Angular sensitivity of light and dark adapted locust retinula cells

Summary Angular sensitivity functions for light and dark-adapted locust retinula cells have been measured. The angular sensitivity for light-adapted cells follows a Gaussian function but in the dark-adapted condition the measured function is broader than Gaussian at its base. Acceptance angles for light-adapted cells are 1.5°±0.2° (horizontal), 1.4°±0.1° (vertical) and for dark-adapted cells, 2.4°±0.5° (horizontal) and 2.5°±0.4° (vertical). These values are significantly smaller than those previously reported and show the locust eye to be closer to its theoretical limit of resolution than was previously indicated.The angular sensitivity of light-adapted cells agress well with the minimum resolvable stripe period reported by those authors refuting anomalous resolution.     

Fine structural changes in dark-light adaptation in relation to unit studies of an insect compound eye with a crustacean-like rhabdom

Structural changes in dark-light adaptation of the compound eye of the beetle Creophilus erythrocephalus, have been investigated by electron microscopy. Their functional consequences have been studied by means of intracellular electrophysiological recordings from retinula cells. In the dark-adapted state the intercellular spaces between the retinula cells swell considerably and produce a palisade of extracellular origin around the rhabdom. Units from dark-adapted eyes are approximately ten times more sensitive than those from light-adapted animals and have larger acceptance angles (7·037° ± 1·23 S.D. compared with 5·135° ± 0·99 S.D. for the light-adapted insect). A spikegenerating unit, inhibited by light, was found once. Although the rhabdom is of the ‘banded’ type, polarization sensitivity was found to be unexpectedly low: 2·31 ± 1·54 S.D. It is concluded that electrical coupling, useful for dim light conditions and eyes with tiny apertures, most likely appears to be the reason for the low average polarization sensitivity.     

Visual behaviour and the structure of dark and light-adapted larval and adult eyes of the New Zealand glowworm Arachnocampa luminosa (Mycetophilidae: Diptera)

Both larval and adult New Zealand cave glowworms exhibit reactions to light; their photoreceptors must, therefore, be regarded as functional. The two principal stemmata of the larva possess large biconvex lenses and voluminous rhabdoms. Approximately 12 retinula cells are present. In light-adapted larvae the diameter of the rhabdom is 8 μm and that of an individual microvillus is 49.5 nm. Dark-adapted eyes have rhabdoms that measure 14 μm in cross section and microvilli with an average diameter of 54 nm. The compound eye of the adult comprises approximately 750 ommatidia, each with a facet diameter of 27–28 μm. A facet is surrounded by 1–6 interommatidial hairs which are up to 30 μm long. The interommatidial angle is 5.5°. Cones, consisting of 4 crystalline cone cells, are of the ‘acone’ type. Pigment granules in the primary pigment cells are twice as large as those of the retinula cells which measure 0.6–0.75 μm in diameter. The rhabdom is basically of the dipteran type, i.e. six open peripheral rhabdomeres surround 2 central rhabdomers arranged in a tandem position. The microvilli of cells 1–6 and cell 8 have diameters ranging from 68 to 73 nm, but those of the distally-located central rhabdomere 7 are 20% larger. This is irrespective of whether the eye is dark or light-adapted. In the latter the cones are long and narrow, the screening pigment granules closely surround the rhabdomeres, and the rhabdom is less voluminous than that of the dark-adapted eye.     

The ultrastructure of the compound eye of Munida rugosa (Crustacea: Anomura) and pigment migration during light and dark adaptation

The galatheid squat lobster, Munida rugosa, has compound eyes of the reflecting superposition type in which a distal cone cell layer and a proximal rhabdom layer are separated by an extensive clear zone. The eye is shown to have certain unique features. In all other reflecting superposition eyes, the clear zone is traversed by crystalline tracts formed by the cone cells. In M. rugosa a thin distal rhabdom thread, formed by the eighth retinula cell, connects the cones to the proximal fusiform rhabdoms. The cytoplasm of the other retinula cells also crosses the clear zone in a complex pattern. Fully light-adapted ommatidia are optically isolated by limited migrations of distal shielding pigments. A reflecting pigment multilayer lines each cone to facilitate the formation of a superposition image. This also shows a light-induced change which may limit the acceptance angle of the eye during light adaptation.     

