This review provides insight into the roles of trypsin and its peptide products in signaling and communication in marine animals and in organizing marine communities. Examples include predator attractants, shell cues for hermit crabs, larval settlement cues for gregarious animals like barnacles and oysters, larval release pheromones, curing and signals from biological glues, biofouling management, feeding stimulants and mechanisms of deposit feeding, body odors and molecules that organize breeding aggregations, I argue trypsin should be considered a keystone enzyme because it and its products are central to functioning of marine ecosystems.