Is social aggregation in aquatic crustaceans a strategy to conserve energy?

Antarctic krill, Euphausia superba, is preeminently a gregarious animal. It lives for almost the whole of its existence from the late furcilia stage in aggregations. Despite this, laboratory study of schooling and swarming behaviour has been seriously neglected and critical emergent properties of group dynamics may have been overlooked. Using different-sized groups of gregarious mysids, I show that weight-specific oxygen uptake is reduced by about seven times when they form cohesive aggregations compared with when they are in uncohesive small groups. If this is true for E. superba, it casts doubt on all previous measurements of metabolic rate and suggests that estimates of the metabolic cost of swimming and perhaps feeding are much too high. The reason that groups conserve energy compared with isolates or small groups is hypothesised to be at least partly due to hydrodynamic processes, which serve to minimise sinking rates. Dye plumes revealed updrafts generated by mysid swarms, which could be exploited by individuals to reduce their sinking rate. These circulation patterns might also increase the efficiency of particle capture by aggregations. I propose that aggregation in aquatic crustaceans is a strategy to optimise energy expenditure and maximise food capture. Measuring behavioural and physiological rate processes in isolated animals will produce only artifacts.