Fatty acid compositions of arthropod and cephalopod photoreceptors: interspecific,seasonal and developmental studies

Fatty acid compositions of the compound eyes of insects (soldier-bug, Hemiptera, and silk moth, Lepidoptera), crustaceans (crayfish and grapsid crab, Decapoda) and inner and outer segments of visual cells of a squid (Cephalopoda, Mollusca) were analyzed by gaschromatography for interspecific comparison. Fatty acid compositions showed great variation among species. In insect compound eyes, 16:0 and 18:0 were the main saturated fatty acids, and 18:1 was the dominant unsaturated fatty acid. Silk moth eyes contained, in addition, considerable amounts of 18:2 and 20:5. In crustacean compound eyes, the main saturated fatty acids were 16:0 and 18:0, and 14:0 (5.0%) was only detected in grapsid crabs; the main unsaturated fatty acids were 20:4, 20:5 and 22:6. Both whole eyes and rhabdom fraction of crayfish showed similar profiles of fatty acid compositions. Both inner and outer segments of squid retinae were characterized by high amounts of unsaturated fatty acids, especially 22:6. Compound eyes of grapsid crabs were used for the experiments on seasonal changes of fatty acid compositions. UFA/SFA ratios (weight in % of unsaturated fatty acids saturated fatty acids) were lowest (1.0) in July and highest (2.5) in March, and unsaturation indexes (average number of double bonds per molecule) were lowest (1.5) in July and highest (2.3) in March. Fatty acids 18:0 and 20:1 showed a significant correlation with the changes of seasonal temperature. Fatty acid analysis of the developing compound eyes of silk moths during the pupal stage revealed that eicosapentanoic acid (20:5) increased remarkably in parallel with the development of photoreceptive membranes, the rhabdoms. This suggests that eicosapentaenoic acid may play an important role in formation and function of rhabdoms.     

Retinular fine structure in compound eyes of diurnal and nocturnal sphingid moths

Summary Retinular fine structure has been compared in the superposition compound eyes of three sphingid moths, one nocturnal, Cechenena, and two diurnal, Cephonodes and Macroglossum. Cechenena and Cephonodes have tiered retinas with three kinds of retinular cells: two distal, six regular and one basal. The distal retinular cells in Cechenena are special in having a complex partially intracellular rhabdomere not present in Cephonodes. Macroglossum lacks the distal retinular cell. In Cephonodes a unique rhabdom type, formed by the six regular retinular cells in the middle region of the retinula, is divided into three separate longitudinal plates arranged closely parallel to one another. Their constituent microvilli are consequently all nearly unidirectional. The ratio of rhabdom volume to retinular cell volume in the two diurnal sphingids is 10–27%; this is about the same as that (25%) of skipper butterflies, but significantly smaller than in the nocturnal Cechenena (60%). In the diurnal sphingids retinular cell membranes show elongate meandering profiles with septate junctions between adjacent retinular cells. From the comparative fine structure of their eyes the diurnal sphingids and the skippers would appear to be phylogenetically closely related.Supported in part by grants from Ministry of Education Japan (Special Project Research in Animal Behaviors)     

Ultrastructural study of the naupliar eye of the ostracodeVargula graminicola (Crustacea,Ostracoda)

Summary Ostracodes, like other crustaceans, have a simple naupliar eye that is built upon a theme of three eye cups surrounded by a layer of screening pigments. The single naupliar eye of the ostracodeVargula graminicola is situated medially on the dorsal-anterior side of the body and has three fused eye cups, two dorso-lateral and one ventral. Each eye cup has the following components: (1) pigment cells between the eye cups, (2) tapetal cells, (3) retinular cells with (4) microvillar rhabdomeres, and (5) axons extending into the protocerebrum. Typically two retinular cells contribute lateral microvilli to each rhabdom. The two dorso-lateral eye cups have about 40 retinular cells (20 rhabdoms) and the ventral eye cup has about 30 retinular cells (15 rhabdoms). Typical of myodocopid naupliar eyes (as reported from light microscopic studies), no lens cells or cuticular lenses were observed. The presence of tapetal cells identifies theVargula eye as a maxillopod-ostracode type crustacean naupliar eye. It is unlikely that the naupliar eye ofV. graminicola functions in image formation, rather it probably functions in the mediation of simple taxis towards and away from light.     

