Photoreceptors and diurnal variation in spectral sensitivity in the fiddler crab Gelasimus dampieri

ABSTRACTColour signals, and the ability to detect them, are important for many animals and can be vital to their survival and fitness. Fiddler crabs use colour information to detect and recognise conspecifics, but their colour vision capabilities remain unclear. Many studies have attempted to measure their spectral sensitivity and identify contributing retinular cells, but the existing evidence is inconclusive. We used electroretinogram (ERG) measurements and intracellular recordings from retinular cells to estimate the spectral sensitivity of Gelasimus dampieri and to track diurnal changes in spectral sensitivity. G. dampieri has a broad spectral sensitivity and is most sensitive to wavelengths between 420 and 460 nm. Selective adaptation experiments uncovered an ultraviolet (UV) retinular cell with a peak sensitivity shorter than 360 nm. The species’ spectral sensitivity above 400 nm is too broad to be fitted by a single visual pigment and using optical modelling, we provide evidence that at least two medium-wavelength sensitive (MWS) visual pigments are contained within a second blue-green sensitive retinular cell. We also found a ∼25 nm diurnal shift in spectral sensitivity towards longer wavelengths in the evening in both ERG and intracellular recordings. Whether the shift is caused by screening pigment migration or changes in opsin expression remains unclear, but the observation shows the diel dynamism of colour vision in this species. Together, these findings support the notion that G. dampieri possesses the minimum requirement for colour vision, with UV and blue/green receptors, and help to explain some of the inconsistent results of previous research.

The morphology and physiology of a “mini-ommatidium” in the median optic nerve of Limulus polyphemus

Examination of the Limulus median optic nerve with low-magnification light microscopy allows clear visualization of an ultraviolet-sensitive mini-ommatidium enshrouded by pigment cells, glial cells, and guanophores. Serial 1-μm sections of median optic nerves containing mini-ommatidia revealed the presence of a single, heavily pigmented photoreceptor (retinular) cell and a single, unpigmented arhabdomeric cell. Computer-assisted serial reconstructions from 1-μm sections confirmed the presence of two cells, each bearing a nucleus, and two axons leaving the mini-ommatidium. The retinular cell is morphologically similar to retinular cells from the median and lateral eyes. Its rhabdomere appears to be a continuous sheet of microvilli with much infolding. The structure of the arhabdomeric cell is nearly identical to those found in the median ocellus. As in other photoreceptors in Limulus, the retinular cell of the mini-ommatidium is innervated by efferent fibers from the brain. Each mini-ommatidium generates a single train of nerve impulses in response to light, presumably from the arhabdomeric cell. Measurement of the spectral sensitivity of the mini-ommatidium based upon a constant-response criterion indicated that the retinular cell is maximally sensitive to near ultraviolet light with λmax = 380 nm. Comparison of intensity-response functions revealed that those of the mini-ommatidium are significantly steeper than those of the ocellus almost certainly as the result of neural processing in the ocellus which is absent in the mini-ommatidium.

Localization of actin filaments and microtubules in the cells of the Limulus lateral and ventral eyes

The ommatidia of the lateral eye of the horseshoe crab, Limulus polyphemus, undergo rhythmic changes in structure that are driven by diurnal lighting and efferent neural activity from a circadian clock in the brain. This study uses cytochemical probes to investigate the cytoskeletal elements mediating these responses and to develop models for their control. Antibodies to actin and phalloidin, a specific F-actin probe, label the rhabdom of lateral eye ommatidia, the cone cells of the ommatidial aperture, the ommatidial sheath, and the peripheral regions of the photoreceptor (retinular cell) cytoplasm. These probes also label the rhabdomere of ventral photoreceptors. Antibodies to tubulin label the eccentric cell dendrite and soma in each lateral eye ommatidium, the cone cells of the aperture, and the peripheral retinular cell cytoplasm. Models are proposed for the cytoskeletal mechanisms involved in controlling aperture and rhabdom shape, pigment movement, and shedding of rhabdomeral membrane.

