Diurnal changes in compound eye fine structure in the blue crab Callinectes. 1. Differences between noon and midnight retinas on an LD 11:13 cycle

Putative photosensitivity in internal light organs (organs of Pesta) of deep-sea sergestid shrimps

Many marine species can regulate the intensity of bioluminescence from their ventral photophores in order to counterilluminate, a camouflage technique whereby animals closely match the intensity of the downwelling illumination blocked by their bodies, thereby hiding their silhouettes. Recent studies on autogenic cuticular photophores in deep-sea shrimps indicate that the photophores themselves are light sensitive. Here, our results suggest photosensitivity in a second type of autogenic photophore, the internal organs of Pesta, found in deep-sea sergestid shrimps. Experiments were conducted onboard ship on live specimens, exposing the animals to bright light, which resulted in ultrastructural changes that matched those seen in crustacean eyes during the photoreceptor membrane turnover, a process that is crucial for the proper functioning of photosensitive components. In addition, RNA-seq studies demonstrated the expression of visual opsins and phototransduction genes in photophore tissue that are known to play a role in light detection, and electrophysiological measurements indicated that the light organs are responding to light received by the eyes. The long sought after mechanism of counterillumination remains unknown, but evidence of photosensitivity in photophores may indicate a dual functionality of light detection and emission.

Light adaptation mechanisms in the eye of the fiddler crab Afruca tangeri

A great diversity of adaptations is found among animals with compound eyes and even closely related taxa can show variation in their light-adaptation strategies. A prime example of a visual system evolved to function in specific light environments is the fiddler crab, used widely as a model to research aspects of crustacean vision and neural pathways. However, questions remain regarding how their eyes respond to the changes in brightness spanning many orders of magnitude, associated with their habitat and ecology. The fiddler crab Afruca tangeri forages at low tide on tropical and semi-tropical mudflats, under bright sunlight and on moonless nights, suggesting that their eyes undergo effective light adaptation. Using synchrotron X-ray tomography, light and transmission electron microscopy and in vivo ophthalmoscopy, we describe the ultrastructural changes in the eye between day and night. Dark adaptation at dusk triggered extensive widening of the rhabdoms and crystalline cone tips. This doubled the ommatidial acceptance angles and increased microvillar surface area for light capture in the rhabdom, theoretically boosting optical sensitivity 7.4 times. During daytime, only partial dark-adaptation was achieved and rhabdoms remained narrow, indicating strong circadian control on the process. Bright light did not evoke changes in screening pigment distributions, suggesting a structural inability to adapt rapidly to the light level fluctuations frequently experienced when entering their burrow to escape predators. This should enable fiddler crabs to shelter for several minutes without undergoing significant dark-adaptation, their vision remaining effectively adapted for predator detection when surfacing again in bright light.

Chemical cues from fish heighten visual sensitivity in larval crabs through changes in photoreceptor structure and function

Several predator avoidance strategies in zooplankton rely on the use of light to control vertical position in the water column. Although light is the primary cue for such photobehavior, predator chemical cues or kairomones increase swimming responses to light. We currently lack a mechanistic understanding for how zooplankton integrate visual and chemical cues to mediate phenotypic plasticity in defensive photobehavior. In marine systems, kairomones are thought to be amino sugar degradation products of fish body mucus. Here, we demonstrate that increasing concentrations of fish kairomones heightened sensitivity of light-mediated swimming behavior for two larval crab species (Rhithropanopeus harrisii and Hemigrapsus sanguineus). Consistent with these behavioral results, we report increased visual sensitivity at the retinal level in larval crab eyes directly following acute (1-3 h) kairomone exposure, as evidenced electrophysiologically from V-log I curves and morphologically from wider, shorter rhabdoms. The observed increases in visual sensitivity do not correspond with a decline in temporal resolution, because latency in electrophysiological responses actually increased after kairomone exposure. Collectively, these data suggest that phenotypic plasticity in larval crab photobehavior is achieved, at least in part, through rapid changes in photoreceptor structure and function.

Compound eyes of insects and crustaceans: Some examples that show there is still a lot of work left to be done

Similarities and differences between the 2 main kinds of compound eye (apposition and superposition) are briefly explained before several promising topics for research on compound eyes are being introduced. Research on the embryology and molecular control of the development of the insect clear-zone eye with superposition optics is one of the suggestions, because almost all of the developmental work on insect eyes in the past has focused on eyes with apposition optics. Age- and habitat-related ultrastructural studies of the retinal organization are another suggestion and the deer cad Lipoptena cervi, which has an aerial phase during which it is winged followed by a several months long parasitic phase during which it is wingless, is mentioned as a candidate species. Sexual dimorphism expressing itself in many species as a difference in eye structure and function provides another promising field for compound eye researchers and so is a focus on compound eye miniaturization in very small insects, especially those that are aquatic and belong to species, in which clear-zone eyes are diagnostic or are tiny insects that are not aquatic, but belong to taxa like the Diptera for instance, in which open rather than closed rhabdoms are the rule. Structures like interommatidial hairs and glands as well as corneal microridges are yet another field that could yield interesting results and in the past has received insufficient consideration. Finally, the dearth of information on distance vision and depth perception is mentioned and a plea is made to examine the photic environment inside the foam shelters of spittle bugs, chrysales of pupae and other structures shielding insects and crustaceans.

