oligotrophic oceanic waters of the East. This study employed genome re-sequencing to investigate the population structure and genetic diversity of three widely distributed octocoral species, Dendronephthya gigantea, Dendronephthya spinifera, and Echinomuricea spinifera, along the water gradient of Hong Kong to investigate the effect of a marginal turbid environment on the population structure of octocorals. Two octocoral genera exhibited differences in vertical distribution in response to sedimentation and turbidity, in which the soft coral Dendronephthya in western waters were restricted to shallower depths while the gorgonian Echinomuricea could occupy deeper waters in the western region despite heavy siltation. A low level of genetic differentiation was reported in D. gigantea in the turbid western region while no significant population differentiation was observed for D. spinifera and E. spinifera between sampling regions. Moreover, low genetic diversity in terms of nucleotide diversity and heterozygosity with highly negatively skewed Tajima’s D was reported for all three studied octocoral species in Hong Kong. The lack of population differentiation and low genetic diversity could reflect a small founder population selected by turbid marginal conditions settled in Hong Kong and expand the population with the aid of asexual reproduction. Despite their low genetic diversity, which suggests low evolutionary potential and intrinsic vulnerability toward diseases and disturbances, octocoral populations in Hong Kong could be selected under a marginal environment and exhibit resilience to suboptimal conditions. This study underscores the necessity of prioritizing marginal octocoral communities in conservation frameworks, as they represent critical units for understanding species persistence under environmental stress and may serve as essential gene sources for regional biodiversity in an increasingly unstable global climate.