Abstract

This study aimed to clarify the taxonomic status of Mactra antiquata sensu lato, which has long been controversial due to morphological and molecular variations. Samples newly collected from four Chinese coastal provinces were analyzed using integrated morphological and molecular methods. Results showed two distinct groups (N-group and S-group) with significant differences in shell ratios, pallial sinus orientation, and escutcheon shape. Molecular analyses (phylogeny, haplotype networks, genetic distances, ASAP, and ABGD) confirmed obvious genetic differentiation exceeding intraspecific thresholds. The S-group was identified as a new species, Mactra haiboensis sp. nov., distributed in the East and South China Seas. This study provides diagnostic criteria for the new species and lays a foundation for its targeted conservation and further research. Mactra antiquata sensu lato, a commercially important clam species in China, exhibits remarkable morphological and molecular diversity, which has led to the proposal of cryptic species within this complex. In the present study, specimens of M. antiquata sensu lato were collected from four coastal provinces (Shandong, Guangdong, Guangxi, and Hainan) of China, and an integrated comparative analysis was performed based on morphological traits and partial sequences of two mitochondrial genes (cytochrome c oxidase subunit I, COI ; and 16S rRNA). Our results revealed that M. antiquata sensu lato could be clearly delineated into two distinct clades: the N-group (comprising specimens collected from Shandong in this study) and the S-group (including specimens collected from Guangdong, Guangxi, and Hainan), with significant intergroup differences. Morphologically, S-group individuals possessed relatively narrower shells (mean shell width-to-length ratio = 0.465) and shorter shells (mean shell height-to-length ratio = 0.781) compared to N-group conspecifics. Additionally, the pallial sinus of S-group clams extended direct