Collective exploitation of large prey by group foraging shapes aggregation and fitness of cnidarian polyps

Group living is widespread and beneficial to metazoans. It improves protection and survival opportunities, reinforcing interspecific competitiveness. Benthic cnidarians often colonize large surfaces. Evidence of collective capture and exploitation of large prey by small, clumped polyps suggests that aggregation is functional to access food resources hardly achievable by isolated individuals. In turn, the chance to catch large prey may represents a driver of aggregation in polyps, whether beneficial to their fitness. Here, the effects of group foraging on aggregation, asexual reproduction, and growth rates of Aurelia coerulea von Lendenfeld 1884 polyps were experimentally tested by providing them with either small or large prey, or a mix of both to simulate the co-occurrence of preys at sea. As expected, some polyps were not able to reach the large prey. Hence, the population was a posteriori divided into group-foragers and solitary-feeders. In general, the large prey diet resulted in higher population fitness and when simultaneously supplied with the small prey represented an energetic booster resulting beneficial for all group-members. The decrease of interindividual distances was reported among group-foragers, that converged towards each other. Cnidarians are basal in metazoan evolution, and the comprehension of their collective foraging behavior, as well as the processes leading to the selective feature driving them to forage in group or not, may be essential to better understand the evolution and spread of social foraging in animals. Moreover, the access to large prey by sessile polyps of Aurelia coerulea could be pivotal in determining the increase in abundance of adult bloom-forming medusae.