Introduction

Cold seeps are areas of the seafloor, where hydrocarbon-rich fluid and gases leak from fissures and emerge through the sediments and into the water column, creating unique habitats. Such seepage was first discovered on the Florida Escarpment in the Gulf of Mexico (Paull et al. 1984). Since then, hundreds of cold-seep sites have been discovered and observed globally (e.g., Feng et al. 2018; German et al. 2011; Suess 2014), unveiling a specialist seepage macrofauna. Cold-seep macrofauna, being sustained by chemosynthetic primary production, typically consists of a high abundance of symbiotrophic organisms (Barry et al. 1996; Carney 1994; Levin 2005; Sibuet and Olu 1998; Washburn et al. 2018). Hence, cold seeps exhibit a community structure that is distinct from that seen in the surrounding seafloor environment. During the past three decades, numerous studies have been performed, driven by efforts to explore these special habitats and their associated organisms. Some studies have focused on the taxonomy and phylogeny of organisms associated with cold seeps, aiming to discover new species and new distribution records, evaluate phylogenetic relationships, and reconstruct the origins and evolutionary histories of seepage faunas (e.g., Chen et al. 2018; Dong and Li 2015; Xu et al. 2019). Other studies have focused on the community structures and temporal dynamics of cold seeps, analyzing the correlation between pattern of biodiversity in the infaunal assemblages and environmental factors such as depth and seepage types (e.g., Bourque et al. 2017; Cunha et al. 2013; Levin et al. 2015; Washburn et al. 2018).

The South China Sea is a marginal sea in the western Pacific Ocean with passive continental margins in the west and north, where various cold-seep sites have been discovered, in particular on the continental slope (Fang et al. 2019; Feng and Chen 2015; Li 2015, 2017; Niu et al. 2017). The first active seepage site to be discovered in the South China Sea is known as Site F (also called Jiaolong Seep No. 1, Formosa Ridge or Taixinan cold seep), located in the northeastern region (Lin et al. 2007). In the summer (June–July) of 2013, the corresponding author (Xinzheng Li) participated in the first cruise with experimental applications of the Chinese-manned submersible “Jiaolong” in the South China Sea (China Ocean Voyage No. 31). The epibenthic community at Site F was described as being dominated by the alvinocaridid shrimp Alvinocaris longirostris Kikuchi & Ohta, 1995, the mytilid mollusk Gigantidas platifrons (Hashimoto & Okutani, 1994), the galatheid squat lobster Shinkaia crosnieri Baba & Williams, 1998, and other squat lobster and mollusk species (Li 2017). From 2013 to 2018, various Chinese research institutions carried out a series of surveys at this site and gathered numerous macrobenthic specimens using remote operated vehicles (ROV) and the manned submersible Jiaolong. To date, 33 epibenthic macroinvertebrate species have been reported from Site F and its rim area, six of which are new to science, revealing high biodiversity and most likely a high level of species endemism (Chan and Komai 2017; Dong and Li 2015; Gong et al. 2015; Li 2017; Sha 2019; Zhang et al. 2016; Zhang and Zhang 2017).

In 2013, a new active cold-seep site was discovered on Four-Way Closure Ridge, not far from Site F (Klaucke et al. 2015). In 2015, another active cold-seep field in the South China Sea was identified on the northwestern continental slope, characterized by patches of authigenic carbonate rocks protruding from the muddy seafloor, which contains at least two seepage sites (Fang et al. 2019; Liang et al. 2017). This area was named the Haima cold seeps and has attracted increasing attentions since its discovery. To date, four new bivalve species have been described based on specimens collected through surveys made at Haima in the past four years (Chen et al. 2018; Jiang et al. 2019; Xu et al. 2019). However, information on other taxonomic groups and community structure found in this field is scarce.

The expedition (Fig. 1) to the Haima cold seeps conducted in May 2018 was organized by Tongji University and Chinese Academy of Sciences. Additional cold-seep sites were discovered, including one on the southwestern slope of Ganquan Plateau, close to the Haima field. The expedition also explored an adjacent mud volcano field, and the slopes of Ganquan Plateau (Fig. 2). The corresponding author (Xinzheng Li) took part in the expedition and dived with the submersible “Shenhai Yongshi” (Deep-Sea Warrior) at the Haima cold-seep sites. Abundant epibenthic macroinvertebrate specimens were collected or observed in situ using the manned submersible. Some stalked barnacles were subsequently described by Gan and Li (2019). The present study summarizes and reports the taxonomic findings on epibenthic macrofauna from Haima cold seeps and nearby ecosystems, based on specimens collected and photographed during the expedition, to provide an overview of the species composition and biodiversity characteristics of the macrofaunal assemblages in these deep-sea habitats.

