The Value of Neighbourhood: Collective predation upon large prey by benthic Cnidaria and its influence on individual predator behaviour, group fitness and population dynamics

Benthic cnidarian species often dominate the community from the intertidal zone to the deep seafloor. They feed through ingestion of suspended particles and small planktonic organisms. Furthermore, they can ingest animals many times larger than their size, in a solitary action referred to as protocooperation with many polyps simultaneously engaged in the capture. The aim of this PhD thesis was to evaluate to what extent these organisms benefit from aggregations large prey exploitation and, when the capture is shared among many polyps, what are the costs and advantages of cooperation over intra-specific competition. The field observations and literature review highlighted that large prey are most represented by large gelatinous plankton, generally considered as trophic endpoints. Their capture requires a particular feeding behaviour, that fits the term “macro-predation”, opposite to “micro-predation”. This neglected trophic relationship could play an important role in bentho-pelagic coupling and in the nutrient cycle of benthic systems. Laboratory experiments on Aurelia coerulea polyps showed an improvement of the fitness and a trend to decrease the interindividual distances to perform protocooperative predation. Polyps can detect the prey-produced molecules and to behave accordingly, mimicking the necessary behaviour to capture the large prey. This observation indicated the involvement of learning processes and memory acquisition. Cnidaria represents an intriguing phylum that is often considered the precursor of many biological and physiological processes that later evolved in Bilateria. The protocooperative interactions among individuals, even though not mediated by cognition, and the derived benefits of large prey feeding, could represent the baseline from which sociality evolved in metazoans, leading to highly coordinated societies that characterize many vertebrate and invertebrate taxa.