Bivalves play vital roles in ocean conservation and food security by acting as ecosystem engineers and underlying >20% of global aquaculture production. Robust stocks are required for restoration or aquaculture breeding programmes, h owever, our understanding of bivalve biology and evolution is limited. Due to high levels of heterozygosity and repeated regions, generating genome assemblies for bivalves was hindered until advances in long read sequencing technology. This has led to a great increase in the number of bivalve assemblies in recent years. By analysing the genomes of 32 species representing each molluscan class, we identify gene families that have undergone expansion during bivalve evolution. These included ancestrally-retained expansions in redox, chaperone and protein recycling gene families and more recent expansions in innate immune response gene families . This mirrors adaptation strategies of other sessile organisms such as plants and reflects the high level of tolerance bivalves require during constant pathogen exposure.