5.1 Background

5.1.1 Cold Seeps in the South China Sea

Since the first report of hydrocarbon seeps in the South China Sea (SCS) in 2005, more than 40 seep sites have been discovered, mostly distributed in the northern SCS (Suess et al. 2005; Chen et al. 2006; Niu and Feng 2017; Wang et al. 2022). Among them are three active seep areas—two located off southwest Taiwan (Yam seep and Site F), and the other off Hainan (Haima cold seep) (Fig. 5.1). The Yam seep (Tseng et al. 2023), located on the Four Way Closure Ridge with a depth around 1500–1700 m, is characterized by low seepage intensity. Little is known about the epifaunal community of the Yam seep, except for the presence of the bathymodiolines Gigantidas platifrons and Bathymodiolus securiformis (Klaucke et al. 2016; Kuo et al. 2019). Site F, located at the summit of Formosa Ridge—a small area of about 180 m × 180 m with well-developed authigenic carbonates at water depths around 1120 to1150 m, has been extensively surveyed in recent years (Lin et al. 2007; Feng and Chen 2015). The Haima cold seep is the largest active seep region in SCS, covering an area of around 350 km2 at water depths between 1350 to 1525 m. It is located in the Qiongdongnan Basin at the northwest continental margin of the SCS. Several epifaunal surveys have been conducted at Site F and Haima during the past decade, which have shed light on their epifaunal compositions and potential trophic relationships (Feng et al. 2015, 2018a; Zhao et al. 2020; Ke et al. 2022).

Fig. 5.1
A map of South China Sea for the distribution of active deep-sea hydrocarbon seep fields. They are located at Haima, Site F, and Yam.

Distribution of active deep-sea hydrocarbon seep fields in the South China Sea

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5.1.2 Seep Macrofauna of the South China Sea

Suess et al. (2005) was the first to systematically survey the SCS cold seeps in 2004, but in this cruise only inactive cold seeps were found in the northeastern slopes. He reported evidence of past seep activities, as shown by the extensive development of authigenic carbonate rocks at depths from 500–800 m. He also reported empty shells of bivalves that are supposed to host endosymbiotic chemosynthetic bacteria, including shells from a mud bottom with high concentrations of methane at 3000 m depth in Haiyang 4 area that were later identified as Archivessica nanshaensis (Li et al. 2023), and some vesicomyid clam and shells, and coral skeletons on authigenic carbonate rocks at the Jiulong methane reef.

Since the first discovery of an active cold seep in the SCS in 2007 (Lin et al. 2007; Machiyama et al. 2007), several studies of the cold seep epifauna have been conducted in this region, including Mollusca (Chen et al. 2018; Xu et al. 2019; Lin et al. 2022a), Annelida (Zhang et al. 2018; Xu et al. 2022), Crustaceans (Dong and Li 2015; Li 2015), Echinodermata (Li et al. 2021; Nethupul et al. 2022) and other taxa (Gong et al. 2015). Six studies have attempted to describe the epifaunal communities inhabiting the Site F and Haima cold seep (Feng et al. 2018a; Zhao et al. 2020; Xu et al. 2020; Dong et al. 2021; Ke et al. 2022; Wang et al. 2022). Feng et al. (2018a) presented a list of 30 species from six phyla, including eight species of Mollusca, 12 species of Arthropoda, three species of Annelida, two species of Porifera, one species of Cnidaria and four species of Chordata. They found only three common species (Gigantidas platifrons, Bathyacmaea lactea and Munidopsis verrilli) in two seep areas. Zhao et al. (2020) expanded the list of epibenthic animals at Site F to 28 species, including 10 new records. In the Haima region, Xu et al. (2020) reported six seep sites with various methane seepage strengths, and different macrofaunal communities, including HM-1 with clusters of Archivesica marissinica and empty Archivesica shells along with mussels and holothuroids, HM-2 with a dense G. haimaensis mussel bed and scattered Paraescarpia tubeworms at the edge, HM-3 with massive mussel beds and Phymorhynchus buccinoides snails, HM-4 with authigenic carbonates with empty Archivesica shells and a few holothuroids, HM-5 with carbonate mounds as a deep-sea fish habitat, and HM-6 with several patches of mussels. They reported species from 11 families and seven classes observed by video or photographic records. Dong et al. (2021) reported a total of 34 species, including 12 species of Mollusca, seven species of Arthropoda, two Annelida and one species of Echinodermata. More recently, Ke et al. (2022) reported a total of 30 species, including 13 species of Mollusca, six species of Arthropoda, seven species of Annelida and three other taxa. Among these, six were new records from the Haima area.

Besides reporting the biodiversity of cold-seep macrofauna, several studies have determined their stable isotopes (i.e., δ13C, δ15N, δ34S) (Feng et al. 2018b; Lin et al. 2020; Zhao et al. 2020; Ke et al. 2022; Wang et al. 2022), which contributed our understanding of the trophic modes of seep specialists and habitat generalists. For instance, Zhao et al. (2020), using a two-source stable isotope mixing model, estimated that methane contributed from 30% in the king crab Lithodes longispina to 91% in the mussel Gigantidas platifrons to the carbon pool of the common animals at Site F. Lin et al. (2020) constructed trophic models on seep and non-seep fauna off Taiwan. They found that the seep megafauna had specialized diet niches, whilst the seep-associated L. longispina was the top predator. Moreover, they also indicated that this king crab utilized energy from the neighboring deep-sea ecosystems.

Studies have also been conducted to unveil the genetic divergence and connectivity of seep macrofauna within the SCS (Yao et al. 2020; Xu et al. 2021). Therefore, more information on the reproductive strategies, larval biology and life history of the seep- and/or vent-endemic species, coupled with better deep-sea hydrodynamic models, is crucial to assess the population connectivity of deep-sea macrofauna in the Northwest Pacific.

5.4 Perspectives

In this chapter, we summarized and updated the profiles of cold-seep macrofauna in the SCS. Integrative morphological and molecular analyses have resulted in the description of nine new species and plenty of new records from these areas over the last decade. However, there are still some knowledge gaps in our understanding of the biodiversity of SCS seeps: (1) as new species have been discovered with more sampling efforts, there should be still undescribed species waiting our discovery, especially those hiding under the sediment surface; (2) among the collected species, quite a few have not been identified to the species level, which hinders our understanding of their phylogenetic position and divergence history; (3) many of the collected species have not been subjected to stable isotope analysis, therefore their trophic levels remain unclear; (3) among the symbiont-hosting species, only a few (i.e. G. haimaensis, G. platifrons, A. marissinica and P. echinospica) have been subjected to detailed studies of symbiotic relationships, therefore more such studies should be conducted to reveal the diversity of symbiosis in these seep-dwelling animals, and understand how they may have exploited the ecological niches; (4) although quite a few species found on the SCS seeps are widely distributed in the Northwest Pacific, only three species (i.e., G. platifrons, S. crosnieri and B. nipponica) have been subjected to population genetic studies in order to understand the divergence and gene flow among their different populations. For the remaining species, population genetic studies should be conducted to define the biogeographic regions of deep-sea chemosynthetic ecosystems, and devise conservation plans for these ecosystems that are under increasing threats from human activities, especially mining for metal-rich sulfur deposits from vent fields and extraction of methane hydrate from cold seeps (Levin et al. 2016).