Social Predation by a Nudibranch Mollusc.

Synopsis: Social predation is a common strategy used by predators to subdue and consume prey. Animals that use this strategy have diverse methods of finding each other, organizing behaviors, and capturing prey. There is wide variation in the extent to which these behaviors are coordinated and in the stability of individual roles. This study characterizes social predation by the nudibranch mollusc, Berghia stephanieae, which is a specialist predator that eats only the sea anemone, Exaiptasia diaphana. A combination of experimental and modeling approaches established that Berghia consistently preys upon E. diaphana in groups, even when resources are abundant. However, this preference for social foraging does not appear to be a fixed personality trait, as individuals did not exhibit stable roles such as leader or follower. Instead, the population exhibited fission-fusion dynamics with temporary roles during predation. The extent of this social feeding was not altered by length of food deprivation, suggesting that animals are not shifting strategies based on hunger state. Furthermore, classic gastropod cues-such as slime trails, attraction to injured anemones, or preference for conspecifics feeding-did not facilitate group formation. Thus, Berghia provides an example of a specialist predator of dangerous prey that loosely organizes social feeding, independent of hunger state and fixed individual roles, while the mechanism of aggregation remains unknown.

Significance statement: Social predation is an adaptive strategy that enables predators to subdue dangerous prey while minimizing injury. Many nudibranchs specialize to predate upon cnidarians, which pose unique challenges due to their potent defenses. Although nudibranchs are often characterized as solitary hunters, our study reveals that Berghia stephanieae exhibits social predation behaviors, forming temporary, fluid groups to feed on sea anemones. These groups lack stable social structures, with individuals adopting temporary roles such as joining or initiating feeding. Interestingly, we found no evidence that aggregation is driven by simple cues such as slime trails, conspecific activity, or prey injury, suggesting that group formation may depend on more complex or context-specific mechanisms. This work highlights the need for further research into the ecological and sensory factors underlying social predation in nudibranchs and other marine predators.