Diurnal changes in the fine structure of photoreceptors in an abalone, Nordotis discus

In order to examined a diurnal (24 h) rhythm in the fine structure of photoreceptors of abalone, Nordotis discus, the animals were kept on a daily light-dark cycle (light period: 6:00 a.m.--6:00 p.m., 2--5 lx) for 10 days; subsequently their eyes were fixed at nine specified times of the day. At 5:30 a.m. the photoreceptors consisted of numerous terminal microvilli growing on the surface of a dome-shaped apical cytoplasmic process (AP). Illumination induced breakdown of the photoreceptive microvilli into many vesicles. These vesicles were densely aggregated on almost the entire surface of enlarged APs at 7:00 a.m. After 1 h they markedly decreased in number. The cell at 10:00 a.m. possessed sparse, short and irregularly-contoured microvilli. The microvilli gradually increased in number from the middle through the later half of the light period. Only within 1 h after deprivation of light shorter but characteristic terminal microvilli were observed. By midnight they became similar in fine structure to those at 5:30 a.m.