Cephalic structures in the nemertine Parborlasia corrugatus-Are they really eyes?

The opinion as to whether tiny, approximately 0.1 mm large spots around the innermost margin of the cephalic slits in the Antarctic nemertine worm Parborlasia corrugatus represent photoreceptors or not has fluctuated over the years. This first electron microscope study of the enigmatic spots fails to detect any screening pigment granules, rhabdomeres, or lamellae, but reveals that the structure in question is principally made up of two types of cell, characterized by vesicular and vacuolar material of approximately 80 nm and 0.3 mum in diameter, respectively. Filamentous connective tissue strands with gaps for axons surrounds the 'eye-spot' and it is suggested that either exposure to the bright Antarctic summer light has led to a total disintegration of all visual membranes or these structures do not represent eyes at all.

Structure of the subrhabdomeric cisternae in the photoreceptor cells of Drosophila melanogaster

The structure of subrhabdomeric cisternae (SRC) and related structures in the photoreceptor cells (retinular cells) of Drosophila melanogaster in normal flies and visual mutants were compared by electron microscopic observation of semithin sections of osmium-impregnated specimens. The three-dimensional organization of SRC and the other cell organelles was demonstrated by stereoscopy. Both light- and dark-adapted normal retinular cells contained elaborate networks of anastomosing tubules of SRC immediately beneath the rhabdomeres. Tubules connecting the SRC and rough endoplasmic reticulum were frequently seen. The SRC were absent from the retinular cells of rdgAKS60 whose rhabdomeres degenerate gradually after eclosion. Instead, numerous smooth vesicles were observed in the subrhabdomeric regions. In rdgBEE170, in which rhabdomere degeneration is light dependent, the SRC appeared normal in the dark-adapted flies. But their SRC gradually disintegrated after exposure to light. In norpASB37, whose rhabdomeres are small but do not degenerate, SRC appeared normal. These results suggest that the SRC is a significant structure for the maintenance of the structure of photoreceptive membrane in the retinular cells of Drosophila.

Daily changes of structure, function and rhodopsin content in the compound eye of the crab Hemigrapsus sanguineus

The compound eye of the crab hemigrapsus sanguineus undergoes daily changes in morphology as determined by light and electron microscopy, both in the quantity of chromophore substances studied by HPLC and in visual sensitivity as shown by electrophysiological techniques. 1. At a temperature of 20 degrees C, the rhabdom occupation ratio (ROR) of an ommatidial retinula was 11.6% (maximum) at midnight, 8.0 times larger than the minimum value at midday (1.4%). 2. Observations by freeze-fracture revealed that the densities of intra-membranous particles (9-11 nm in diameter) of rhabdomeric membrane were ca. 2000/microns 2 and ca. 3000/microns 2 for night and daytime compound eyes, respectively. 3. Screening pigment granules migrated longitudinally and aggregated at night, but dispersed during the day. Reflecting pigment granules migrate transversally in the proximal half of the reticula layer i.e. cytoplasmic extensions containing reflecting pigment granules squeeze between neighbouring retinula cells causing optical isolation (Fig. 4). Thus the screening pigment granules within the retinula cells show longitudinal migration and radial movement so that the daytime rhabdoms are closely surrounded by the pigment granules. 4. At 20 degrees C, the total amount of chromophore of the visual pigment (11-cis and all-trans-retinal) was 1.4 times larger at night than during the day i.e. 46.6 pmol/eye at midnight and 33.2 pmol/eye at midday. Calculations of the total surface area of rhabdomeric membrane, total number of intra-membranous particles in rhabdomeric membrane and the total number of chromophore molecules in a compound eye, indicate that a considerable amount of chromophore-protein complex exists outside the rhabdom during the day. 5. The change in rhabdom size and quantity of chromophore were highly dependent on temperature. At 10 degrees C both rhabdom size and amount of chromophore stayed close to daytime levels throughout the 24 hours. 6. The intracellularly determined relative sensitivity of the dark adapted night eye to a point source of light was about twice as high as the dark-adapted day eye. Most of the increase in the sensitivity is attributed primarily to the effect of reflecting pigment migration around the basement membrane and, secondarily, to the changes in the amount and properties of the photoreceptive membrane. The results form the basis of a detailed discussion as to how an apposition eye can function possibly as a night-eye.

Phagocytosis of rhabdomeral membrane by crab photoreceptors (Leptograpsus variegatus)

Many crabs possess fused rhabdoms which are partly broken down at dawn and re-synthesised at dusk. The cross-sectional area of the rhabdom is therefore smaller during the day than at night. The only previously described mechanism of membrane removal from the rhabdomere in Crustacea involves the formation of pinocytotic vesicles at the bases of the microvilli. The geometry of the rhabdom is such that uniform pinocytosis across the base of each rhabdomere would result in a stack of orthogonally oriented rectangles. In the process described here, microvilli from the outer edges of the rhabdomeres are engulfed by adjacent retinula cells, reducing the number as well as the length of the microvilli and maintaining the smooth longitudinal profile needed for optimal functioning of the rhabdom.