Antarctic krill takes its cue from its swarm neighbors | Polarjournal
Antarctic krill can only survive in the wild under the protection of the swarm. Photo: Uwe Kils via Wikipedia

Antarctic krill (Euphausia superba) is the keystone species in the Southern Ocean on which virtually all life in the marine ecosystem around Antarctica depends. The centimeter-sized crustaceans, which form huge swarms, are correspondingly well researched, but still quite little is known about their swarm behavior. A team of researchers has now been able to answer some questions in a recent study that appeared in the journal Proceedings of the Royal Society.

In general, animals form swarms to be better protected from predators in the mass and to easier find food. This is also true for Antarctic krill, which moreover saves energy when swimming in a swarm. Thus, swarming behavior has a crucial importance for its survival. However, scientists were still puzzling over how krill form and maintain swarms.

Australian scientists have now investigated these questions for the first time by closely observing a small captive krill swarm of about 3000 to 4000 individuals. They used two cameras to record the krill’s movements and then converted the picture into 3D images. Using mathematical software, the researchers then studied the movements of individual krill individuals.

The team found that Antarctic krill respond strongly to the behavior of their neighbors, with the animals using clear rules of interaction, much like schooling fish. This social attraction is coupled with a flow of information between individuals, such as matching their speed to that of their neighbors. According to the researchers, this behavior suggests that krill swarms are not just formed due to good food availability or ocean currents, but rather are actively formed.

According to the study results, krill in the swarm orients itself to its nearest neighbors, even if the movements seem more like a wild jumble. Video: U.S. National Ocean Service

The distance from an individual to its nearest neighbors in the experiment was two to three body lengths, and similar to fish shoals or bird flocks, it largely aligns itself with them. The team also observed that an individual accelerates when its near neighbors are in front or behind it. The information provider for the change in velocity could be the propulsion jet from the near neighbors, the authors speculate.

Compared to other school-forming animals, Antarctic krill thus shows some similarities, but the researchers also observed characteristics that appear to be unique: Krill responded differently to its neighbors depending on which vertical layer they were in. “Focal krill [a krill individual, editor’s note] turned towards near neighbours who were ahead and below and up but away from those ahead and above”, the authors said.

One explanation for this behavior could be the hydrodynamics of krill shoals. Furthermore, the differential response to neighboring individuals could be related to predator avoidance strategies, as many predators attack from above or below, but not from the side.

Having been able to answer important questions regarding the schooling behavior of Antarctic krill with this study, the researchers see an urgent need to corroborate their laboratory observations with further studies of free-living krill in the Southern Ocean.

Julia Hager, PolarJournal

Link to the study: Burns Alicia L., Schaerf Timothy M., Lizier Joseph et al. 2022 Self-organization and information transfer in Antarctic krill swarms. Proc. R. Soc. B. 289: 20212361. 20212361. http://doi.org/10.1098/rspb.2021.2361

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