Distal Pigment Research Articles

The ommatidia of Arca noae: a three-tier structure with a central light-guiding element for the receptor cell

The compound eyes of ark clams appear to function as an optical system to trigger shell closure against predators. We have analyzed the structure of the ommatidia of Arca noae by thin section electron microscopy and serial sectioning, Concanavalin A-gold labeling and acid phosphatase cytochemistry. Our results demonstrate that the ommatidia are a three-tier structure composed of a central single receptor cell, surrounded and covered by proximal pigment cells followed by rows of distal pigment cells. The receptor cells of Arca noae have no lens and the disks of their receptive segment are derived from sensory cilia. The distal mitochondrial segment in the cytoplasm between the nucleus and the receptive segment is surrounded by a mass of Concanavalin A-reactive glycogen particles. Although both, proximal and distal pigment cells have numerous microvilli, only those of the proximal pigment cells form a well-aligned brush border. The microvilli of the latter are ≈9-11μm long and have a diameter of ≈70-80nm. Numerous microlamellar bodies cover them. The microlamellar bodies are stored in acid phosphatase-negative secretory granules of the pigment granule-free apical cytoplasm of proximal pigment cells before their secretion. Observation of living compound eyes indicated that the apex of proximal pigment cells transmitted significantly more light than the surrounding distal pigment cells. Hence, the regular geometry of the brush border seems to be a light-guiding structure for receptor cells similar to an optical fiber.

Serotonin expression in the optic lobes of cavernicolous crustaceans during the light–dark transition phase: Role of the lamina ganglionaris

The expression pattern of serotonin neurotransmitter (5-HT) was investigated in the optic lobes of the marine cave mysid Hemimysis margalefi (Crustacea, Mysidacea) during the dark to light and light to dark transitions by fluorescence immunohistochemistry. The intensity of the expression was evaluated according to the number of immunoreactive elements. Experiments were carried out under laboratory controlled conditions of illumination. The results show that some structures as the X-organ and medulla terminalis are always immunoreactive during light or dark periods. Other ones as the rhabdom of the retinular cells (rh), the distal pigment cells (dpc), the internal medulla (im), the external medulla (em), and the sinus gland (sg) are immunoreactive during light exposure. In these latter structures, immunoreactivity is quickly detected after illumination (30 min) while, back to darkness, it takes longer to disappear (1 h for rh and dpc, 2 h for im and more for em and sg). The 5-HT expression pattern in the lamina ganglionaris is different. During the dark to light transition, immunoreactive cells are already detected after 30 min of light exposure; after 1 h of exposure, the lamina ganglionaris expresses many immunoreactive cells, then, they are less numerous after 2 h; finally, the detection limit is reached after 3 h of exposure. During the light to dark transition, 5-HT is expressed in the lamina ganglionaris only after 1 h of darkness. The number of 5-HT neurones was increased after 2 h of darkness but no reactivity is detected after one night. To go further, same experiments were carried out in an other mysid species, Leptomysis lingvura that lives at same depth but in a shallow lagoon. In contrast to H. margalefi, L. lingvura is not subject to nycthemeral migrations. A different 5-HT expression pattern in the lamina ganglionaris has been observed: immunolabelling was detected as soon as 30 min, whatever the considered transition phase, dark to light or light to dark. These results are discussed considering the behaviour of each species. We suggest that the expression pattern of 5-HT in the lamina ganglionaris can play the role of a photoreception signal integrator during the light–dark transition phases and that cavernicolous mysid crustaceans can be considered as a good model to study the mechanisms of determination and regulation of circadian migrations of zooplankton.

Red pigment-concentrating hormone induces a calcium-mediated retraction of distal retinal pigments in the crayfish.