龟纹瓢虫成虫的复眼形态及其显微结构

利用光镜、组织切片法观察了龟纹瓢虫Propylaea japonica(Thunberg)成虫的复眼形态及其显微结构。结果如下:(1)头正前方观,复眼外形似半球,且后方稍向内合拢。每个复眼约包括630个小眼。(2)每个小眼是由1套屈光器(1个角膜和1个晶锥)、6至8个小网膜细胞及其特化产生的视杆和基细胞等几部分组成。晶体周围及小网膜色素细胞内均含有丰富的色素颗粒。(3)小眼整体纵切显示,其上、下段色素颗粒分布相对较多,中段分布较少。(4)明、暗适应状态对小眼的色素颗粒分布有影响,性别对其分布无明显影响。明适应状态下,其色素颗粒较均匀地分布于视杆两侧上下,暗适应状态时色素颗粒则主要分布在视杆部位的上侧,显示其具有一定的重叠眼性质;而在相同的明、暗适应状态下其雌、雄成虫复眼的色素颗粒分布间无明显差异。     

Comparative Studies on Dark Adaptation in the Compound Eyes of Nocturnal and Diurnal Lepidoptera

A comparative analysis has been carried out of the time course and range of dark adaptation in the compound eyes of some common butterflies and noctuid moths (Lepidoptera). The change in sensitivity of the eye during dark adaptation was determined by measurements of the intensity of illumination necessary to elicit an electrical response of a given magnitude of the eye. It was found that the curve for dark adaptation in the diurnal species was smooth. The range of adaptive change varied in different species but usually did not cover more than 1 to 1.5 log units. In the nocturnal species the dark adaptation was found to proceed in two phases. The first phase was usually completed in less than 10 minutes and covered a range of 1 to 1.5 log units. The second phase was more prolonged and covered a range of 2 to 3 log units. In some of the experiments on nocturnal species the second phase failed to appear. Measurements of the size of the response at different intensities showed that the intensity/amplitude relationship was the same in the light-adapted eye as in the dark-adapted eye. In the nocturnal insects the response of the eye in the light-adapted condition was about 20 per cent of that in the dark-adapted eye, while in diurnal insects it was about 60 per cent.     

Adaptations for vision in dim light: impulse responses and bumps in nocturnal spider photoreceptor cells (<Emphasis Type="Italic">Cupiennius</Emphasis><Emphasis Type="Italic">salei</Emphasis> Keys)

The photoreceptor cells of the nocturnal spider Cupiennius salei were investigated by intracellular electrophysiology. (1) The responses of photoreceptor cells of posterior median (PM) and anterior median (AM) eyes to short (2 ms) light pulses showed long integration times in the dark-adapted and shorter integration times in the light-adapted state. (2) At very low light intensities, the photoreceptors responded to single photons with discrete potentials, called bumps, of high amplitude (2–20 mV). When measured in profoundly dark-adapted photoreceptor cells of the PM eyes these bumps showed an integration time of 128 ± 35 ms (n = 7) whereas in dark-adapted photoreceptor cells of AM eyes the integration time was 84 ± 13 ms (n = 8), indicating that the AM eyes are intrinsically faster than the PM eyes. (3) Long integration times, which improve visual reliability in dim light, and large responses to single photons in the dark-adapted state, contribute to a high visual sensitivity in Cupiennius at night. This conclusion is underlined by a calculation of sensitivity that accounts for both anatomical and physiological characteristics of the eye.     

Volume and surface changes of smooth endoplasmic reticulum in crayfish retinula cells upon light- and dark-adaptation

Summary Adaptation to light and darkness involves major transformations of the smooth endoplasmic reticulum (SER) in the retinula cells of the crayfish and other invertebrates, the mechanisms of which are unknown. This paper presents measurements of SER stereological parameters at three levels along the main axis of light-adapted and dark-adapted retinula cells of the crayfish. Both the volume density and the surface density of SER in the perinuclear region of dark-adapted cells are more than double the values found in light-adapted cells. This relationship is inverted in the axoplasm above and below the basement membrane, where SER volume and surface in dark-adapted cells are approximately half of the quantities measured in light-adapted cells. Although this proportional correspondence between changes in separate regions of the cells might suggest merely an intracellular shifting of SER membrane, calculation of the approximate absolute amounts of membrane involved makes this possibility very unlikely. It is concluded that the vast increase of SER in the perinuclear region of dark-adapted cells implies a large input of membrane, which is subsequently removed during light-adaptation.Abbreviation SER smooth endoplasmic reticulum     

