Starfish are important predators in bottom-culture farming systems for the Yesso scallop (Mizuhopecten yessoensis). We studied M. yessoensis and its predator the northern Pacific seastar (Asterias amurensis) in order to understand the impact of A. amurensis predation on M. yessoensis farming and survival. We observed and quantified the movement behavior and shell closure ability of different sized M. yessoensis when exposed to stimulation from A. amurensis. Furthermore, we investigated changes in the activities of the enzymes superoxide dismutase (SOD), catalase (CAT), arginine kinase (AK), and octopine dehydrogenase (ODH) in the gills, adductor muscles, and mantle tissues of M. yessoensis after continuous stimulation by A. amurensis. Based on enzyme activity results, we performed transcriptome deep sequencing on the differentially expressed enzymes in the various scallop tissues to identify the key genes for further functional validation.