The octapeptide red pigment-concentrating hormone is capable of eliciting the aggregation of intracellular pigment granules in distal retinal pigment cells of isolated retinas of the crayfish Procambarus clarkii (Girard). The final level and the time course of pigment aggregation are dose dependent within a range of 10(-10) mol l(-1) to 10(-4) mol l(-1). The effect of red pigment-concentrating hormone is prevented by previous incubation with an anti- red pigment-concentrating hormone antibody; however, application of the antibody after the onset of the red pigment-concentrating hormone effect, does not prevent its full development. A similar effect to that elicited by red pigment-concentrating hormone is induced by the calcium ionophores ionomycin and A-23187. Red pigment-concentrating hormone evokes entry of 45Ca2+ to retinal cells. However, the red pigment-concentrating hormone-induced pigment aggregation persists in the presence of the calcium channel blocker verapamil and in Ca2+-free solutions. Caffeine and thapsigargin, known to release calcium from intracellular stores, elicit distal pigment aggregation, while ryanodine and dantrolene, blockers of intracellular calcium release, as well as the intracellular calcium chelator bapta-AM suppress the effect of red pigment-concentrating hormone. These results suggest that red pigment-concentrating hormone elicits distal retinal pigment aggregation by increasing intracellular calcium concentration, acting via a dual mechanism: (1) promoting calcium entry, and (2) releasing intracellular calcium.

Morphogenesis and pattern formation in the retina of the crayfish Procambarus clarkii.

Pattern formation and ommatidial differentiation in the crayfish retina were analyzed using confocal, light and electron microscopy. Optic primordia first appear in the embryo as round elevations covered by a surface epithelial layer. Retinal differentiation begins with a wave of mitotic activity that moves across this epithelium from lateral to medial. Ommatidial cell clusters are visible at the surface along a transition zone, which lies at the interface of the medial undifferentiated retina and the lateral patterned retina. This zone is 8-10 cells wide and composed of small uniform cell profiles. Lateral to the transition zone the initial ommatidial cell clusters form staggered rows across the surface. Each first row cluster contains eight retinula cells surrounded by four cone, two corneagenous and two distal pigment cells. Ommatidial clusters in the first nine rows show significant changes in their organization, which are visible at the surface of the retina. In row 10 the retinula cells recede from the surface and the cone cells close in above them creating a constant cell pattern at the surface. Rhabdome development begins distally and extends downward as the retinula cluster recedes from the surface. Movement of the retinula cells inward and enlargement of the cone and corneagenous cells at the surface creates a two-tiered organization characteristic of each ommatidium. Comparison of retinal pattern formation and differentiation in the crayfish with retinal morphogenesis in Drosophila and other insects show several similarities between the two arthropod groups.

Physiological mechanisms in the control of bioluminescent countershading in a midwater shrimp

In the oceanic midwater environment, most animals have evolved an extraordinary anti‐predation behavior using bioluminescent countershading (counterillumination) to help them remain cryptic to visual predators. For the midwater penaeid shrimp, Sergestes similis, the interaction of both hormonal and neural systems may be involved in the control of counterillumination. S. similis responds to downward‐directed illumination, detected by the eyes, with light emission from five hepatic light organs. Dark‐adapted specimens undergo a slow induction process prior to production of the conventional counterillumination response. The induction of bio‐luminescence may involve a hormonal pathway mediated by the light‐adapting retinal distal pigment dispersing hormone. Once induced, the rapid control of counterillumination may involve a neural pathway. Because counterilluminating animals directly respond to their optical environment, an understanding of the control of bioluminescence provides an insight into the poorly u...

Modulation of crayfish retinal function by red pigment concentrating hormone

The role of the crustacean octapeptide red pigment concentrating hormone (RPCH) in the control of crayfish retinal activity was explored. RPCH injection into intact animals resulted, after a latency of 10­30 min, in a dose-dependent enhancement of electroretinogram (ERG) amplitude lasting 60­120 min. RPCH was able to potentiate ERG amplitude in both light-adapted and dark-adapted animals. Following light-adaptation, responsiveness to RPCH was five times higher than following dark-adaptation. In conjunction with ERG enhancement, in light-adapted animals, RPCH injection elicited a dose-dependent retraction of distal retinal pigment, but did not affect proximal retinal pigment position. The effects of RPCH were blocked by a polyclonal antibody raised against a tyrosinated form of RPCH (A-tyr-RPCH). The antibody was also capable of partially blocking the nocturnal phase of the circadian rhythm of ERG amplitude and the darkness-induced retraction of distal retinal pigment. These results suggest that RPCH acts both on the retinal photoreceptors and on the distal pigment cells, playing a physiological role as a mediator of the effects induced by darkness and by the nocturnal phase of the circadian rhythm.

