Many animals escape predators by hiding. Hiding decisions are economic in that individuals trade off the physiological costs of hiding with the benefits of increased security. The number of conspecifics may increase competition, security, or attract predators, influencing predation risk. We studied hiding time in Christmas tree worms (Spirobranchus giganteus), sessile marine invertebrates, which lived with 0–17 other worms within 20cm. Competition and predation risk reduction both predict a shorter latency to re-emerge given the necessity to feed and potential for safety in numbers, respectively. In contrast, if grouped worms attract more predators, individuals should hide longer. We experimentally induced hiding in 174 worms and found a significant, positive relationship between hiding time and number of conspecifics. We repeated the test 4 consecutive times in 1 day on a subset of 30 worms that were either solitary or lived with one other untested worm. We found that worms with longer hiding times habituated more quickly than worms with shorter hiding times, and that the individual worm explained 55.8% of the variation in hiding time. When we conducted the trials for 4 days, we found that the individual worm explained 41.75% of the variation, but no evidence of behavioral plasticity. Worm antipredator responses were consistently individualistic, but behavioral plasticity was only evident over short time scales. For sessile marine invertebrates, higher densities may attract predators, enhancing rather than diluting predation risk. Worms that cannot move away from their neighbors, thus seemingly modify antipredator behavior in consistent ways.