Key indicators of circadian regulation include the persistence of
physiological rhythmicity in the absence of environmental time cues and
entrainment of this rhythmicity by the ambient light cycle. In some
teleosts, the inner segments of rod and cone photoreceptors contract and
elongate according to changes in ambient lighting and the circadian cycle.
Pigment granules in the retinal pigment epithelium (RPE) disperse and
aggregate in a similar manner. Collectively, these movements are known as
retinomotor movements. We report the histological characterization of
diurnal and circadian retinomotor movements in zebrafish, Danio
rerio. Adult fish subjected to a 14:10 light:dark (LD) cycle,
constant darkness (DD), or constant light (LL) were sacrificed at
1–13 h intervals and processed for semithin sectioning of the
retina. Using bright-field microscopy, 15 measurements of pigment granule
position and the inner segment lengths of 30 rods and 30–45 cones
were collected from the central third of the dorso-optic retina per time
point. In LD, rods and cones followed a clear diurnal rhythm in their
inner segment movements. Short-single, UV-sensitive cones were found to
contract significantly 1 h before light onset in LD conditions. In DD
conditions, the inner segments movements of short-single and double cones
displayed statistically significant rhythms. RPE pigment granule movements
are rhythmically regulated in both LD and DD although fluctuations are
damped in the absence of photic cues. No significant retinomotor movements
were observed in LL. These findings indicate retinomotor movements in
zebrafish are differentially regulated by an endogenous oscillator and by
light-dependent mechanisms.