Evidence for a pathway of distal screening pigment granules across the basement membrane of the crayfish photoreceptor.

The distal pigment cells of Orconectes limosus contain two layers of large electron lucent vacuoles that are separated by layers of small right-angled platelets adjacent to the crystalline cones. The crystalline cones of the dioptric apparatus of this species have evaginations into the distal pigment cell cytoplasm. In photoreceptors of Orconectes limosus and Procambarus clarkii a dark pigment accumulation site was detected just distal to the basement membrane at the edges of each retina. These pigment accumulations occurred independent of the state of light adaptation. Ultrastructurally the pigment granules at this accumulation site resemble distal screening pigment granules according to their size (up to 1.2 microns in diameter) and fibrous structure. Distal screening pigment granules were also found in tube-like cell processes or extracellularly within and proximal to the retinal basement membrane, indicating pigment transport to and across the basement membrane. Proximal to the basement membrane screening pigment granules were also observed disintegrated to a gravel-like electron dense material in widely branched cells. Evidence was found that an electron dense material, probably resulting from disintegrating screening pigment granules, was incorporated in the integument of the eyestalk. Four hours after injection of gold particles into the eye stalk distal to the retina they were detected inside and proximal to the retinal basement membrane.

Steady‐state and dynamic properties of photomechanical light and dark adaptation in the eye of the shrimp Eualus gaimardii (crustacea, natantia)

AbstractA reflectometric method was used to study dark and light dark adaptation in shrimps (Eualus) by recording the reflectometric response to alternating periods of light‐off and light‐on that varied in intensity. We found that 1) During light‐off and light‐on periods the response slowly attained an upper and lower asymptotic level, respectively, followed by a steady‐state phase of these levels. The steady‐state level correlated strongly with the position of the distal screening pigment. 2) Steady‐state response plotted against log intensity defined an intensity‐response function representing the dynamic range of the distal pigment and consequently that of the effective aperture. The threshold where the pigment departed from its fully light‐ and dark‐adapted position was 10−5.5 and 10−9.5 W/cm2, respectively. Over this interval the intensity‐response function was linear and the increase in effective aperture equalled about .5 log unit gain in image brightness per log unit change in intensity. 3) The intensity‐response function showed cyclic shifts along the intensity continuum caused by endogeneous factors. 4) The duration of light adaptation was constant while rate of change increased linearly as a function of log intensity. Dark adaptation had similar properties at low intensities, thereafter its rate of change was constant while the duration increased linearly. These findings are consistent with the idea that the distal pigment is controlled by two hormones, the dark‐adapting hormone being subordinated to the light‐adapting one. Both the size of the dynamic intensity range and the temporal properties of light and dark adaptation showed that the photomechanical mechanism is well adapted to cope with twilight viewing conditions.

Light‐induced damage to the dioptric apparatus ofnephrops norvegicus(L.) and the quantitative assessment of the damage

In Nephrops norvegicus damage to the compound eye following exposure to excess light is not restricted to the photoreceptor layer, but is followed by morphological changes to the dioptric apparatus. Retinula cell damage results in the disruption of the cone cell processes leading to distal retraction of the crystalline tracts. The shape of the cones is also affected and there is a redistribution of the distal pigments. Within two months of exposure the reflective properties of subcorneal elements are changed and damage may be seen in the intact eye. A method based on external observations of the intact eye has been developed for quantitatively estimating the damage to the dioptric layer. The figures obtained were compared with estimates of retinula cell damage in the same eyes calculated using measurements from serially sectioned material. The estimation of ommatidial damage without recourse to sectioning should be of immediate practical benefit.