The Fine Structure of the Compound Eyes of Shallow-Water Asellotes,Jaera albifrons Leach and Asellus aquaticus L. (Crustacea: Isopoda)

Abstract Both species have small sessile compound eyes. The dioptric apparatus of J. albifrons consists of a biconvex lens and a pyriform crystalline cone, the latter formed by two principal and two accessory cone cells. A. aquaticus has a reduced lens and a round cone formed by two to four principal cone cells with two to no accessory cone cells. Distal pigment cells and pigmented retinular cells lie between the ommatidia in J. albifrons. A. aquaticus has only the pigmented retinular cells. Both species have a fused, continuous (unhanded) rhabdom formed by eight retinular cells (R1—8), one of which (R8) is situated distally. The retinular cells R1—7 form, in J. albifrons, a cylinder-shaped middle portion with three microvillar directions (60° apart) and a proximal star-shaped portion. The entire rhabdom of A. aquaticus is star-shaped. Distal pigment-cell processes and basal cells form the fenestrated membrane in J. albifrons and “eye-cup cells” in A. aquaticus.     

The Ultrastructure of the Compound Eye of Two Species of Marine Ostracodes (Crustacea: Ostracoda: Cypridinidae)

Ultrastructurally, the compound eyes of the luminescent marine ostracodes Vargula graminkola and V. tsujii are similar. These ostracodes have two lateral compound eyes, with relatively few ommatidia (13 and 20 respectively). They exhibit apposition type compound eyes as seen in many other arthropods. Each ommatidium includes: a flat, ectodermal cuticular covering, corneagen cells, two long cone cells that give rise to a large conspicuous crystalline cone, retinular cells, pigment cells, a microvillar rhabdom and proximal axonal neurons. The axons merge to form an optic nerve that extends into the brain through a short, muscular stalk that is surrounded externally by a cuticle. The number of retinular cells is typically six per ommatidium in V. graminicola and eight per ommatidium in V. tsujii. Screening pigment cells surround each ommatidium forming a layer that is about 5–15 pigment granules thick. In addition to pigment cells, the cytoplasm of the retinular cells includes numerous screening pigment granules. In light/dark adaptation, there are no obvious morphological differences in the orientation of the rhabdom or in the organization of the screening pigments. Both Vargula species studied are nocturnally active and bioluminescent suggesting that these eyes are capable receptors of the bright conspecific luminescence.     

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。     

Daily changes of structure,function and rhodopsin content in the compound eye of the crabHemigrapsus 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.     

The Fine Structure of the Compound Eye of Tanais cavolinii Milne-Edwards (Crustacea: Tanaidacea)

Abstract The compound (apposition) eyes of Tanais cavolinii are not well developed: the number of ommatidia is small and there are certain irregularities in structure. The refractive components are formed by the cornea and the cone. The latter is built up by two cone cells. In addition, there are two accessory cone cells confined to the distal part of the cone. The eight pigmented retinular cells extend from the cornea to the basement membrane. Proximal to the cone, they form a fused continuous rhabdom, which in cross section has a rectangular outline. In the middle part of the rhabdom, the microvilli are arranged perpendicular to the long axis of the rhabdom when seen in cross section. The microvilli outside of this area can be arranged either parallel or perpendicular to the microvilli of the middle part. Other irregularities occur in the ommatidium, e.g. the position of the retinular cell nuclei, which are found at different levels. Extensions from the cone cells fuse and form a mesh proximal to the rhabdom. Between the mesh and basal lamina is a basal cell type enveloping the proximal parts of the retinular cells and their axons. These cells also form the basal lamina, which delimits the compound eye from the haemocoel. No special pigment cells are present in the compound eye of Tanais cavolinii.     