Structure and function of the larval eye of the sawfly, Perga

The visual properties of the stemmata of fourth and fifth instar sawfly larvae have been investigated by means of intracellular electrophysiological recordings, interference-, light-, scanning, and transmission electron microscopy, and ray tracing techniques. Each larva possesses one pair of lateral ocelli consisting of biconvex lens, a clear-zone of corneagenous cells devoid of pigment, and a retina made up of groups of 8 retinula cells with a central and fused rhabdom. Migration of pigment during the night is predominantly radial, i.e. directed towards the periphery of the retinula cell. Vertical pigment movement occurs after exposure to bright sunlight for several hours. A circadian rhythm controls the sensitivity of single units, which at night show an increased sensitivity by 2 to 3 log units. The difference between the mean values of acceptance angles for light-adapted units (11·46 ± 5·1° S.D.) and dark-adapted ones (13·83 ± 6·8°) is not statistically significant. The wide range of acceptance angles, with 4·5° being the narrowest and 31° the widest, is explained by the optics of the single lens: there is a small region of highest resolution where light is well focused to a spot, but towards the outer edge of the cup-shaped eye the focusing becomes less accurate. Sawfly larvae turn their heads towards an approaching object if it subtends at least 4° of arc. This limit does not change over a range of 3 log units. A polarization sensitivity of up to 10 : 1 was determined electrophysiologically. Electron microscope studies of the rhabdom show a system of highly oriented microvilli which is thought to be responsible for the polarization sensitivity. Two different waveforms, both occurring after resting potentials of 40 to 70 mV, were found in electrophysiological recordings: (a) hyperpolarizations and (b) depolarizations. Throughout the study only depolarizing units were taken into consideration. These showed characteristics of both the compound eye and the ocellus.     

Nachweis einer Pupillenreaktion im Auge der Schmeißfliege

Zusammenfassung Elektronenoptische Untersuchungen mit der Gefrierätzmethode an Augen der Normalform von Calliphora erythrocephala (Meig.) ergeben, daß im dunkeladaptierten Zustand die Rhabdomere 1–6 in den Retinulazellen dicht von Vesikeln umgeben sind, die wahrscheinlich durch Pinocytose an den Mikrovilli der Rhabdomere entstehen. Diese Bläschen fehlen im helladaptierten Zustand nahezu völlig. Die optische Dichte des umgebenden Mediums der lichtleitenden Rhabdomere wird dadurch im Dunkel-Auge herabgesetzt, was mit Hilfe der Becke-Methode im Lichtmikroskop festgestellt werden konnte.Physikalische Berechnungen mit den Gleichungen der geometrischen Optik ergeben, daß durch diese Brechzahländerung in den Sinneszellen im dunkeladaptierten Auge der physiologisch wirksame Lichtfluß in den Rhabdomeren l–6 im grünen Spektralbereich um etwa 30% erhöht werden kann.

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Evidence for a longitudinal pupil in the blowfly eye from studies with light and electron microscopes

Summary Eyes of the wild-type blowfly, Calliphora erythrocephala (Meig.), were investigated by using freeze-etching and the light microscope. In the dark-adapted eye a layer of vesicles 1 m thick borders on the rhabdomeres in sense cells Nos. 1 to 6. These vesicles may arise by pinocytosis at the bases of the rhabdomeric microvilli and decrease the optical density of the sense cell medium adjacent to the rhabdomeres. In the light-adapted sense cells only a few vesicles could be seen. No differences could be observed to exist between the two states of adaptation in the retinular cells Nos. 7 and 8 and in the axial retinular space.Calculations based on geometrical optics show that when rhabdomeres Nos. 1 to 6 are in the dark-adapted state they can transmit about 1.3 times more light energy than when they are in the light-adapted state. The results show the existence of a light-induced pupil reaction correlated to ultrastructural changes in the photoreceptor cells of the blowfly eye.

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Der Deutschen Forschungsgemeinschaft danke ich für die Gewährung eines Ausbildungsstipendiums.     

Photocycles of bacteriorhodopsin in light- and dark-adapted purple membrane studied by time-resolved absorption spectroscopy.

