The iridescence from the cornea of the sand goby (Pomatoschistus minutus) occurs because of thin layer interference from the platelet‐like cells in the stroma. It is suggested that ionic pumps across the epithelium control the water content in the stroma and thus the spectral reflection. A saline was perfused over goby eyes and simple ion manipulation was carried out to observe any changes in the iridescent characteristics. It was found that removal of CI−and K+ions reduced the peak reflected wavelength to the blue end of the spectrum, whereas Na+had little effect. The removal of K+also caused a dramatic change to the normal shift in reflected spectral intensity. The iridescence was also found to be sensitive to pH, and the buffer HEPES was detrimental to the cornea compared to controls. These results suggest similarities to amphibian and mammalian corneal hydration cont