Cellular identification of the violet receptor in the crayfish eye

Summary Ten violet receptors in the retinas of crayfish (Procambarus) were injected intracellularly with the fluorescent dye Lucifer Yellow-CH and subsequently identified in histological preparations. All had their cell body located distal to the main rhabdom, in the position of the small, 8th retinular cell. In nine cases it was possible to trace the axon of the violet receptor beyond thelamina ganglionaris, and in four cases, to its termination in themedulla.By contrast, 22 green receptors similarly injected were all found to contribute to the main rhabdom, which is formed by retinular cells 1–7. Their axons synapsed in thelamina ganglionaris.Microspectrophotometry of the 8th cell reveals an absorption peak at 440 nm. As previous microspectrophotometric observations indicated that retinular cells 1–7 all contain a visual pigment with _max at 530 nm, the microspectrophotometric data confirm that the violet receptor is cell 8.This work was supported by USPHS grant EY00222 to Yale University. D.C. is a USPHS predoctoral trainee supported by National Research Service Award 5-T32-GM07527. We are grateful to Dr. W.W. Steward for a gift of Lucifer Yellow-CH and J.D. Collins for technical assistance.     

Photopigment gene expression and rhabdom formation in the crayfish (<Emphasis Type="Italic">Procambarus clarkii</Emphasis>)

This study examines the expression of the photopigment gene in the developing retina of the freshwater crayfish Procambarus clarkii(Crustacea, Malacostraca, Decapoda). Both sense and anti-sense RNA probes were used for in situ hybridization (ISH) of whole embryos collected at various stages during development. A characteristic of retinal development is the formation of screening pigment in the retinular cells of the retinal ommatidia. This pigmentation is seen as a band that begins at the lateral side of the retinal field and progresses medially. At hatching the retina is approximately 50% pigmented. ISH of whole embryos shows that expression of the photopigment gene by the retinular cells correlates with the extent of the screening pigment band in the retina and with the presence of rhabdoms within the ommatidia. Sections taken through embryos after being hybridized indicate that staining is localized in the cytoplasm of the retinular cells and in the axonal region below the basement membrane. No staining reaction was seen in the rhabdoms of older ommatidia. ISH staining was also seen at the anterior midline of the protocerebrum where extraretinal photoreceptors have been reported. The data presented here show a close correlation of opsin expression within the retinular cells of the ommatidia and the formation of the very early rhabdoms, similar to Drosophila. The results will be discussed in relation to recent studies in Drosophila that suggest rhodopsin plays a role in effecting the organization of the terminal web-like cytoskeleton at the base of the developing rhabdom microvilli.     

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.     

Twisted rhabdomeres in the compound eye of a tipulid fly (Diptera)

Summary The individual rhabdomeres of the outer retinular cells (R1–6) in the tipulid fly, Ptilogyna, twist about their long axes. Proximally, the rhabdoms become partitioned off by processes from the retinular cells, so that the basal region of each rhabdomere is enclosed in a pocket formed by its own cell (Fig. 2). This organisation of the rhabdom enables each rhabdomere to twist while supported within its own retinular cell, and while the cell itself maintains its orientation with respect to the entire ommatidium. Theory predicts that the rhabdomeral twisting should significantly reduce the polarisation sensitivity of R1–6, but have little effect on the efficiency with which unpolarised light is absorbed.     

Photoreceptor spectral sensitivities of the Small White butterfly <Emphasis Type="Italic">Pieris rapae crucivora</Emphasis> interpreted with optical modeling

The compound eye of the Small White butterfly, Pieris rapae crucivora, has four classes of visual pigments, with peak absorption in the ultraviolet, violet, blue and green, but electrophysiological recordings yielded eight photoreceptors classes: an ultraviolet, violet, blue, double-peaked blue, green, blue-suppressed-green, pale-red and deep-red class. These photoreceptor classes were identified in three types of ommatidia, distinguishable by the different eye shine spectra and fluorescence; the latter only being present in the eyes of males. We present here two slightly different optical models that incorporate the various visual pigments, the light-filtering actions of the fluorescent, pale-red and deep-red screening pigment, located inside or adjacent to the rhabdom, and the reflectance spectrum of the tapetum that abuts the rhabdom proximally. The models serve to explain the photoreceptor spectral sensitivities as well as the eye shine.     