Nanosecond time-resolved absorption spectra have been measured throughout the photocycle of bacteriorhodopsin in both light-adapted and dark-adapted purple membrane (PM). The data from dark-adapted samples are interpretable as the superposition of two photocycles arising independently from the all-trans and 13-cis retinal isomers that coexist in the dark-adapted state. The presence of a photocycle in dark-adapted PM which is indistinguishable from that observed for light-adapted PM under the same experimental conditions is demonstrated by the observation of the same five relaxation rates associated with essentially identical changes in the photoproduct spectra. This cycle is attributed to the all-trans component. The cycle of the 13-cis component is revealed by scaling the data measured for the light-adapted sample and subtracting it from the data on the dark-adapted mixture. At times less than 1 ms, the resulting difference spectra are nearly time-independent. The peak of the difference spectrum is near 600 nm, although there appears to be a slight (approximately 2 nm) blue-shift in the first few microseconds. Subsequently the amplitude of this spectrum decays and the peak of the difference spectrum shifts in two relaxations. Most of the amplitude of the photoproduct difference spectrum (approximately 80%) decays in a single relaxation having a time constant of approximately 35 ms. The difference spectrum remaining after this relaxation peaks at approximately 590 nm and is indistinguishable from the classical light-dark difference spectrum, which we find, in experiments performed on a much longer time scale, to peak at 588 nm. The decay of this remaining photo-product is not resolvable in the nanosecond kinetic experiments, but dark adaptation of a completely light-adapted sample is found to occur exponentially with a relaxation time of approximately 2,000 s under the conditions of our experiments.     

The eye of the soldier beetle Chauliognathus pulchellus (Cantharidae).

The soldier beetle eye is unusual in having large optically isotropic corneal cones which project inwards from a thick isotropic cornea. Refraction is mainly at the corneal surface. Calculation shows that the first focal plane is near the tip of the cone, from which the optical pathway continues as a crystalline tract. At the distal end of the crystalline tract, 3 micrometer in diameter, the four cone cells enclose the proximal tip of the corneal cone; at the proximal end they enclose the distal tip of a long fused rhabdom rod. The eye is remarkable in that there are two classes of retinula cells; four cells contribute to the long thin axial rhabdom, 2 micrometer in diameter and 120 micrometer long, and the other four cells form two rounded rhabdoms, 10 x 4 micrometer in cross-section and 20 micrometer deep, which lie to one side of the optical axis. The physiological properties of individual retinula cells were measured by intracellular recording. The retinula cells are of three spectral types with peaks near 360, 450 and 520--530 nm. Except by the criterion of spectral sensitivity, the retinula cells sampled could not be sorted into more than one class. The measured value of the acceptance angle, near 3 degrees in the dark-adapted state, is consistent with the hypothesis that all sampled cells were of the anatomical type that participate in the central rhabdom rod. A calculation of the theoretical field size of individual retinula cells from measurments of refractive index and lens dimensions predicts that cells which participate in the central rhabdom will have acceptance angles near 3 degrees. The conclusion, therefore, is that only one anatomical type of cell has so far been sampled.     

A crustacean compound eye adapted for low light intensities (Isopoda)

Summary The optical performance of the apposition compound eye of the marine isopodCirolana borealis Lilljeborg (Crustacea) was investigated. The ommatidia comprise large lenses (diam. ca. 150 m), spherical crystalline cones and hypertrophied rhabdoms. The 7 rhabdomeres are fused distally and open proximally. We have designated this rhabdom type as semifused. Distal pigment cells screen neighbouring ommatidia, and a well developed reflecting pigment layer surrounds the rhabdom. The focal length was determined in situ and refractive index measurements, raytracings, and eye mappings were made. The focus was found to lie well below the distal rhabdom tip. A theoretical acceptance function was constructed and a 50% acceptance angle of 45 ° was estimated. The eye parameter (p, according to Snyder 1977) of different ommatidia was between 44 and 14. This together with the anatomy demonstrate an optimation to extremely low light intensities. TheCirolana eye provides an example where acuity is sacrificed for the eye to be able to see at the low light intensities of the inhabitat.The investigation has been supported by a grant from the Swedish Natural Science Research Council (grant no. 2760-103). Our thanks are due to the staff of the marine biological station in Espegrend (Norway). The skilled technical assistance of Miss Inger Norling, Mrs. Rita Wallen, and Miss Maria Walles is gratefully acknowledged. And finally, we would like to express our deep appreciation to Professor Rolf Elofsson for constructive discussions and for his interest and encouragement throughout the investigation.     

