Cathepsins,a superfamily of hydrolytic enzymes produced and enclosed within lysosomes,function in vertebrates;however,their function within the innate immune system of invertebrates remains largely unknown.In this study,we investigated the immune functionality of cathepsin D(catD) in Chinese mitten crab (Eriocheir sinensis),a commercially important and disease vulnerable aquaculture species.Spiroplasma eriocheiris is an emerging pathogen in the commercially exploited Chinese mitten crab.We studied the connection between the catD and S.eriocheiris.