Effects of Chloroquine on the Photosensory Membrane Turnover and the Ultrastructure of Lysosome-Related Bodies of the Crayfish Photoreceptor

The effect of chloroquine in combination with bright light on the ultrastructure of crayfish (Orconectes limosus R.) photoreceptors was investigated in vivo. Chloroquine had several effects upon the crayfish retina. Multivesicular bodies (MVB) that are involved in lysosomal degradation of photosensory membrane were altered in ultrastructure. MVB frequently contained smaller MVB in a state of more advanced membrane degradation. Additionally screening pigment-like granules appeared in MVB. MVB accumulated in and filled the retinular cell cytoplasm just proximal or distal to the basement membrane which indicated inhibition of photosensory membrane degradation. Under chloroquine treatment rhabdom degradation appeared to be inhibited, as rhabdom diameter was less reduced under these conditions. Also chloroquine caused accumulation of screening pigment granules in glial cells within the lamina ganglionaris.

Dual controls for screening pigment movement in photoreceptors of theLimuluslateral eye: Circadian efferent input and light

The radial and longitudinal distribution of retinular screening pigment in the lateral eye of the horseshoe crabLimulus polyphemuswas quantified under a variety of experimental conditions. Pigment position was characterized by the center and width of the radial distribution at four levels in the ommatidium.Under diurnal lighting, intact animals show movement of pigment granules from the periphery of the retinular cell at night towards the junction of the arhabdomeral and rhabdomeral segments of the retinular cell in the day. In constant darkness, intact animals exhibit the same circadian rhythm in pigment migration. Animals with bilaterally cut optic nerves do not receive circadian efferent input from the brain and show little pigment movement in diurnal lighting. In all of these cases, pigment was either aggregated in a band just peripheral to the rays of the rhabdom or dispersed to the periphery of the retinular cell.When dark-adapted animals are exposed to a sudden large light increment, pigment moves inward between the rays of the rhabdom. During the day, this inward response begins immediately and reverses as the ommatidial aperture begins to close. At night, the onset of the inward movement is delayed, but then occurs more rapidly than during the day. No significant longitudinal movement of photoreceptor screening pigment was detected under any of these experimental conditions.Two opposing mechanisms control the movement of screening pigment in these cells. Release of neurotransmitters from the circadian efferents causes outward movement; large increments of light cause inward movement. In the absence of sudden changes in light intensity, circadian efferent input, not cyclic lighting, appears to be the major determinant of screening pigment position. A sudden and large increment of light triggers the rapid inward movement which appears to be a protective mechanism optimized for daytime performance.

Circadian rhythms in Limulus photoreceptors. I. Intracellular studies.

The sensitivity of the lateral eye of the horseshoe crab, Limulus polyphemus, is modulated by efferent optic nerve impulses transmitted from a circadian clock located in the brain (Barlow, R. B., Jr., S. J. Bolanowski, and M. L. Brachman. 1977. Science. 197:86-89). At night, the efferent impulses invade the retinular, eccentric, and pigment cells of every ommatidium, inducing multiple anatomical and physiological changes that combine to increase retinal sensitivity as much as 100,000 times. We developed techniques for recording transmembrane potentials from a single cell in situ for several days to determine what circadian changes in retinal sensitivity originate in the primary phototransducing cell, the retinular cell. We found that the direct efferent input to the photoreceptor cell decreases its noise and increases its response. Noise is decreased by reducing the rate of spontaneous bumps by up to 100%. The response is increased by elevating photon catch (photons absorbed per flash) as much as 30 times, and increasing gain (response per absorbed photon) as much as 40%. The cellular mechanism for reducing the rate of spontaneous quantum bumps is not known. The mechanism for increasing gain appears to be the modulation of ionic conductances in the photoreceptor cell membrane. The mechanism for increasing photon catch is multiple changes in the anatomy of retinal cells. We combine these cellular events in a proposed scheme for the circadian rhythm in the intensity coding of single photoreceptors.