The cone oil droplets of avian retinas

The cone oil droplets of 19 species of birds from 11 families were examined by microspectrophotometry. Individual droplets were expanded with mineral oil, suspended in aqueous glycerol, and absorbance spectra measured between 700 and 320 nm. A classification of oil droplets is proposed, in which objective measurements of their carotenoids are related to the size, position and visual appearance of the droplets under the microscope. Some droplets contain no carotenoid and are transparent at wavelengths longer than 320 nm. Other droplets appear colorless but contain carotenoids absorbing at 385 or 402 nm. The pale droplets that have traditionally been described as greenish contain a mixture of two carotenoids. All of these types are distinct from yellow and red droplets. Red droplets contain astaxanthin esters, and yellow droplets contain a carotenoid with a spectrum similar to zeaxanthin. The 402 nm chromophore is galloxanthin, a C27 apo-carotenoid with 8 double bonds. The in vivo optical densities are 1-4 in the paler droplets, range up to about 8 in the yellow droplets, and can exceed 20 in the red droplets. All droplets that contain carotenoid can exhibit substantial absorption in the near u.v. The frequencies of the several droplet types in the retinas of different species suggests that these organelles respond readily to natural selection and may be involved in more than one function.