Geometrical optics of a galatheid compound eye

The eyes of galatheid squat lobsters (Munida rugosa) are shown to be of the reflecting superposition type. In the dark-adapted state corneal lenses focus light at the level of the rhabdoms and light from more than 1000 facets is redirected to the superposition focus by the reflecting surfaces of the crystalline cones. When the eye is light adapted, apposition optics are used. In this state paraxial light is focused by the corneal lens and the parabolic proximal end of the cone onto the distal end of a rhabdomeric lightguide. The latter transmits light across the clear zone to the rhabdom layer. In the dorsal part of the eye the individual ommatidia become progressively shorter until the cones and rhabdoms are no longer separated by a clear zone. Although formerly considered to be developing ommatidia, they are shown to be retained specifically for scanning the downwelling irradiance.Abbreviations RI refractive index - SEM scanning electron microscope     

Chick eye optics: zero to fourteen days

Ocular dimensions and refractive state data for chicks 0 to 14 days of age were obtained from 234 untreated control eyes of birds treated unilaterally in previous work involving various defocussing lenses and/or translucent goggles. Refractive state and corneal curvatures were measured in vivo by retinoscopy and ophthalmometry respectively. Intraocular dimensions were measured by A-scan ultrasonography, after which the eyes were removed, weighed and measured. In some cases (n=52) intraocular dimensions and lens curvatures were obtained from frozen sections of enucleated eyes. The hyperopia of hatchling chicks (+6.5+4.0 D) initially decreases rapidly and then more gradually to + 2.0 ± 0.5 D by 16 days. The distribution of refractive errors is very broad at Day 0, but becomes leptokurtotic, with a slight myopic skew, by Day 14. Corneal radius is constant for the first four days, possible as a result of pre-hatching lid pressure, and then increases linearly, as do all lens dimensions, axial diameter and equatorial diameter. Schematic eyes were developed for Days 0, 7, and 14.     

Adaptations of macrophytes to life in moving water: upslope limits and mechanical properties of stems

Structural properties of vascular aquatic macrophyte stems were studied from a mechanical perspective using data from Jackson Lake, Grand Teton National Park, Wyoming. Stem tensile strength, extensibility and cross sectional width were measured for seven vascular freshwater species. Mean values for these properties were significantly different when all species were compared.Moreover, tensile strength and extensibility were significantly different when deep water species were compared with shallow water species. Extensibility was significantly different when species duplicating by clone production were compared with species switching between clonal and sexual duplication, but tensile strength was not statistically different. Force to break a stem was modeled as a significantly increasing power function of stem cross sectional area (R² = 86.0%). Species specific tensile properties are considered as factors that can regulate the upslope distribution of macrophytes in moving water.     

Visual optics: The sandlance eye breaks all the rules

The eyes of the sandlance differ from those of other fish, both optically and in the kinds of movements they make. The predatory behaviour of these tiny fish not only makes their lifestyle similar to that of a chameleon, but has led to several extraordinary examples of convergence in the visual system.     

Adaptations to the acoustic environment by the squirrelfishes Myripristis violaceus and M. Pralinius

Three sounds naturally produced by squirrelfish of the genus Myripristis were recorded and analyzed sonographically. Captive Myripristis violaceus responded acoustically and behaviorally to playbacks of calls by conspecifics. Acoustic characteristics (velocity and pressure levels, and their attenuation as a function of frequency and distance) of grunt sounds, produced by hand held fish, were determined for M. violaceus and M. pralinius. Background noise components were analyzed for four different environments of these fish. Background noise and grunt sounds had high velocity levels, relative to pressure levels, expected in acoustic near fields, but attenuated at rates characteristic of acoustic far fields. Electrophysiological recordings from the lateral line organs of M. violaceus indicated that the lateral line system is directionally sensitive to a vector component (e.g., displacement or velocity) of the sound field, and is capable of mediating the observed behavioral responses.     

Adaptations of Anomopoda crustaceans (Cladocera) to the benthic mode of life

Crustaceans of the order Anomopoda inhabit both pelagic and littoral zones of fresh water bodies. Different adaptations to the benthic mode of life in representatives of different families and genera of this order are described. They include incomplete molting, the presence of a mucous film on the body surface, protection of the head from frontal damage, large lateral projections on the valves and additional sculpture on the head and valves, well-developed lateral armature of the postabdomen, the presence of projections on the postabdomen preanal margin, specialization of setae and spines of the second antenna, especially large exopodites on limbs IV and V, a reduced compound eye, and a large, well-developed ocellus. Similar adaptations to the benthic mode of life in the Anomopoda and Ostracoda are discussed.     

