Direct and indirect biotic interactions driving community structure, can be positive, increasing the fitness of both partners, or negative, often increasing the fitness of only one of the participant to the interaction. Moreover, partner interaction may produce an integration among them, thus giving rise to an association defined as symbiosis, with parasitism and mutualism as the two extremes of a symbiotic continuum. In the past, negative interactions have been considered more widespread within communities and responsible for most of their structures. They have also been studied by an experimental approach. By contrast, cooperative interactions have mainly approached with a descriptive aspect. Negative interactions are more abundant in benign environments, whilst positive ones develop according to the environmental harshness with different organisms cooperating to cope environmental stress. Both parasitism and mutualism can evolve from a commensal interaction depending from the ecological context and have a different role in promoting evolution. In turn commensalism can evolve from epibiosis, a widespread phenomenon in marine benthic environment where the high spatial competition promotes the overgrowth of organisms. Therefore epibiosis is the starting point for further evolution of different kind of interactions. We describe some examples of mutualisms in marine environment, underlining how such interactions can lead to the increase of biodiversity.