"Mutualistic" means a relationship between two or more organisms where they both benefit from each other's presence or actions. It's like a teamwork or cooperation between them for their own advantages.
Nature abounds with examples of mutualistic relationships. (sciencedaily.com)
From their experiments, the researchers developed a simple model to predict the type of mutualistic relationship that would develop between the two strains, given how their populations changed over time. (sciencedaily.com)
Such mutualistic associations, where both host and symbiont evolve to accommodate one another abound in the history of life. (fossilmuseum.net)