Caudal photoreceptors synchronize the circadian rhythms in crayfish—II. Functional relationships between caudal and visual photoreceptors

1. 1. Mechanisms involved in ERG cireadian rhythm synchronization were studied. 2. 2. Functional communication between the 6th abdominal ganglion photoreceptors and some structures involved in the ERG circadian rhythm, i.e. the visual photoreceptors and the shielding pigmentary cells, was analyzed. 3. 3. Spontaneous and evoked light potentials in the abdominal chain showed differences depending on day hour with maxima by night. 4. 4. Reduction induced by photostimulation of the 6th abdominal ganglion upon visual photoreceptors activity and shielding pigments position, was also maximum by night. 5. 5. Our results support an anatomical-funtional relationship between the caudal photoreceptors and visual photoreceptors, related to ERG circadian rhythm synchronization and involving the distal pigment dispersing hormone released from the sinus gland.

Eye structure and optics in the pelagic shrimp Acetes sibogae (Decapoda, Natantia, Sergestidae) in relation to light-dark adaptation and natural history

The structure and optics of the compound eyes of the neritic sergestid shrimp, Acetes sibogae , are described. The eyes are nearly spherical and heavily pigmented. The facets are square, indicating that the eye operates by the recently recognized mechanism of reflecting superposition. The most distal portion of each ommatidium is the corneal lens, which is secreted by two underlying corneagenous cells. These two cells surround the crystalline cone and cone stalk and the four cells of which they are composed and extend proximally at least as far as the distal rhabdom. Near the base of the cone stalk the extensions of the corneagenous cells swell and enclose spheres which bear on their surfaces small particles similar to ribosomes in appearance. Beneath the corneagenous cells lie four crystalline cone cells, parts of which differentiate to form the crystalline cone and cone stalk. The latter structures are compound, one quarter of each being contributed by each crystalline cone cell. Distally the crystalline cone cells send a small projection, which is surrounded by the corneagenous cells, to the cornea. Proximal extensions of each of the four parts of the cone stalk extend between the retinula cells and meet within the basement membrane. Between the base of the cone stalk and the regularly layered rhabdom lies the distal rhabdom. It is surrounded by a cell that we have termed retinula cell eight (R8), by analogy with other crustacean systems, and consists of unordered microvilli projecting from the cell membrane into the extracellular space above the layered rhabdom. In addition to R 8, which contributes only to the distal rhabdom, seven other retinula cells contribute to the proximal rhabdom, which consists of alternating ordered layers of orthogonally arranged microvilli. Four of these retinula cells are arranged orthogonally and extend far distally along the crystalline tract. The other three do not extend as far distally and alternate with the first four in their position around the axis of the ommatidium. R8 is located still further proximally at the level of the distal rhabdom. All seven of the retinula cells which contribute to the proximal rhabdom contain proximal pigment and extend through the basement membrane. The basement membrane consists of a meshwork grid with each intersection supporting a rhabdom so at this point the retinula cell axons project into different squares of the meshwork. Tapetal pigment cells are present in the vicinity of the basement membrane and extend downward to the lamina. The granules of tapetal pigment are covered or exposed by movements of the proximal pigment and also change their intracellular distribution depending on illumination. In addition to the proximal (retinula cell) pigment and the tapetal pigment the eye contains four types of distal pigment. Moving inward from the cornea these are the distal yellow pigment (DYP) which surrounds the entire eye; the distal reflecting pigment (DRP), which forms a thin layer and is continuous with the tapetal pigment at the edge of the eye; and the black distal pigment and the mirror pigment (MP) both contained within distal pigment cells (DPC). In the light-adapted state the proximal pigment moves distally, surrounding the rhabdoms, and the tapetal pigment granules move proximally so that they are mainly found beneath the basement membrane. Movements of the distal pigments are less clearcut, but they all appear to move somewhat proximally in the light-adapted state. Multivesicular bodies are more abundant in the retinula cells shortly after dawn, and are possibly related to membrane turnover. Interommatidial angle, as measured on both fixed and fresh material, varied from 2.8 to 3.8° in different parts of the eye. The crystalline cones were found to have a uniform refractive index radially, which, combined with their square shape, indicates that they function by reflecting superposition. Total internal reflection from the sides of the cones is adequate to explain the maximum diameter of the eyeshine from the dark-adapted eye at night without the need for additional mirrors. Nevertheless, from its organization and appearance the mirror pigment could act as a reflector in the dark-adapted eye. Also, the size of the glow patch indicates that there would be a gain of nearly two log units in image brightness in going from the light-adapted to the dark-adapted state. Each corneal facet was found to act as a weak converging lens, with a focal length of approximately 300 μm. The eye structure of Acetes is discussed in relation to that of other shrimp and to the natural history of Acetes .