Adaptations of the reed frog Hyperolius viridiflavus (Amphibia: Anura: Hyperoliidae) to its arid environment

Summary Hyperolius viridiflavus possesses one complete layer of iridophores in the stratum spongiosum of its skin at about 8 days after metamorphosis. The high reflectance of this thin layer is almost certainly the result of multilayer interference reflection. In order to reflect a mean of about 35% of the incident radiation across a spectrum of 300–2900 nm only 30 layers of well-arranged crystals are required, resulting in a layer 10.5 m thick. These theoretical values are in good agreement with the actual mean diameter of single iridophores (15.0±3.0 m), the number of stacked platelets (40–100) and the measured reflectance of one complete layer of these cells (32.2±2.3%). Iridescence colours typical of multilayer interference reflectors were seen after severe dehydration. The skin colour turned from white (0–10% weight loss) through a copper-like iridescence (10–25% weight loss) to green iridescence (25–42%). In dry season state, H. viridiflavus needs a much higher reflectance to cope with the problems of high solar radiation load during long periods with severe dehydration stress. Dry-adapted skin contains about 4–6 layers of iridophores. The measured reflectance (up to 60% across the solar spectrum) of this thick layer (over 60 m) is not in keeping with the results obtained by applying the multilayer interference theory. Light, scattered independently of wavelength from disordered crystals, superimposes on the multilayer-induced spectral reflectance. The initial parallel shift of the multilayer curves with increasing thickness and the almost constant (white) reflectance of layers exceeding 60 m clearly point to a changing physical basis with increasing layer thickness.     

An owl's eye: Schematic optics and visual performance inStrix aluco L.

Summary The schematic eye ofStrix aluco, a nocturnal owl species, is described. A comparative and ecological context is used to examine the relationships between optical parameters of the eye and its light gathering and resolving powers. It is concluded that the essentially nocturnal feature of the owl eye does not lie in either its light gathering power or the sensitivity of individual rod receptors. Differences in visual performance at low light levels between the owl and the diurnal pigeon appear to be attributable to differences in the retinal neural integration mechanisms of the two species. However, it is hypothesised that the neural mechanisms which mediate the extraction of spatial information from the retinal image throughout the nocturnal luminance range, can function in the owl eye only because of its absolutely large sized retinal image. Thus the primarily nocturnal feature of the owl eye is its absolutely large posterior nodal distance, retinal image brightness is maximised only as a secondary feature.Abbreviation PND posterior nodal distance     

Adaptations of Crocodylus acutus and Alligator for life in saline water

• 1.1. American crocodiles (C. acutus) weighing less than 200 g are unable to grow when kept in 35 ppt sea water in the laboratory. Yet paradoxically there are some highly saline areas in south Florida where rapid growth occurs. It is possible that these conflicting observations can be reconciled by behavioral osmoregulation of young crocodiles.
• 2.2. Hatching occurs during the rainy season and small crocodiles may drink from the brackish “lens” available during and after rainfall.
• 3.3. Using a weekly regime of alternating exposure to 35 ppt (6 days) and 4 ppt (12–24 hr), it has been demonstrated that growth of small crocodiles occurs. Feeding takes place primarily when brackish water is available. Salinities as high as 18 ppt were drunk when crocodiles were dehydrated by 15–20% of initial mass.
• 4.4. C. acutus and Alligator have a rather low rate of water efflux in sea water (0.2ml/100g-hr).
• 5.5. Sodium influx in sea water of C. acutus is low, but higher than efflux. Thus there is no evidence yet for a significant role of the lingual salt glands in sodium excretion.
• 6.6. The major adaptations to saline water of hatchling C. acutus are a low intake of sodium, an ability to selectively drink water of lower salinities, and to grow very rapidly (within 3–4 months) to a size much more tolerant of immersion in 35 ppt sea water.

