The Manila clam Ruditapes philippinarum is the world’s second most important bivalve mollusk commercially farmed, whose indigenous populations are mainly distributed in the coastal areas of East Asia. However, with the development of commercialization, mixture of populations and loss of local germplasm have become prominent problems. Here, genetic differentiation of seven Manila clam populations from East Asia was investigated through analyzing the polymorphism of the mitochondrial cytochrome C oxidase subunit I (COI) gene as well as 20 simple sequence repeat (SSR) molecular loci. In total, 40 haplotypes were identified, among which 31 were unique. Moreover, two main haplotypes were detected with several radiating derived haplotypes. Populations in Japan-North Korea shared haplotype Hap_31, and populations in China shared haplotype Hap_7, suggesting that the natural geographical isolation of the Yangtze River and the Yalu River might have divided the East Asian indigenous populations into three groups, which were located in South China, North China, and Japan-North Korea, respectively. The Aquaculture breeding activities from South to North in China might have promoted gene exchange among Manila clam populations. Population in Laizhou had the highest genetic diversity and therefore could be an excellent germplasm source.

The Manila clam, Ruditapes philippinarum, is an ecologically and economically important marine bivalve species. In this study, we conducted transcriptomic sequencing of two different shell color strains (O and Z) before color appearance (uncolored juvenile clam) and pigmented shell color (colored juvenile clam) and investigated the analysis of the differential expression patterns of specific genes associated with pigmentation by RNA-seq and time course qPCR analysis. The transcription level of 16 differentially expressed genes (DEGs) related with shell color was analyzed by qRT-PCR to validate the performance of RNA-seq from Illumina sequence data where most of them were up-regulated. Two genes were down-regulated after the occurrence of zebra clam stripes compared with uncolored zebra clam. The trend of gene expression obtained by qPCR was basically consistent with that of RNA-seq. The synthesis of melanin in bivalves plays potential roles in the pigmentation of the shell and is closely related to the formation of the surface pattern. The porphyrin metabolism combined with tyrosinase and melanogenesis signaling pathway is a novel finding in shell color determination of R. philippinarum. This study sheds light on the pigmentation and coloration mechanism of the Manila clam.