In many coral reefs, anemonefish gather in exceptionally dense groups within immense carpets of anemones. These aggregations, called megacolonies, can contain more than 100 individuals and around ten breeding couples, challenging the classic social model based on a strict size hierarchy. The project "Megacollectif - Megacolonies, a new type of social interactions and collective behavior in anemonefish," supported by the MITI-CNRS 2025 call for projects "Complex Interactions and Collective Behaviors 2025," aims to study this alternative social organization and explore the hypothesis of multi-level societies, a type of structure never before described in fish. This interdisciplinary program mobilizes approaches from ecology, ethology, genomics, and comparative endocrinology, as well as telemetry, to map social and spatial interactions within megacolonies, based on sites in Japan, French Polynesia, and Australia. It reveals surprising social plasticity in anemonefish and questions the evolution of collective behavior in marine ecosystems.