Degeneration of the compound eye of the termite Neotermes jouteli (Isoptera) in darkness during the phase of reproduction

Summary Long-term light deprivation of the royal pair of Neotermes jouteli during the phase of reproduction leads to a dramatic change in the organization within the compound eye. In a swarming alate, investigated with scanning and transmission electron microscopy, the eye consists of about 200 ommatidia. No differences between male and female eyes are observed. Each ommatidium is composed of a biconvex cornea, a cone of the eucone type, and a rhabdom which is located directly beneath the Semper cells. The rhabdom consists of eight rhabdomeres which are fused along the ommatidial axis. In the central part of the compound eye the rhabdom measures roughly 20 m in length. Concealed life of the imagines causes a dismantling of the cone and the rhabdom until complete destruction. This is accompanied by an increase in the number of pigment granules and a decrease in the number of mitochondria.     

A comparison of the second harmonic generation from light-adapted, dark-adapted, blue, and acid purple membrane.

The second order nonlinear polarizability and dipole moment changes upon light excitation of light-adapted bacteriorhodopsin (BR), dark-adapted BR, blue membrane, and acid purple membrane have been measured by second harmonic generation. Our results indicate that the dipole moment changes of the retinal chromophore, delta mu, are very sensitive to both the chromophore structure and protein/chromophore interactions. Delta mu of light-adapted BR is larger than that of dark-adapted BR. The acid-induced formation of the blue membrane results in an increase in the delta mu value, and formation of acid purple membrane, resulting from further reduction of pH to 0, returns the delta mu to that of light-adapted BR. The implications of these findings are discussed.     

Immunochemical evidence for the light-regulated modulation of phosphatidylinositol-4,5-bisphosphate in rat photoreceptor cells

Immunocytochemical localization of phosphatidylinositol-4,5-bisphosphate (PIP2) in the rat rod photoreceptor outer segments (OS) was investigated with rabbit antiPIP2 antibodies. The OS of the light-adapted rat eye showed little or no staining, whereas the OS of the dark-adapted eye were intensely stained for PIP2. The immunoreactivity of photoreceptor PIP2 in the eye exposed to a brief flash of light was markedly reduced. However, subsequent dark-adaptation of the flash-bleached eye resulted in a rapid recovery of PIP2 immunoreactivity; dark-adaptation for 5 min was sufficient for recovery to the fully dark-adapted level. In dark-adapted eyes exposed to graded light intensities, the PIP2 immunostaining varied with light levels and was correlated with unbleached rhodopsin concentrations. These results suggest that PIP2 in the rat photoreceptor cells is rapidly hydrolyzed upon light exposure and rapidly synthesized in the dark and that the decrease of PIP2 level is triggered by photic bleaching of rhodopsin.     

Fatty acid composition and ultrastructure of photoreceptive membranes in the crayfish Procambarus clarkii under conditions of thermal and photic stress

The ultrastructural state of the crayfish visual membrane is correlated with its fatty acid composition during times of photic and thermal stress and the period over which the dynamic events occur is investigated. Crayfish kept at 4 °C under constant darkness contain in their rhabdoms significantly increased amounts of unsaturated fatty acids such as 16:1, 18:1, 20:5, and 22:6 compared with individuals kept at 25 °C. The ratio of unsaturated/saturated fatty acids (UFA/SFA-ratio) amounts to 2.17 in the cold-water- and 1.46 in the warm water-acclimated animals. The visual membranes of crayfish suddenly transferred from 4 °C to 25 °C exhibited ultrastructural modifications such as membrane collapse and disappearance of microvillar dense␣core-filaments most clearly 3 h post-transfer. Parallel to the structural changes a significant increase in fatty acid 18:0 was observed, while the amounts of 16:1 and 20:1 decreased. When 4 °C, dark-adapted crayfish were exposed to light alone and not a temperature increase, only fatty acid 22:6 showed a significant reduction to 10% of its pre-experimental level within 2 h of exposure. Thereafter, it slowly increased again. In cold water-acclimated crayfish that had been exposed to light of 5000 lx for 3␣weeks no significant change of the UFA/SFA ratio was observed, although fatty acid species 18:0, 20:4, and 20:5 had increased at the expense of fatty acids 14:0, 16:0, 16:1, 18:1, 20:1, and 22:6. The total amount of fatty acids, however, had become significantly smaller (from 0.058 ng g⁻¹ body weight in the dark-adapted to 0.048 ng g⁻¹ in the light-adapted crayfish). Morphologically the rhabdom volume had decreased by approx. 20%, but ultrastructurally rhabdom microvilli remained almost unchanged. The amount of peroxidized lipids in the retina following irradiation with bright white light in the cold-adapted crayfish fell during the first 2 h of exposure from 0.4 nmol g⁻¹ to 0.32 nmol g⁻¹, but after 12 h of exposure had reached a level of 0.48 nmol g⁻¹. Greatest structural abnormalities to the visual membranes occurred when dark-adapted, cold-acclimated crayfish were suddenly subjected to bright light and an increase in water temperature. Under such conditions the microvillar arrangement was disrupted and membrane collapse and disappearance of core-filaments were apparent. Our results provide evidence that the fatty acid composition of the membranes determines to a considerable extent the structural integrity of the photoreceptor, but that it is too simplistic a model to think that peroxidation of membrane lipids alone is responsible for the disintegration of the photoreceptive membranes in the crayfish eye following exposure to bright light. Accepted: 4 July 1996     

