Among the various effector mechanisms involved in immune responses, the complement system is one of the most ancient, deeply-rooted and important for its ability to orchestrate different cells and factors of both innate and adaptive immunity. The comprehension of the evolution of the main complement components can provide clues to understand changes related to adaptations to new environmental conditions and life-cycles or, in the case of vertebrates, to interactions with the adaptive immunity. Data on tunicates, evolutionary close to vertebrates, are of primary importance for the elucidationof the changes associated with the invertebrate-vertebrate transition. In our model tunicate Botryllus schlosseri, we described both a lectin and an alternative pathway of complement activation, similar to those of vertebrates. All the described complement-related genes such as c3, bf, ficolin, mbl and masp are transcribed by morula cells, i.e., the immunocytes involved in cytotoxic responses and immunomodulation. Functional data suggest the presence of a complement-related immunocyte dialogue during the immune response. When B. schlosseri immunocytes are incubated with yeast (Saccharomyces cerevisiae) we observed an over-expression of C3 by morula cell that led to an increased amoebocyte-mediated phagocytosis. When a specific C3 inhibitor (compstatin) is added to the medium, this activity decreases. These new data pave the way for a better comprehension of the evolution lof complement-system behavior during immune responses. In the next future, our efforts will focus on the regulation of complement system in tunicates to shed light on the complement system function in a pre-adaptive immunity scenario.