Accessory pigment distribution and migration in the compound eye of Nephrops norvegicus (L.) (Crustacea: Decapoda)

The Norway lobster, Nephrops norvegicus (L.) has compound eyes of the reflecting superposition type. Four types of accessory pigment were identified and located within the eye: distal shielding pigment and distal reflecting pigment in the distal pigment cells, proximal shielding pigment in the retinula cells and proximal reflecting pigment in the tapetal cells. We also identified a lipid layer localized within the retinula cell axons between the basement membrane of the eye and the lamina ganglion. Of the four accessory pigments only the proximal shielding pigment moves with light and dark adaptation, the rest are stationary. Rates of light and dark adaptation were measured at dawn (0600) and dusk (1800), respectively, in phase with the circadian rhythm. Light and dark adaptation were studied at times out of phase with the circadian rhythm. The rate of light adaptation was the same for in- and out-of-phase animals. For dark adaptation there was a marked difference with out-of-phase animals showing virtually no pigment movement for the first 3 h following the onset of darkness. In-phase dark adaptation results in a proximal migration of pigment to a position level with the bottoms of the rhabdoms within 30 min. In animals dark adapted out of phase with the normal L: D cycle, it takes 5 to 6 h for the pigment to reach this position. The rate of light adaptation was the same at three different light intensities (0.02, 0.24, and 1.80 μmol·m −2·s −1). Lightinduced damage was evident 2 h after the onset of illumination at the highest intensity. The distribution of the proximal shielding pigment at various stages of in-phase dark and light adaptation was investigated using a microdensitometer. When fully dark adapted, the pigment is confined to a thin band in the vicinity of the basement membrane. During light adaptation the pigment moves distally, eventually accumulating just above the rhabdoms. Dark adaptation is complete within 2 h but during light adaptation pigment is still moving distally up to 6 h after the onset of illumination.

Diurnal changes in photoreceptor sensitivity in a reflecting superposition eye

1. Intracellular recordings of intensity response functions (V/log I curves) were made at different adaptation states at different times during a 24th period, from single photoreceptors in the central region of the eye of the Australian crayfishCherax destructor (Clark 1936). 2. The intensity that elicits a criterion response (50% of the maximum) is less at night than in the day in both the dark-adapted (DA) and lightadapted (LA) states. 3. Within the first 30 s of light-adaptation there is an extremely fast initial decrease in sensitivity, while in the latter stages of dark-adaptation there is a slow but steady increase, neither of which can be correlated with the position of the screening pigments. 4. The time courses for the movement of screening-pigment on lightor dark-adaptation during both day and night were measured using light microscopy. As found by de Bruin and Crisp (1957) in several species of prawn and Frixione et al. (1979) in the crayfishProcambarus, the proximal pigment migrates faster than the distal pigment on light-adaptation. On dark-adaptation, the distal pigment migrates faster than the proximal pigment, the latter only carrying out the first stage of dark-adaptation during the day. 5. It is concluded that there are three variable systems affecting sensitivity in a reflecting superposition eye: (a) The effective rhabdom cross-sectional area as determined by the position of proximal pigment; (b) the blur circle diameter and intensity determined by the position of the distal pigments and the width of the clear zone, and (c) transduction gain.