Orthogonal microvillus pattern in the eighth rhabdomere of the rock crab Grapsus

Summary The eighth retinular cell (R 8) of Grapsus lacks cytoplasmic pigment granules and basically resembles those previously known in the ghost crab Ocypode and the mysid Praunus. Distally located, R 8 comprises four lobes inserted between the outer ends of the seven regular retinular cells (R 1–R 7). A thin cytoplasmic bridge connects these lobes. One lobe adjacent to R 1 contains the nucleus of R 8 and gives rise proximally to the cell's axon. The short distal eighth rhabdomere consists of microvilli (mvl) protruding axially from all four lobes. Similar R 8's were found also in two other crab families and in two other genera of mysids.In Grapsus the eighth rhabdomere is extraordinary in possessing mvl oriented in two orthogonal directions parallel to the mvl of R 1–R 7. The distal 20% of the rhabdom consists of mvl originating exclusively from R 8. These appear in somewhat irregular bands and are alternately oriented parallel to the animal's vertical or horizontal axis. More proximally the retinula contains eleven sectors but the rhabdom still comprises bands of alternating mvl with those from R 8 joined respectively by the rhabdomeres of R 1, 4, and 5 (horizontal) and R 2, 3, 6 and 7 (vertical). The rest of the rhabdom shows typical decapod organization with seven interdigitating rhabdomeres.This research has been aided by grants from the United States Public Health Service (5 RO1 EY 00405) and the National Geographic Society. The authors are grateful to Mabelita Campbell for her helpful assistance.     

钙离子浓度对暗适应时的中华绒螯蟹光感受器超微结构的影响

用透射电镜研究了暗适应时中华绒螯蟹的光感受器超微结构与外界钙离子浓度的关系,结果显示出与培育在生理溶液中的光感受器相比,细胞外钙离子浓度升高,使得感杆束的直径急剧缩小,感杆束周围胞质增厚,胞饮泡增加,膜下猪泡囊极度减小。胞质中多囊体的数量和直径减小,而板模体和溶酶体的数量增加,同时细胞内的色素颗粒增多。分布在小网膜细胞的远端。细胞的结构表现为类似光适应状态,与之相反,细胞外钙离子浓度降低时小眼的感     

A phospholipase inhibitor, manoalide, and a G-protein activator, Mas-7, both affect the turnover of phototransductive membranes by crab retinas in darkness With a model for the regulation of rhabdomeral membrane turnover

(1) In vitro retinas of a crab, Leptograpsus, were treated with a phospholipase inhibitor, manoalide, or a G-protein activator, Mas-7. Both drugs address early stages of the phototransduction cascade. (2) Manoalide inhibited the light-dependent reduction of rhabdoms during the `day' phase of the light cycle, but did not induce rhabdom overgrowth. Following a period of darkness manoalide failed to affect the diminution of illuminated rhabdoms. (3) The diminution of rhabdoms that follows photoreceptor depolarisation induced by 100 mmol · l<sup>−1</sup> K<sup>+</sup> in darkness was not affected by 2␣μmol · l<sup>−1</sup> manoalide. (4) When retinas in the `night' phase were treated with Mas-7 in darkness, rhabdom diameters were augmented, concurrently with endocytosis of photoreceptor plasma membranes. (5) The results of combining manoalide and Mas-7 with actinomycin D, U-57908 or okadaic acid, drugs used in previous studies to manipulate steps notionally lower in the transduction cascade, lead to a hypothetical model for the regulation of phototransductive membrane turnover by arthropods. Accepted: 3 October 1996     

The eyes of Macrosoma sp. (Lepidoptera: Hedyloidea): a nocturnal butterfly with superposition optics

