Meeting Abstract
Animals may take up certain positions within collectively swimming or flying groups in order to reduce their cost of locomotion or to enhance their ability to sense prey, predators, or conspecifics. Antarctic krill (Euphausia superba) form massive schools which may extend for several kilometers horizontally and for hundreds of meters vertically. The three-dimensional structure of such schools may reveal their adaptive advantage. We used stereophotogrammetric videos of Antarctic krill schooling in the laboratory to determine the internal structure of such schools. For krill with a mean body length of 34 mm (measured from telson tip to the front of the eyeball), the mean school density was 4244 animals m-3 at a mean swimming speed of 68 mm s-1 and mean nearest neighbor distance of 47.1 mm. With a polarity of 34°, the krill reached a similarly high level of organization as fish schools. The nearest neighbor position distribution is anisotropic and shows a preference for Antarctic krill to swim in the propulsion jet of their nearest neighbor, likely to sense its hydrodynamic signal. The distribution also shows that Antarctic krill avoid having a nearest neighbor overhead, possibly to avoid obstruction of the light it needs for orientation. Structure in the nearest neighbor position distribution is also likely created by the tight packing of elongated krill bodies within the school. Further, the distributions shed light on how various hydrodynamic mechanisms might lead to energy savings for krill swimming in schools.
