What is the ideal shape to assume for a school of fish? This was the question that Andrew Brierley and Martin Cox of the University of St Andrews tried to answer in their study, which is to be published in a forthcoming issue of the journal Current Biology.
The authors used multibeam sonar to examine the three-dimensional sizes and shapes of more than 1000 schools of Antarctic krill (Euphausia superba), as well as more than 900 schools of five different species of sardines and anchovies worldwide.
They found that the ideal shape of the schools of krill and clupeid fishes were largely identical – in the form of an irregular crystal (a shape the authors described as a 'multifaceted lozenge').
The assumption of a common shape for schooling organisms living in habitats as diverse as the polar seas and the tropics suggested that a set of identical factors were involved.
The authors found that the schools always maintained a fixed surface area:volume ratio.
Using a series of computer models, the authors calculated that the optimal shape and packing density of the school is determined largely by a model in which the individuals in the school juggle for access to oxygen-rich water and the exposure to predation.
Each individual is safest in the centre of the school and at the highest risk of predation at the edges of the school. However, the water in the centre of the school is poorest in oxygen, while that at the edge of the school is richest.
The shape of the school observed provides individual members of the school with an optimal trade-off between predator avoidance and oxygen acquisition.
For more information, see the paper: Brierley, AS and MJ Cox (2010) Shapes of krill swarms and fish schools emerge as aggregation members avoid predators and access oxygen. Current Biology doi:10.1016/j.cub.2010.08.041