Fig. 1
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Location map of the study area in the South China Sea. H Haima cold-seep field; N northeastern slope of Ganquan Plateau; M mud volcano field, S southwestern slope of Ganquan Plateau

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Fig. 2
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Haibitats investigated during the expedition. a Southwestern slope of Ganquan Plateau; b northeastern slope of Ganquan Plateau; c Haima cold-seep site; d mud volcano field

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Results and discussion

Faunal composition and species list

A total of 41 species (Table 1) were identified to genus or species level from specimens either collected or observed in situ during the expedition to the slopes of Ganquan Plateau, the Haima cold-seep sites, and a mud volcano field. These comprise 16 species of Crustacea; 14 Mollusca; 4 Cnidaria; 3 Porifera; 3 Annelida and 1 Echinodermata (Figs. 3, 4, 5 and 6). Some other specimens that are pending identification may include undescribed taxa and therefore are not listed or reported here.

Table 1 Checklist of the species currently identified to genus or species levels collected from deep-sea habitats of Haima cold seeps, Ganquan Plateau and a mud volcano field in May of 2018
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Fig. 3
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In situ photographs of epibenthic macrofauna taken during the expedition. a Rhodaniridogorgia sp., mud volcano field; b Acanthogorgiidae gen. et sp. indet., southwestern slope of Ganquan Plateau; c Actinostolidae gen. et sp. indet., mud volcano field; d Paraescarpia echinospica, Haima cold-seep site; e Plicifusus sp., on muddy seafloor around Haima cold-seep site; f Alvinocaris longirostris) and squat lobster (probably Munidopsis lauensis), on mussel bed in Haima cold-seep site; g Nematocarcinus sp., on muddy seafloor around Haima cold-seep site; h Metanephrops neptunus, mud volcano field; i Paralomis sp., in rim of mussel bed in Haima cold-seep site; j, Cyrtomaia sp., on sponge Chonelasma sp., northeastern slope of Ganquan Plateau; k Chaceon sp., mud volcano field; l Echinothuriidae gen. et sp. indet., on muddy seafloor around Haima cold-seep site

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Fig. 4
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Colour images of freshly collected specimens. a Caulophacus sp.; b Pleurochorium sp.; c Chonelasma sp.; d Enallopsammia rostrata; e Actinernus sp.; f Branchipolynoe pettiboneae; g Paraescarpia echinospica. Scale bar = 5 cm (a, c, e); 1 cm (b, d, f, g)

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Fig. 5
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Colour images of freshly collected specimens. a Leptochiton tenuidontus; b Malletia sp.; c Solemya sp.; d Gigantidas haimaensis; e Calyptogena marissinica; f Lucinoma sp.; g Tristichotrochus ikukoae; h Provanna glabra; i Scabrotrophon scitulus. Scale bar = 1 cm (a, b, f, g, h, i); 5 cm (c, d, e)

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Fig. 6
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Colour images of freshly collected specimens. a Glyptelasma gigas; b Poecilasma litum; c Poecilasma obliqua; d Eurythenes maldoror; e Bathynomus jamesi; f Alvinocaris longirostris; g Uroptychus setosidigitalis; h Munidopsis lauensis; i Munidopsis pilosa. Scale bar = 5 mm (a); 1 mm (b, c); 1 cm (d, f, g, h, i); 5 cm (e)

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The investigation area of the southwestern slope of Ganquan Plateau has a variety of microhabitats. The substrate basically comprised boulders on steep slopes and flat rock on gentle slopes. In addition, a small cold seep at the foot of the plateau was discovered for the first time. Six macrofaunal species were identified based on the specimens collected from this area, including two species of sponges (Fig. 4a, b), one species of cold-water coral (Fig. 4d), and one gastropod species (Fig. 5g) which was associated with the sponge. Paraescarpia echinospica, a symbiotrophic tubeworm, was the only organism observed in the small cold-seep site within this study area.

