  This work focuses on the formation of mating aggregates in zooplankton. In
particular, sexual encounters are behaviourally supported by males actively
swimming in search for females, and approaching them for mating once they are
found. While the random search leads to a diffusive flux of individuals, the
approaching for encounter supports attraction. Thus, we ask whether these
competing mechanisms of diffusion and attraction can support aggregation and
lead to the formation of mating clusters. To answer our question we formulate a
model in which particles performing random walks can briefly make contact with
other particles if they are found within a particular distance from each other.
Our analysis shows that this model supports clustering in a way analogous to
the process of colloid aggregation. Following that, we analyze a dataset of 3D
trajectories of swimming copepods and show that the results compare well with
our model. These results support the hypothesis that pair-interactions promote
mating aggregates in zooplankton and are sufficient to overcome the diffusive
nature of their mate searching behavior. Our results are useful for
understanding small-scale clustering of zooplankton, which is crucial for
predicting encounter rates and reproduction rates in the ocean.
