In the vast immensity of the sea, finding a space to live can be highly competitive for animals that live fixed to the bottom. The ability to grow and live on living substrates, known as epibiosis, is a direct consequence of this competition for space, and can offer different advantages and disadvantages to associated organisms. Hydroids are colonial cnidarians attached to various living substrates such as algae, sponges, corals and crustaceans. In this project, we ask whether the great diversity of forms, habits and life histories observed among hydroids can be explained by their interactions with different living substrates. Thus, we seek to understand the role of epibiosis in the diversity patterns of marine hydroids and the ecological and/or evolutionary factors that can explain these interactions.