The investigation area of the northeastern slope of Ganquan Plateau is characterized by flat and rocky slopes extending across a large area. Six macrofaunal species were identified in this habitat, including two sessile organisms (a Chonelasma sponge (Fig. 4c) and a Pseudochrysogorgia cold-water coral), a squat lobster of the genus Uroptychus (Fig. 6g) that was associated with the cold-water coral, and a Glycera polychaete associated with an unidentified sponge. A Cyrtomaia crab (Fig. 3j) was collected from an unidentified sponge, but nature of their relationship is not clear. One species of Palaemonella shrimp was observed in high abundance on the sea bottom.

Five cold-seep sites were surveyed during the expedition. Large areas of mussel beds (dominated by Gigantidas haimaensis) were observed at these seeps, which is a typical feature of an active seepage ecosystem (Feng et al. 2018). In contrast, patches of relatively small authigenic carbonate hardgrounds were common around these sites, indicating an early stage of ecological succession in some places (Feng et al. 2018). In total, 24 species were identified from the specimens collected at Haima cold seeps, and most were mollusks (13 species) (Figs. 3e, 5a–f, h, i) and crustaceans (seven species) (Figs. 3f, g, i, 6d–f, h, i). The large vesicomyid clam species Calyptogena marissinica, which was described first time by Chen et al. (2018) based on samples from the Haima cold seeps, was found and collected again by the corresponding author (Xinzheng Li) in the expedition of 2018. It is a very common species in the cold-seep areas. A red bloody fluid flows from freshly collected specimens of C. marissinica. This appearance is very similar to other species of the genus Calyptogena (Fujikura et al. 2012). The amphipod Eurythenes maldoror (Fig. 6d) and the isopod Bathynomus jamesi (Fig. 6e), both of which are opportunistic scavengers, were captured in cage traps. No coral species were observed in this habitat. The symbiotrophic tubeworm Paraescarpia echinospica, (Figs. 3d, 4g) occurred in small clusters surrounding authigenic carbonate rocks. The scaled polychaete Branchipolynoe pettiboneae (Fig. 4f) and the sea anemone Actinernus sp. (Fig. 4e) were very common within and around the cold-seep mussel beds.

A mud volcano field was newly found during the expedition, and its adjacent seabed was investigated. Here, a large area of flat rocks was present on the otherwise muddy seafloor, providing a hard substrate that harbored abundant corals. Eight species were identified based on faunal collections at this mud volcano field. One cold-water coral, Rhodaniridogorgia sp. (Fig. 3a), was identified from video images. Six crustacean species, including three species of stalked barnacle (Fig. 6a–c), were either collected or observed by video. A scallop bivalve of the genus Propeamussium was the only mollusk observed in this habitat.

Diversity and distributions of the major macrofaunal groups

Nearly all the sponges observed and collected during the expedition were distributed on rocky slopes of the Ganquan Plateau, whereas no sponges were found at the Haima cold seeps. Similarly, all cold-water corals were collected or observed on the slopes of Ganquan Plateau or at the mud volcano field, where the seafloor was largely composed of boulders or continuous flat reefs, respectively. This is in line with the sessile lifestyle of these two animal groups. Two anemone species were observed in the cold-seep habitat: Actinernus sp., which was present in high abundance attached to empty shells, and a species of Hormathiidae, which was found on the seafloor adjacent to one of the seepage site.

Most of the mollusk and polychaete species were found living in the seepage area, where the seabed was mainly mud sediment, providing an ideal habitat for mud-preferring mollusks and polychaetes. High chemosynthetic primary production in cold-seep sites sustains a flourishing seepage community in which chenosynthetic-specialist mussels predominate with high biodiversity and abundance, typically forming large areas of mussel beds. At least eight chenosynthetic-specialist mollusks or polychaetes were collected during the expedition. Only two gastropod species, namely Tristichotrochus ikukoae and a species of Cancellariidae, were present on the slope of Ganquan Plateau, associated with a sponge and a cold-water coral, respectively.

