Small marine crustaceans, such as krill, and larger clupeid fish, which include sardines and anchovies, swim together in large shoals of a strikingly similar 'lozenge' shape.
Andrew Brierley and Martin Cox at the University of St Andrews in Fife, UK, used multibeam sonar to survey populations of krill (Euphausia superba; pictured) off the western Antarctic Peninsula. When they compared their observations with published studies on fish behaviour, they found a fairly constant ratio between shoals' surface area and volume, even though the size and packing density of krill and clupeid fish shoals varies widely.
The duo's modelling work suggests that the consistent shoal shape balances the creatures' needs, maximizing their exposure to oxygen while minimizing their risk of predation.
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Animal behaviour: Same-shaped shoals. Nature 467, 370 (2010). https://doi.org/10.1038/467370a
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DOI: https://doi.org/10.1038/467370a
