Non-consumptive effects of predators on prey populations have received increased interest in recent years. For Crassostrea virginica (Eastern Oyster), much of the focus has been on induced morphological defenses (e.g., shell thickening). Here, we provide in situ documentation of a behavioral response of Eastern Oysters (valve closure) to the threat of predation on a natural reef. This behavioral response, while intuitive, has been largely ignored in the literature despite potential impacts on individual oyster health by affecting feeding and subsequently energy assimilation, reproductive condition, and growth. In situ photographs revealed that, under natural conditions, Eastern Oysters closed during the passive presence of a crab mate-guarding pair and took ∼5 minutes to reopen to pre-predator gapes. Given that multiple oysters in our photos reacted similarly, this behavioral response may scale up to have effects on the population and the ecosystem services that Eastern Oysters provide. Ultimately, our observations open the door to a number of testable hypotheses regarding a predator's non-consumptive effects on oyster reefs.