The visual system of nocturnal Hedyloidea butterflies was investigated for the first time, using light and electron microscopy. This study was undertaken to determine whether hedylids possess the classic superposition eye design characteristic of most moths, or apposition eyes of true butterflies (Papilionoidea), and, to gain insights into the sensory ecology of the Hedyloidea. We show that Macrosoma heliconiaria possesses a superposition-type visual mechanism, characterized by long cylindrical crystalline cones, a lack of corneal processes, 8 constricted retinular sense cells, rhabdoms separated from the crystalline cones forming a translucent 'clear zone', and tight networks of trachea that form a tapetum proximal to the retina and which also surround the rhabdoms to form a tracheal sheath. Dark-adapted individuals of M. heliconiaria, M. conifera, and M. rubidinarea exhibited distal retinular pigment migration, forming an eye glow. Correspondingly, light-exposure induced pigment to migrate proximally, causing the eye glow to be replaced by a dark pseudopupil. Other characteristics of the visual system, including relative eye size, facet size, and external morphology of the optic lobes, are mostly 'moth like' and correlate with an active, nocturnal lifestyle. The results are discussed in relation to the evolution of lepidopteran eyes, and the sensory ecology of this poorly understood butterfly superfamily.     

The visual pigment of a stomatopod crustacean,Squilla empusa

Summary Stomatopod crustaceans are visually active animals which have large, mobile compound eyes of unique design. Aspects of their ecology and behavior suggest they may be able to discriminate hues. Isolated rhabdoms of the squillid stomatopod,Squilla empusa, were investigated using microspectrophotometry and fluorometry. A single rhodopsin, of _max507 nm, exists in the main rhabdom. Its stable metarhodopsin, with _max503 nm, possesses typical arthropod fluorescence characteristics. No evidence was found for a visual pigment with peak absorption in the ultraviolet. Vision in this animal might therefore be monochromatic.Abbreviation ASW artificial sea water     

Neurosecretory fibres in the median eyes of the scorpion,Androctonus australis L.

Summary The retina of the median eyes of the North African scorpion, Androctonus australis L., is supplied with numerous neurosecretory nerve fibres which establish synaptoid contacts on the retinula cells. The number of fibres or profiles of varicosities of fibre terminals associated with a retinular unit (five retinula cells with a fused rhabdom) varies between 10 and 20. Electron-opaque vesicles with a diameter of 80–100 nm are abundant within the axonal profiles. The synaptoid junctions are characterized by postsynaptic electron-dense material on the inner leaflet of the retinula cell membrane and, frequently, presynaptic submembranous dense material. Because of these ultrastructural features, the junctions observed here resemble typical interneuronal synaptic contacts. Hence this kind of neurosecretory junction appears to be unique among arthropods.It is suggested that the neurosecretory fibres within the retina represent the efferent pathways for the control of the circadian pigment movements within the retinula cells.Supported by the Deutsche Forschungsgemeinschaft (F1 77/7)     

Cellular basis for polarized light perception in the spider crab,Libinia

Summary Differential increases in the numbers of pinocytotic vesicles, multivesicular bodies and total complex bodies occurred in the cytoplasm of specific photoreceptor cells in the compound eye of the crab Libinia exposed for six hours to polarized light with various e-vector orientations. These data coupled with previous results on the same species proved that the seven retinular cells in each ommatidium formed two functional groups selectively light adaptable by e-vectors oriented 90° apart. One group (Channel I, comprising Cells 1, 4 and 5) was more affected by horizontal polarization; the other (Channel II, comprising Cells 2, 3, 6 and 7) was more affected by vertical polarization.This confirmed by a quite independent technique the conclusion reached from electrophysiological experiments on the crab Cardisoma that decapod compound eyes have two orthogonal polarization analyzer channels. In addition the present data showed that both channels occur in each ommatidium as hypothesized on previous electron microscopic evidence and that the axes of maximum absoprtion in the two retinal channels were parallel to the long axes of their cells' rhabdom microvilli, horizontal in Channel I and vertical in Channel II. The latter relations in turn supported the hypothesis that the dichroism of rhodopsin was fundamental to the analyzer mechanism.This research has been supported by U. S. Air Force Grant AFOSR 1064 and NASA Grant NGR 07-004-055. The authors wish to thank Professor Joseph G. Gall for generously sharing his electron microscope facilities.