Mysids have previously been shown to consume oxygen at much lower rates when in a swarm (cohesive group in which individuals are evenly spaced but not polarized) than when in uncohesive small groups. Thus the swarm represents a strategy for conserving energy. In this study different swarm sizes of mysids were forced to perform escape responses by exposing them to jets of ammonium hydroxide, while measuring their oxygen uptake. Swarms of 200 individuals showed no significant difference in oxygen consumption with or without the ammonium treatment. In contrast, swarms of 100 showed a significant increase of around 45% oxygen uptake when exposed to ammonium. In conjunction with earlier demonstrations of increased feeding success and lower oxygen uptake when aggregated, this work clearly confirms the energetic benefits of being in a larger social group rather than a smaller one or remaining solitary.