Cold-Seep Fauna in the South China Sea: Composition and Biogeography

Determining faunal composition is a prerequisite for delineation of biogeography and defining conservation units, but many deep-sea seep- and vent-fauna remain poorly characterized. Over the last two decades, progress has been made in our understanding of the seep fauna in the South China Sea (SCS), filling a knowledge gap in the biogeography of Indo-Pacific chemosynthetic ecosystems. Roughly 40 seep areas have been identified SCS, but only three are active and distributed on the northern continental shelf. Among them, the relatively well-characterized Site F and Haima seeps illustrate different levels of seep development and faunal endemism. Site F is a small area (180 m x 180 m) located in the eastern part, with well-developed authigenic carbonate rocks harboring a community dominated by bathymodioline mussels and Shinkaia squad lobsters shared with the seep- and vent-systems of the Okinawa Trough, Ryukyu Arc and Sagami Bay. By contrast, Haima seeps located in the inner SCS show less-developed authigenic rocks and host more heterogenous epifaunal communities scattered over an area of ~ 3 km, and dominated by vesicomyid clams, siboglinid tubeworms, and bathymodioline mussels, respectively. Identifying the epifauna revealed ~30 species epibenthos from Site F and ~65 from Haima, 12 shared between the two areas, and 4 and 35 appeared endemic to Site F and Haima, respectively. Genetic studies of the bathymodioline mussel, squat lobster, and patellogastropod limpet populations revealed their different levels of connectivity with the respective vent- and seep-populations in the northwestern Pacific, but the SCS populations were always discovered as a distinct genetic group. Genus-level analyses revealed a higher similarity between SCS and North Indian Ocean seep fauna than between SCS and other western Pacific seep or vent fauna, indicating high historical connectivity between seep-fauna in the SCS and Indian Ocean. Stable isotope and genetic analyses unveiled a diversity of chemosymbioses among the epibenthos, with the symbionts being either methanotrophic bacteria or sulfur oxidizing bacteria, and associated with their hosts as endosymbionts and digested by the host cells, or epibionts with or without direct cellular integration with host cells. Overall, the diversity, genetic distinctiveness, different levels of endemism, and different modes of symbioses of in the SCS seep fauna highlight their roles in the global biogeography and the need for their conservation.