Solemya johnsoni Dall, 1891; Recent, North Pacific.

Holotype : Complete, Haima cold seep cruise, HOV Shen Hai Yong Shi 71, 1390 m, coll. crew of R/V Tan Suo Yi Hao, 17 May 2018, MBM287872.

Figure 2. 

Holotype of Acharax haimaensis sp. nov.

Shell : the shell is elongate, rectangular, equivalve, and robust. Shell length 103.32 mm, width 48.68 mm, height 45.20 mm. The length/width ratio and length/height ratio are 2.12 and 2.29, respectively. The beak is positioned at approximately the posterior quarter of the shell. The anterior margin is broadly rounded and compressed medially, while the posterior margin is tightly rounded. The ventral margin is straight and shallowly concave towards the posterior. The periostracum extends well beyond the calcified shell margin, folding inwards, and reddish brown but gradually paler towards the prodissoconch. The shell has radial ridges with strong, flattened summits. The ridges are more crowded anteriorly and posteriorly, and vary in both width and colour. There are four closely spaced ridges over the posterior area, and the median area is almost smooth, with two or three low ribs. The anterior region with eight or nine deeply cut ribs. Hinge teeth are absent, and the ligament is mainly external. The posterior siphon aperture is lined by two rings of papillae. The foot is large, with a broadly oval sole, and its margin interdigitates between large and small blunt papillae.

Adductor muscle scar and pallial scars : the anterior adductor-muscle scar has a D-shaped, irregularly and posteriorly oriented straight face. There is a loop of muscle scar that extends from the hinge plane, reaching halfway along the valve and connecting ventrally to adductor scar. The posterior adductor scar is ovoid, with irregular contours, and exhibits faint radial sculpture. It is separated by a line that extends from the scar to the beak along a subtle depression. The foot is flattened at anterior end when open. The flattened face is longitudinally bifurcated, displaying horizontal lamellae, and is fringed with approximately 32 short, adze-shaped ridges. These ridges interlock when the foot is closed, and each ridge is topped with a small, rounded papilla. The mantle is fused along the ventral margin and covers large gills that extend about three-fifths of the total length of the body.

Named after the type locality, the Haima cold seep in the northwestern slope of the South China Sea.

Currently, Acharax haimaensis sp. nov. is known only from the type locality, in the northwestern slope of the South China Sea, at a depth of 1,390 m.

The genus Acharax has a worldwide distribution in cold seep habitats with sulfide present (Sibuet and Olu 1998) at depths ranging from approximately 400 m to 6,000 m (Neulinger et al. 2006). Acharax has approximately nine extant species (WoRMs 2024) and more than 20 fossil seep species (Amano and Ando 2011; Saether et al. 2016; Isaji and Kato 2017; Hansen et al. 2020). Acharax clarificata Dell, 1995 closely resembles to our specimen, but the new species differs from A. clarificata in the shape of the shell. Our specimen has an elongate, nearly rectangular shell with parallel dorsal and ventral margins, whereas A. clarificata has a shallowly concave ventral margin and is particularly more deeply concave towards the posterior end.

The genus Acharax exhibits distinct morphological differences from Solemya, including a prominent large external ligament located on a narrow nymph and larger size (Walton 2015). However, due to the striking similarities in gross morphology with Solemya, there is a possibility that Acharax species have been misclassified as Solemya in the past (Sibuet and Olu 1998). Even within the genus Acharax, certain species share morphological similarities, but molecular data suggests the presence of cryptic speciation (Neulinger et al. 2006). Therefore, gene-sequence analysis can provide valuable information for the classification of solemyid bivalves.

The genetic divergence between Acharax haimaensis sp. nov. and the solemyid species analyzed ranged from 21.4% (A. johnsoni) to 28.2% (Solemya pusilla) (Table 1). It is evident that the lowest genetic distance was observed between the two Acharax species. The phylogenetic tree of the family Solemyidae, reconstructed using maximum likelihood based on mitochondrial COI,16S rRNA and 18S rRNA sequence data, is shown in Fig. 3. Both Acharax and Solemya formed monophyletic clades with strong support values (≥95%).

Figure 3. 

Phylogenetic relationships of Solemyida by the ML analysis of mitochondrial (COI+16S rRNA+18S rRNA) sequences. The black triangles demonstrate bootstrap values ≥95% for the node.

The placement of the new species, Acharax haimaensis sp. nov., within the genus Acharax is supported by both the morphological and molecular phylogenetic analyses. Its discovery at the Haima cold seep contributes to the known diversity of solemyids in chemosynthesis-based ecosystems. The Protobranchia represent an intriguing group of Bivalvia in terms of their early evolution, unique anatomy, larval development, and ecological diversification. However, there are still controversies surrounding the origin and evolutionary process of Protobranchia, particularly the phylogenetic relationships among higher taxa. Currently, research into the morphological taxonomy and molecular systematics of the Protobranchia is relatively limited. More comprehensive taxon collections in the future will be necessary to lead us closer to the goal of reconstructing the evolutionary history of protobranch bivalves.