Color Discrimination Conditioning in Two Octopus Octopus aegina and O. vulgaris

In order to investigate the color vision in Octopoda, color discrimination conditioning and transposition tests were done with two octopuses, Octopus aegina and O. vulgaris in tanks. The animals were trained to discriminate by touch between blue (25mm in diameter, reflectance λmax=460nm) and white or light shaded balls (negative balls) for a food reward. The balls were presented simultaneously in front of a shelter wherein each animal resided. After the completion of the conditioning, the negative balls were substituted with ones of different shades of gray (G1 to G5 in the order of reflectance), and transposition tests were done to examine the possibility of darkness-based discrimination. In the transposition tests, O. aegina showed correct responses at 70-100% to the blue ball irrespective of the shades of grey employed, implying the possession of color vision. O. vulgaris changed the choosing response with the shades of gray employed, indicating the darkness-based choosing and lack of color vision; biased to blue at G1, random at G2, biased to gray at G3 and G4 which were darker than the blue, and biased to blue at G5 which was much darker than the blue.