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Attraction between predators and their prey can take several forms. Siphonophores create “lures” with nematocyst batteries on the fishing tentacles to attract the prey (Purcell 1980). In a different form of attraction, medusae have been observed to swim toward areas of high prey concentration (Arai 1991) in the laboratory, as well as aggregating in areas where prey have recently been, suggesting a chemoreceptive mechanism of attraction resulting in higher densities of medusae.

High concentrations of medusae can be achieved by physical aggregating mechanisms or by rapid reproduction in place to form a true bloom (Graham et al. 2001). Physical cues that have been implicated in high concentrations of medusae include light‐mediated migrations such as diurnal vertical migration and aggregations associated with discontinuities in temperature, salinity, and density (pycnoclines) in the vertical plane. The reasons for accumulation of medusae at pycnoclines likely include higher concentrations of prey at the density discontinuities as well as passive mechanisms such as buoyancy at the cline.

Wind, waves, and currents can also act to produce aggregations of medusae. Populations of medusae are often compressed along the shoreline, resulting in rafts of jellies on the beach during periods of onshore winds. Oceanic frontal systems may harbor increased densities of medusae relative to waters outside of the frontal zone, similar to increases in populations observed in fishes and other more mobile species at oceanic fronts. Interestingly, a unique, persistent aggregation of the medusa Chrysaora fuscescens may be found in Monterey Bay California, the result of upwelled water entrained by a coastal prominence in the northern part of the bay (Graham et al. 1992).