Predator-induced hatching plasticity has been demonstrated in many species of amphibians. However, animals from other clades (e.g., marine species of molluscs and annelids) also place their embryos in capsules or gelatinous masses and might also exhibit hatching plasticity to predators. To date there is no evidence of predator-induced hatching plasticity from any marine species or a major clade of bilateria animals, the Lophotrochozoa. We studied predator-induced hatching plasticity of Nucella lamellosa, a carnivorous marine snail that deposits embryos in capsules. We used two experiments to investigate the effects of two types of predator, crabs and isopods, on developing embryos. In the first experiment, we quantified proportion of hatched embryos from capsules through time exposed to water-borne chemicals of crabs and isopods. Crabs delayed time-to-hatching, and the effects of predators were additive. In the second experiment, we quantified proportion of hatched embryos from capsules through time, developmental stage, and size of embryos in capsules exposed to water-borne chemicals of crabs and conspecifics. With this experiment, we wanted to answer: (1) whether a delay in hatching corresponded to embryos developing slower, and (2) whether the general products of metabolic waste from organisms can delay hatching. We unexpectedly observed that adult conspecific snails accelerated hatching but not developmental rate—the few past studies on the effects of conspecifics have all demonstrated that conspecifics delay time-to-hatching and rate of development. The results were also inconsistent with metabolic waste in general causing a delay in hatching, although the effect of conspecifics does weaken this inference. This study demonstrates that predators delay time-to-hatching in a marine mollusc, and suggests that predator-induced hatching plasticity is widespread among animals and likely evolved multiple times within the bilateria. In addition, conspecifics accelerated time-to-hatching in a marine mollusc, which suggests that conspecifics, like predators, might commonly influence when embryos hatch.