Circadian changes in the retinal pigment epithelium of the butterfly fish (Pantodon buchholzi)

The fine structure of the retinal pigment epithelium (RPE) of the butterfly fish (Pantodon buchholzi) has been investigated in both light- and dark-adaptation. In this species which possesses a retinal tapetum lucidum, the RPE melanosomes move basally (sclerally) in dark-adaptation and apically (vitreally) during light-adaptation. In addition a number of other morphological changes are noted which also appear to be influenced by the light cycle. In dark-adaptation when the melanosomes move basally the lipid spheres of the tapetum lucidum move apically to form an efficient reflective layer with the tapetal spheres often coalescing at this time. In light-adaptation when the melanosomes move more apically, the tapetal material moves more basally and is probably masked to some extent by the melanosomes. Other morphological features which appear to change at least to some degree are; the shape and location of the RPE nucleus (more basally located and more pleomorphic in light-adaptation); the localisation and electron density of the mitochondria (more scattered and more electron dense in dark-adaptation) and the number and size of the myeloid bodies (small and relatively numerous in dark-adaptation and larger but more widely scattered in the light-adapted state). Wandering phagocytes between the RPE cells were only noted in light-adaptation when they were situated adjacent to Bruch's membrane. These observations would indicate that a number of morphological changes take place within the RPE cells of this species in addition to be obvious movement of melanosomes and that these changes should perhaps be included in descriptions of photomechanical or retinomotor movements.