The stable dodecameric oxy form of Carcinus aestuarii hemocyanin (Cae oxy-Hc) has been probed by resonance Raman, UV-vis, and CD spectroscopy. A clear signal emerged at 746 cm(-1) (Raman) sensitive to isotopic exchange with (18)O(2) (-40 cm(-1)), thus confirming the presence of a mu-eta(2):eta(2) peroxide coordinated to the cupric centers. The optical fingerprints of Cae oxy-Hc show that symmetry breaking exists at the Cu(II)-O(2)(2-)-Cu(II) core and that exciton coupling effects are present in the visible region of the spectra.