Effect of external level of calcium on ERG circadian rhythm in isolated eyestalk of crayfish

1. 1. The electrical response to light (electroretinogram, ERG) from dissected eyestatks of crayfish placed in normal, calcium-rich and calcium-free Van Harreveld solution was recorded during three or more consecutive days. 2. 2. In these experimental conditions the amplitude of ERG showed both high frequency and circadian cycles, which varied both in period and amplitude depending on the solution employed. 3. 3. In contrast, the distal pigment index did not show circadian activity when eyestalks were treated with the same experimental conditions. 4. 4. The role of calcium in the amplitude of ERG, in periodical activity of eyestalk, and in distal pigment position is discussed. 5. 5. It is proposed that calcium reinforces the coupling mechanisms underlying the ERG circadian rhythm in the crayfish eyestalk.

Ultrastructure of the eyes of the grass shrimp, Palaemonetes pugio

The cone cells and corneagenous cells possess extensive networks of smooth tubular endoplasmic reticulum that may be involved in optical reflectance and light-adaptational responses, respectively. The extracellular basal lamina of the basement membrane is confluent with glial cell capillary walls and may prove to be a viaduct for the transmission of hemolymph-borne substances to the retina or of retinal degradation products to the hemolymph. In addition to dense pigment granules, the distal pigment cells are shown for the first time to contain migratory reflecting platelets that are usually polymorphic in light-adapted eyes but are rectangular in dark-adapted eyes. In the latter these plates become aligned against the crystalline cones and presumably contribute to the reflection superposition optics of the grass shrimp. Dark-adapted retinular cells possess well-developed perirhabdomal cisternae, oblong or ovoid mitochondria, generally vesicular rough endoplasmic reticulum, and occasional, spherical, calcium-like intrarhabdomal inclusions. Light-adapted retinular cells possess poorly developed perirhabdomal cisternae, lamelliform rough endoplasmic reticulum, and condensed mitochondria frequently associated with lipid droplets and pigment granules. The cytoplasmic boundaries of the reflecting pigment cells expand into the extracellular spaces between individual ommatidial retinular cells during dark adaptation and recede to the interommatidial extracellular spaces during light adaptation. Cytoplasmic microfilament bundles found only at the bases of partially light-adapted rhabdomeric microvilli may be involved in microvillar shortening.

Neuropeptide modulation of photosensitivity. II. Physiological and anatomical effects of substance P on the lateral eye of Limulus

A system of efferent substance P-like immunoreactive fibers innervates the ommatidia of the Limulus lateral eye. Thus, we tested the physiological effects of substance P on the lateral eye by measuring the electroretinogram, a population potential reflecting the photoreceptors' response to light, under different experimental conditions. Substance P had no direct effect on the photoreceptors, but it induced an increase in their responsiveness to test flashes of light. The latency, magnitude, and duration of this reversible modulatory effect was dose-dependent. The lateral eye displays an endogenous circadian rhythm in its responsiveness to light. Application of exogenous substance P in the daytime causes an immediate rise as well as an increase in the nocturnal peak, while injection of one of its antagonists (D-Pro2, D-Phe7, D-Trp9 substance P) in the afternoon retards the normal rise in sensitivity and reduces the nighttime levels. Passive incubation with substance P antibodies at midnight caused a drop to diurnal levels of photosensitivity. Short-term changes in photosensitivity, similar in their nature to the substance P-induced ones, were caused by arousing the subjects. Arousal had an effect on the ongoing circadian rhythm similar to that of substance P application. Thus, the substance P efferent system may regulate neural responsiveness in both a short-term, environmentally induced manner, as well as for level setting in a circadian fashion. The mechanism for substance P-induced increases in photosensitivity involves changes in ommatidial structure: contraction of distal pigment cells, resulting in an increased aperture, and contraction of the retinular cells and rhabdom, resulting in a wider diameter of the latter. These structural modifications result in a greater angle of acceptance and increased light quantum catch.