Crusateans generally have a strong ability to move, which allows them to adapt to various deep-sea habitats. A high biodiversity of crustaceans was observed in this expedition. However, few species were shared among the four types of habitat, each of which had a distinct crustacean fauna. The rocky slopes of Ganquan Plateau supported plentiful gorgonians; coral-associated squat lobsters, such as species of Uroptychus and Sternostylus, were also very common. Additionally, the more complicated topographical environment attracted crabs which could shelter and breed under rocks and in gaps. The rocky substrate in the mud volcano area also provides a habitat for crabs and coral-associated crustaceans; solid rocks on an otherwise muddy seabed provided habitat for hole-dwelling species, such as the crayfish Metanephrops neptunus. By contrast, the cold-seep sites were colonized by species of squat lobster and alvinocaridid shrimp that are specialists in reducing habitats. Examples include Munidopsis lauensis and Alvinocaris longirostris, which are generally dominant in cold-seep communities. King crabs, acting as vagrant predators, were also very common in the cold-seep communities.

Members of the Echinodermata constituted an important part of the overall faunal biodiversity of the Haima cold-seep areas. However, the taxonomic identity of these animals was difficult to determine and only one species could be identified to genus level. Ophiuroids (brittle stars) were the most diverse subgroup and were observed in high abundance in the cold-seep assemblages. They included species of Histampica, Ophiacanthidae, and Amphiuridae. Conversely, species of Euryalidae were associated with gorgonians at Ganquan Plateau, and some species of Ophionereididae, Ophiuridae and Ophiacanthidae were associated with sponges. A species of sea cucumber, probably belonging to the Chiridotidae, was common in and around cold-seep sites. Sea urchins were scarcely observed, but one large-sized individual representing a species of Echinothuriidae (Fig. 3l) was captured on the muddy seabed beside a mussel bed.

Preliminary description of community structures and faunal comparison

The community structure of a cold-seep habitat can be influenced by many factors, such as depth, seepage type, and gas composition. Many studies have focused on endobenthic fauna assemblages (micro- and meiobenthos) to quantitatively evaluate their communities in different cold-seep habitats (e.g., Bourque et al. 2017; Cunha et al. 2013; Levin et al. 2015; Washburn et al. 2018).

Although nearly half of the taxa reported here have not been identified to genus or species level, this investigation lends preliminary insights into the community structures of the different types of habitat explored during the expedition. Accordingly, the Ganquan Plateau slopes, characterized by hard substrates, were colonized by high numbers of cold-water corals and sponges along with their associated crustaceans and ophiuroids, revealing faunal community structures similar to those at seamounts. To a large extent, it was the abundance and biodiversity of sessile invertebrates, rather than motile animals, determined the overall epibenthic community structure in these rocky habitats. However, the southwestern slope of Ganquan Plateau, which is deeper and nearer to the Haima cold seeps than the northeastern slope, had a cold-seep microhabitat site; therefore seep-associated faunal species, such as the tubeworm Paraescarpia echinospica, were present. The Haima cold-seep faunal assemblages on the muddy seabed revealed typical seepage communities, composed of species that are specialized for life in reducing habitats and opportunistic predators but devoid of cold-water corals and sponges. The mud volcano field, characterized by flat reefs on an otherwise muddy seafloor, shares some community characteristics with the Ganquan Plateau slopes. However, no seep-associated organism was observed in the mud volcano field, probably because the gas release there is less intensive than that in cold-seep sites and thus does not support a chemosynthesis-based assemblage.