Resonance Raman study of the dark-adapted form of the purple membrane protein.

The resonance Raman spectrum of the dark-adapted form of the purple membrane protein (bacteriorhodopsin) has been obtained and is compared to the light-adapted pigment and model chromophore spectra. As in the light-adapted form, the chromophore-protein linkage is found to be a protonated Schiff base. Electron delocalization appears to play the dominant role in color regulation. The dark-adapted spectrum indicates a conformation closer to 13-cis than the light-adapted spectrum.     

Localization of the pupil trigger in insect superposition eyes

Summary In the superposition eyes of the sphingid moth Deilephila and the neuropteran Ascalaphus, adjustment to different intensities is subserved by longitudinal migrations of screening pigment in specialized pigment cells. Using ophthalmoscopic techniques we have localized the light-sensitive trigger that controls pigment position.In both species, local illumination of a small spot anywhere within the eye glow of a dark-adapted eye evokes local light adaptation in the ommatidia whose facets receive the light. Details of the response pattern demonstrate that a distal light-sensitive trigger is located axially in the ommatidium, just beneath the crystalline cone, and extends with less sensitivity deep into the clear zone. The distal trigger in Deilephila was shown to be predominantly UV sensitive, and a UV-absorbing structure, presumably the distal trigger, was observed near the proximal tip of the crystalline cone.In Ascalaphus we also found another trigger located more proximally, which causes local pigment reaction in the ommatidia whose rhabdoms are illuminated (the centre of the eye glow). The light-sensitive trigger for this response appears to be the rhabdom itself.     

A study of visual pigments in the two antarctic crustaceans Orchomene plebs (Amphipoda) and Glyptonotus antarcticus (Isopoda)

• 1.1. Total chromophore contents as well as the contributions made by 11-cis retinal were determined by high pressure liquid chromatography in light- and dark-adapted eyes of Orchomene plebs and Glyptonotus antarcticus (Amphipoda and Isopoda, respectively).
• 2.2. In O. plebs the highest amount of total chromophore in pmol/eye was found to be 18.5 in 36 hr dark-adapted animals. The lowest amount (11.6 pmol/eye) was recorded in 24 hr light-adapted individuals.
• 3.3. In dark-adapted O. plebs, irrespective of whether dark-adapted for 36 or 60 hr, the percentage of 11-cis retinal was maximally 96.6%. In the light-adapted material it reached 71.2%
• 4.4. In eyes of 20 hr dark-adapted Glyptonotous antarcticus, possibly because of insufficient dark adaptation, a total chromophore content of only 3.2 pmol/eye was found. The percentage of 11-cis retinal was 55.8.
• 5.5. Porphyropsin with its testable 3-dehydroretinal (vitamin A₂-aldehyde) was not encountered in any of our samples.
• 6.6. Calculations of photopigment per gram body weight and a comparison with data from freshwater crayfish show that dark-adapted O. plebs possess approximately 20 times the relative photopigment amount of the crayfish. Absolute sensitivity of the eye of O. plebs is, therefore, expected to be very high.