In 1973, D. Weihs modelled fish schools and predicetd that the diamond shape (shifted rows, so that the single fish is always in the middle between the two nearest neighbours in front of it) is the preferred organization structure. He could show by 2D modelling, that the diamond shape could improve swimming efficiency because of the hydrodynamic interactions with the vortices created by the fishes in front. Intesaaf Ashraf et al. (2017) challenge this widespread idea of prefered diamond shape organization. They studied fish schools of the red nose tetra fish (Hemigrammus bleheri) and showed that the organization structure depends on the swimming speed. Slow swiming fishgroups showed indeed a preference for diamond or T-shape organization. However, when forced to swim faster, so in a situation in which energy efficiency would be most beneficial (for example when escaping a predator), they prefer a “in line” (phalanx-shaped) organization. This phalanx pattern reduces the distance to to the nearest neighbour. Both configurations, diamond-shaped and phalanx-shaped, save energy (reducing tail-flapping frequency) compared to a single fish swimming alone. So why changing to a line when fast swimming is required? Ashraf et al. think this conformation change is connected to the change of swim-movement-synchronization. While slow swimming fishes show no correlation in their tail movement, fast swiming fishes were synchronized (in phase or out-of phase) with their nearest neighbour. Ashraf et al. predict that this synchronized swimming kinetics together with the “side by side” organization is a good strategy to optimize thrust (and therefore speed). Unfortunately, they didn’t modeled this configuration to proof this idea. However, this study shows that maybe Weihs model of energy-saving diamond-shaped fish schools maybe needs some improvement. “Simple phalanx pattern leads to energy saving in cohesive fish schooling“
I. Ashraf et al. (2017) PNAS, doi:10.1073/pnas.1706503114
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