Chaetognaths are considered to be the dominant predators of small zooplankton and fish larvae. Therefore, it is thought that they play an influential role in marine pelagic food webs, transferring most of the zooplanktonic biomass to higher trophic levels. To date, their head armature (teeth and grasping hooks) and the results of laboratory feeding experiments have supported this view. However, analyses of gut contents and other aspects of their feeding behavior suggest that chaetognaths are primarily osmotrophic animals. They gulp seawater, taking in dissolved and thin particulate organic matter produced in abundance by viruses and bacteria. This diet explains the success of chaetognaths in number and biomass and the observed lack of relationship between chaetognaths and prey availability. The role of chaetognaths should be revisited in the global ocean ecological balance, particularly with respect to vertical carbon flux, and also in predicting fishery resources.