Chromium (Cr) is an essential microelement in mammals, involved in modulation of metabolism and immune function, also showing endocrine and anti-depressant activities. Cr(III) is considered the biologically active form, although the underlying mechanisms are still unclear; on the other hand, the oxidized form Cr(VI), that is most abundant in the environment and more easily absorbed in biological systems, is associated with oxidative stress and toxicity. Physiological responses to nM concentrations of Cr(IV) were investigated in of the model aquatic invertebrate Mytilus. The results demonstrate that Cr exerts pleiotropic effects on metabolism, immune function, redox balance and neuroendocrine signaling, supporting its role as an essential metal in biological systems and contributing to the understanding of its modes of action from the molecular to the tissue level.