The epibenthic community structure of the Haima cold-seep field is distinct from that of Site F in the northeastern South China Sea (southwest to Taiwan Island). Site F is notable for its Gigantidas platifrons–Shinkaia crosnieri community, in which the mussel G. platifrons and the squat lobster S. crosnieri thrives in high abundance (Li 2015, 2017). The Haima cold-seep field, which is approximately 1000 km from Site F, was nearly devoid of S. crosnieri and has low occurrence of G. platifrons, with G. haimaensis being the dominant mussel species. Likewise, siboglinid tubeworms were common in the Haima field but scarce at Site F. The squat lobster Munidopsis lauensis and the alvinocaridid shrimp Alvinocaris longirostris were common at both cold-seep areas. The community structures at the Haima cold-seep field and the Site F are depicted in Fig. 7. At first approximation, the community structure at Haima cold seeps are roughly similar to that of chemosynthesis-based communities in Sagami Bay (Fujikura et al. 2012), although the presence of the tubeworm Paraescarpia echinospica links the Haima cold seeps to chemosynthetic haibitats at the Nansei-shoto (Ryukyu) Trench. In contrast, the community structure at Site F is roughly similar to that of the hydrothermal vents in the Okinawa Trough owing to the Gigantidas platifrons–Shinkaia crosnieri assemblage (Feng et al. 2018; Li 2015, 2017). The cause of such differences between cold-seep communities is still unknown. In accordance with the fieldwork experiences of the corresponding author (Xinzheng Li) at Site F in 2013 and at the Haima cold seeps in 2018, the sulfide smell of the substrate samples from the Haima cold-seep areas was much stronger than that from Site F. This phenomenon may imply that there are differences in the substrate materials, concentrations of reducing compounds, or levels of redox potentials, caused by differences in primary productivities of chemosythetic microbes. Quantitative environmental data would be needed to verify this.

Fig. 7
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Schematic diagram of the community structures of the Site F cold-seep site (a) and Haima cold-seep field (b). a I, chemosynthetic microbes; II, flagship species, making up the landscape of the community, left, Shinkaia crosnieri, middle, Gigantidas platifrons, right, Alvinocaris longirostris; III, top predator, Lithodes longispina Sakai, 1971. b I, chemosynthetic microbes; II, flagship species, making up the landscape of the community, above, from left to right, Gigantidas haimaensis, Calyptogena marissinica, Paraescarpia echinospica, indeterminate species of Chiridotidae; below, from left to right, Leptochiton tenuidontus, Bathyacmaea lactea, Histampica sp., Actinernus sp.; III, top predators, Paralomis sp., even ? Plicifusus sp.

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Our future research will build on this preliminary taxonomic work, especially to (1) better determine community connectivity of epibenthic macroinvertebrates among different seepage sites in the South China Sea, and (2) reveal the factors that govern the community structures and ultimately determine the biodiversity at cold seeps. In conclusion, the cold seeps at both Haima and Site F merit further research, with comparisons based on more sampling and field observations, as well as comprehensive analyses that consider biological, chemical, geographic, and physical oceanographic evidence.

Materials and methods

The investigated areas (Fig. 1) in the South China Sea are located on the northwestern continental slope and included three geomorphological environments: (1) the Haima cold-seep field, with five sites investigated, covering a depth range of 1380–1390 m; (2) a mud volcano field, at depths of 500–810 m; and (3) the slopes of Ganquan Plateau, at depths of 586–910 m on the northeastern slope, and at 1300–1412 m on the southwestern slope. At Haima cold-seep field, two sites were investigated on each dive in the manned submersible, and samples from each site were loaded together in a sampling box. Therefore, the macrofaunal specimens collected from all the five sites were examined and analysed as a single bulk sample to represent the Haima cold-seep habitat. On the southwestern slope of Ganquan Plateau, several microhabitats were present, ranging from rocky cliff to a small muddy cold-seep site in a limited area, where only siboglinid tubeworms were observed, but were integrated as part of the overall fauna of the southwestern slope.

Sampling and in situ observations were performed with the manned submersible “Shenhai Yongshi”. Videos and photos (Fig. 2) were taken using high-definition underwater cameras deployed on the submersible. During the cruise, only epibenthic marcoinvertebrates were qualitatively collected and analyzed. The specimens were visually detected and collected, and were therefore mostly larger than 5 mm in size. Cold-water corals, sponges, mollusks, sea urchins, and large crustaceans were directly grabbed using the mechanical arms of the submersible; small crustaceans like squat lobsters and shrimps were sampled using nets manipulated by the mechanical arms. Some adherent organisms, such as small gastropods and ophiurids, were collected together with rocks and corals. Specimens were photographed immediately after being transported to the deck of the mother vessel, and then directly frozen for conservation. Most of the specimens were retained by the research for taxonomic examination; however, specimens deposited in Tongji University and uncollected organisms observed in situ were identified only from videos and photographs.

Taxonomic studies of the specimens were mainly based on morphological method. DNA barcoding was employed to confirm the identification of some species. The samples are deposited in the Marine Biological Museum of Chinese Academy of Sciences in Qingdao, China.