  We present a theory of jellyfish swarm formation and exemplify it with
simulations of active Brownian particles. The motivation for our analysis is
the phenomenon of jellyfish blooms in the ocean and clustering of jellyfish in
tank experiments. We argue that such clusters emerge due to an externally
induced phase transition of jellyfish density, such as convergent flows, which
is then maintained and amplified by self-induced stimuli. Our study introduces
three mechanisms relevant for a better understanding of jellyfish blooming that
have not been taken into account before which are a signaling tracer,
jellyfish-wall interaction and ignorance of external stimuli. Our results agree
with the biological fact that jellyfish exhibit an extreme sensitivity to
stimuli in order to achieve favorable aggregations. Based on our theoretical
framework, we are able to provide a clear terminology for future experimental
analysis of jellyfish swarming and we pinpoint potential limitations of tank
experiments.