The euphausiid compound eye ? a morphological re-investigation (Crustacea: Euphausiacea)

The ommatidia of the compund eye of the krill, Meganyctiphanes norvegica, are composed of the following cell types: corneagenous cells, main cone cells, accessory cone cells, distal pigment cells, and retinula cells. The corneagenous cells secrete the cuticular lenses, below which the crystalline cones are present. The bipartite crystalline cone is formed within the main cone cells, whereas the accessory cone cells invest the distal part of the cone and proceed proximally in the retina to the basement membrane. Six distal pigment cells are arranged around each ommatidium, though shared by adjacent ommatidia. The seven retinula cells form the rhabdom, and penetrate the basement membrane as axons. The basement membrane is formed by the conjoined extended prolongations from the accessory cone cells and sheets from the basal pigment cells.

Fine structure and convergent development of the Cirolana compound eye (Crustacea Isopoda)

Each sessile eye comprises about 60 ommatidia with large lenses (diameter 100–150 μm), spherical crystalline cones, and hypertrophied rhabdoms (diameter about 100 μm). The cones are formed by two main cone cells, and in addition two accessory cone cells are present. Seven retinula cells contribute to the rhabdom, which is fused distally and open proximally (separated rhabdomeres). A special cell type, previously designated ‘hyaline cells’ are shown to be reflecting pigment cells. These cells form a well-developed tapetal layer. Distal pigment cells screen neighbouring ommatidia. Further, the eye is delimited by two membranes homologous to the fenestrated and eye capsule membranes present in several other isopod groups. The hypertrophied rhabdoms and the elaborate tapetal layer separate the morphology of the Cirolana eye from that of other isopod eyes. These structural features makes the Cirolana eye a case of convergent development with several other deep water living crustaceans.

The compound eye of Leptodora kindtii (Cladocera). An adaptation to planktonic life.

Each of the approximately 500 ommatidia in the compound eye of the cladoceran crustacean Leptodora kindtii has a crystalline cone consisting of five cells. Five retinula cells are also present, one of which contributes to the distal 1-2 micrometer of the rhabdom only; the other four retinula cells from a continuous rhabdom. Throughout the rhabdom its cross section displays two separate halves with the axis of the microvilli in one half perpendicular to that in the other (orthogonal pattern). Interferometric analysis of the refractive index of the crystalline cone revealed an inhomogeneous system with one distal and one proximal gradient. The gradient system was found to exclude rays entering from adjacent facets, thus maintaining the optical isolation. Consequently, these optics replace distal screening pigment, which is absent in the eye. The long and unscreened crystalline cones give rise to an almost transparent eye in conformity with the overall transparency of this plank-tonic animal. The morphological characteristics of the eye of this species deviate from other cladoceran eyes, but the optical design closely resembles that of some pelagic marine amphipod crustaceans.

Regionally different ommatidial structure in the compound eye of the water-flea Polyphemus (Cladocera, Crustacea)

The two compound eyes of Polyphemus pediculus are completely fused in the midline to form a single integrated unit containing 130 ommatidia with four different types of rhabdoms. The general features of the eye include a cuticle lacking corneal lenses, crystalline cones composed of five cells and the presence of juxtacrystalline cells and distal pigment cells. The rhabdoms are fused and the palisade, when present, is a part of the extracellular space in which the rhabdom is suspended. Four different types of rhabdoms were found zonally arranged in the eye. (i) A foveal type, in the dorsofrontal region of the eye, is characterized by its long and slender shape. (Only five retinula cells contribute to forming this irregularly layered rhabdom, with the first layer composing the distal half of the rhabdom.) (ii) A second type, located ventrally to the fovea, is conventionally layered and is formed by six retinula cells, one of which is aberrant. (iii) A dorsolateral type is continuous (unlayered) and formed by six retinula cells of which one is aberrant. (iv) A dorsal and ventral edge type is wide and short, and lacking palisade. Six retinula cells contribute to the continuous rhabdom and two of these are aberrant with tiny rhabdomeres. The foveal type of rhabdom has a peculiar arrangement of the microvilli, which is thought to depress the sensitivity to vertically polarized light. This mechanism is believed to enhance the ability to detect prey. The zoned eye, with its specialized receptive apparatus, is interpreted as an adaptation for coping with a diversity of visual tasks by a very small animal.