When Asterropteryx semipunctatus and Gnatholepis anjerensis detect a predator they bob rhythmically, raising and lowering the body. Bobbing and reduced movement was transmitted visually between fish in adjacent tanks, both within and between species. Fish isolated from other fish for at least 5 days still bobbed when they encountered a predator model. G. anjerensis remained resident in one location for an average of 61 days from marking to disappearance or the end of the study. They could, therefore, benefit if bobbing reduced predator hunting success. Three general categories of hypotheses have been proposed to account for predator-recognition behaviour: assessment of the predator, informing the predator that it is detected, and warning the sender's conspecifics. Two of these, informing the predator and warning, seem most likely to account for goby bobbing. These hypotheses are not mutually exclusive.