Bivalve mollusks are economically important invertebrates that exhibit marked diversity in benthic lifestyle and provide valuable resources for understanding the molecular basis of adaptation to benthic life. In this report, we present a high-quality, chromosome-anchored reference genome of the Venus clam, Cyclina sinensis. The chromosome-level genome was assembled by Pacific Bioscience single-molecule real-time sequencing, Illumina paired-end sequencing, 10× Genomics, and high-throughput chromosome conformation capture technologies. The final genome assembly of C. sinensis is 903.2 Mb in size, with a contig N50 size of 2.6 Mb and a scaffold N50 size of 46.5 Mb. Enrichment analyses of significantly expanded and positively selected genes suggested evolutionary adaptation of this clam to buried life. In addition, a change in shell color represents another mechanism of adaptation to burial in sediment. The high-quality genome generated in this work provides a valuable resource for investigating the molecular mechanisms of adaptation to buried lifestyle.

Flatfish undergo extreme morphological development and settle to a benthic in the adult stage, and are likely to be more susceptible to environmental stress. Heat shock proteins 70 (hsp70) are involved in embryonic development and stress response in metazoan animals. However, the evolutionary history and functions ofhsp70in flatfish are poorly understood. Here, we identified 15hsp70genes in the genome of Japanese flounder (Paralichthys olivaceus), a flatfish endemic to northwestern Pacific Ocean. Gene structure and motifs of the Japanese flounderhsp70were conserved, and there were few structure variants compared to other fish species. We constructed a maximum likelihood tree to understand the evolutionary relationship of thehsp70genes among surveyed fish. Selection pressure analysis suggested that four genes,hspa4l,hspa9,hspa13,andhyou1, showed signs of positive selection. We then extracted transcriptome data on the Japanese flounder withEdwardsiella tardato induce stress, and found thathspa9,hspa12b, hspa4l, hspa13, andhyou1were highly expressed, likely to protect cells from stress. Interestingly, expression patterns ofhsp70genes were divergent in different developmental stages of the Japanese flounder. We found that at least onehsp70gene was always highly expressed at various stages of embryonic development of the Japanese flounder, thereby indicating thathsp70genes were constitutively expressed in the Japanese flounder. Our findings provide basic and useful resources to better understandhsp70genes in flatfish.