Diurnal changes in retinula cell sensitivities and receptive fields (two-dimensional angular sensitivity functions) in the apposition eyes of Ligia exotica (Crustacea, Isopoda)

The structural organization of the retinula cells in the eye of Ligia exoti ca changes diurnally, At night, the microvilli elongate, losing the regular and parallel alignment characteristic of the day condition. Crystalline co nes and distal rhabdom tips are not pushed into each other during the day, but at night the rhabdoms protrude into the crystalline cones by up to 5 mu m. Screening pigment granules in the retinula cells disperse during the nig ht, but migrate radially towards the vicinity of the rhabdom during the day . No such displacements of the pigment granules of either distal or proxima l screening pigment cells were observed. The sensitivity of the eye, monitored by electroretinogram (ERG) recordings , changes diurnally: values at midnight are, on average, 10 times those occ urring during the day. However, intracellular recordings from single retinu la cells (50 during the day and 50 at night) indicate that the difference b etween night and day sensitivities is only 2.5-fold. Two-dimensional angular sensitivity curves, indicative of a single unit's s patial sensitivity, had considerably less regular outlines at night than du ring the day. If based on the 50 % sensitivity level, day and night eyes po ssessed receptive fields of almost identical width (approximately 2 degrees ), but if sensitivities below the 50 % limit were included, then receptive fields at night were significantly more extensive. We suggest that the morphological adaptations and diurnal changes in chromo phore content seen in the apposition eye of L. exotica allow this animal to improve its photon capture at night while preserving at least some of the spatial resolving power characteristic of the light-adapted state. This wou ld explain why this animal is capable of performing complex escape behaviou rs in the presence of predators both in bright and in very dim light.