Characterization of retinal oil droplets in diurnal geckos (reptilia, gekkonidae).

Retinal oil droplets have been documented in the retinae of four genera of diurnal geckos (Phelsuma, Gonatodes, Quedenfeldtia, and Pristurus), while other large diurnal genera (Sphaerodactylus and Lygodactylus) lack oil droplets. Where they occur, droplets are found only in the minor members of double cones of type B of the extrafoveal and peripheral regions, whereas in the foveal cones droplets could not be detected. Oil droplets in gecko retinae have neither an internal structure nor an own membrane; this is typical for oil droplets of amphibians and sauropsids. The droplets are nonfluorescent and definitely transparent. Thus, they do not function as filters modifying the spectral composition of the light reaching the outer segments. The constant relationship between the diameters of the oil droplets and those of the outer segment bases might still suggest an optical function of the droplets (e.g., as microlenses focusing light on the outer segments). However, as the ecologically very similar genera Phelsuma, Gonatodes, and Lygodactylus differ in the presence or absence of oil droplets, this potential function seems to be not of physiological significance.