Safety in numbers? Evidence of non-social behaviour in the moon jellyfish Aurelia spp.

Abstract When animals explore a novel environment, they often aggregate in groups rather than staying isolated to reduce the risks of predation. This effect is known as ‘safety in numbers’ and has been widely reported in terrestrial and aquatic species. Jellyfish are often found to be clustered in the same areas in seas and oceans. Here, we asked whether one reason for such behavior might be the tendency to actively join other conspecifics when encountering novel environments. We observed moon jellyfish (Aurelia spp.) in a free-choice test measuring the proportion of time spent near conspecifics when they are inserted into an unfamiliar environment. We found evidence that jellyfish detect the presence of other conspecifics and use this information to actively direct their behavior in a given direction. Contrary to what was expected by the ‘safety in numbers’ effect, moon jellyfish have significantly distanced themselves from their social companions throughout the whole observation period. This result suggests that, although moon jellyfish are commonly observed in large aggregates in nature due to various environmental factors, they may exhibit limited social behavior in unfamiliar environments.