Retinal extracts have been prepared from dark-adapted mudsuckers by treatment of retinal tissue or of isolated outer segments of the visual cells with digitonin solution. The extracts were examined spectrophotometrically and found to absorb light maximally between the wave lengths of 488 and 510 mµ, depending on the proportion of yellow impurities and light-sensitive pigment present. This photosensitive pigment was shown to be homogeneous by partial bleaching of the extracts with monochromatic light of various wave lengths from 390 to 660 mµ. The mudsucker pigment was specifically demonstrated not to be a mixture of rhodopsin and porphyropsin; the adequacy of the method used to analyze such mixtures was shown by performing a control experiment with an artificial mixture of bullfrog rhodopsin and carp porphyropsin.
Comparison of the hydroxylamine difference spectrum and of the absorption maximum of the purest retinal extract located the mudsucker photosensitive pigment maximum at 512 ± 1 mµ. Extraction of retinal tissue with a fat solvent after exposure to white light gave a preparation which after the addition of antimony chloride reagent developed the absorption band maximal near 664 mµ, which is characteristic of retinene1. If an hour intervened between exposure of the retinal tissue to light and extraction of the carotenoid, the antimony trichloride test gave a color band maximal at 620 mµ, characteristic of vitamin A1. No evidence of retinene2 or vitamin A2 was obtained. The euryhaline mudsucker has, therefore, a photosensitive retinal pigment with an absorption maximum halfway between the peaks of rhodopsins and of porphyropsins and belonging to the retinene1 system characteristic of rhodopsins. The pigment is therefore named a retinene1 pigment 512 of the mudsucker, Gillichthys mirabilis. It is uncertain whether this type of photosensitive pigment will be found in other euryhaline fishes.