THE UNSELFISH SWARM

1. Laboratory-reared waterflea, Daphnia longispina (O.F. Muller) clone s have differing swarm-forming tendencies and groups of genetically mi xed individuals never form tight aggregations. We found that 79% of th e individuals in a naturally occurring D. longispina swarm had the sam e allelic genotype. 2. Swarming could work as an anti-predator strateg y: the costs and benefits of a 'swarm-joining' strategy at both indivi dual and group levels depend on predator functional response curves. W e propose group-level benefits from swarming that favour the formation of swarms among clonal sisters and present evidence that some predato rs have the required functional response.