     

Effects of temperature on interactions of physiological and behavioural capacities of larval California grunion: Adaptations to the planktonic environment

The relationship between temperature-specific (11.3–27.1°C) rates of yolk utilization and temperature preference of California grunion, Leuresthes tenuis (Ayres), have been compared. Yolk-sac larvae experienced difficulty metabolizing protein at and above 25°C, and below 15°C they encountered problems with fat metabolism. These temperatures limited the range for normal embryonic development. The oil globule persisted longer than the yolk and may provide buoyancy to the anterior end as well as energy.

Thermoregulatory precision increased with number of days post-hatching. Yolk-sac larvae preferred ≈ 25°C; this temperature minimized duration of the yolk-sac stage but was outside the range for maximum growth efficiency (18–23°C). Following yolk resorption, non-feeding larvae preferred 18°C, which conserved energy but still maximized growth efficiency. Until at least 69 days post-hatching, larvae past the yolk-sac stage selected temperatures that maximized growth efficiency. Physiological and behavioural results indicate that larvae selected cooler temperatures when not feeding. This has been discussed and related to an apparent general adaptive behaviour to maximize bioenergetic efficiency.     

The eye of the humboldt penguin, Spheniscus humboldti: visual fields and schematic optics

Construction of a schematic eye indicates that the eye of Spheniscus humboldti is aquatic in design. The lens has a power of 100 dioptres (D) while (in air) the cornea has a power of 29 D. In air, the eye is myopic (approximately 28 D) but in water it is emmetropic. Minimum pupil size would seem insufficient to allow the pupil to function as a stenopaic aperture and increase depth of focus sufficiently to overcome the eye's aerial myopia. Entry into water reduces maximum image brightness by approximately three times. In air, the maximum width of the retinal binocular field is 45 degrees and this occurs approximately 10 degrees above the line of the bill. The bill intrudes into the retinal field and binocular field width in the plane containing the bill and the optic axes is 28 degrees. The vertical extent of the binocular field is 125 degrees. In the plane containing the optic axes the cyclopean field equals 282 degrees and the optic axes diverge by 116 degrees. In this plane the mean uniocular field is 155 degrees with the temporal hemifield approximately 11 degrees larger than the nasal hemifield. Entry into water reduces the widths of the visual fields such that maximum binocular field width is only 17 degrees and the vertical extent is reduced to about 80 degrees. Binocular vision is lost in the plane of the bill, and the uniocular retinal field is reduced by 32 degrees and the cyclopean field by 36 degrees.     

Microbial transport: Adaptations to natural environments

The cytoplasmic membrane of bacteria is the matrix for metabolic energy transducing processes such as proton motive force generation and solute transport. Passive permeation of protons across the cytoplasmic membrane is a crucial determinant in the proton motive generating capacity of the organisms. Adaptations of the membrane composition are needed to restrict the proton permeation rates especially at higher temperatures. Thermophilic bacteria cannot sufficiently restrict this proton permeation at their growth temperature and have to rely on the much␣lower permeation of Na + to generate a sodium motive force for driving metabolic energy-dependent membrane processes. Specific transport systems mediate passage across the membrane at physiological rates of all compounds needed for growth and metabolism and of all end products of metabolism. Some of transport systems, the secondary transporters, transduce one form of electrochemical energy into another form. These transporters can play crucial roles in the generation of metabolic energy. This is especially so in anaerobes such as Lactic Acid Bacteria which live under energy-limited conditions. Several transport systems are specifically aimed at the generation of metabolic energy during periods of energy-limitation. In their natural environment bacteria are also often exposed to cytotoxic compounds, including antibiotics. Many bacteria can respond to this live-threatening condition by overexpressing powerful drug-extruding multidrug resistance systems.     

N2 fixation in phototrophs: adaptation to a specialized way of life

Phototrophic diazotrophs include the photosynthetic green and purple bacteria, the heliobacteria, many cyanobacteria and the unusual chlorophyll-containing rhizobia that are found in the stem nodules of Aeschynomene spp. In this review, which concentrates on cyanobacteria, the interrelations between photosynthesis and N₂ fixation are discussed. Photosynthesis can, in theory, directly provide the ATP and reductant needed to support N₂ fixation but the link between these two processes is usually indirect, mediated through accumulated carbon reserves. In cyanobacteria, which possess an oxygenic photosynthesis, this serves to separate the O₂ that is produced by photosynthesis from the O₂-sensitive nitrogenase. However, in certain circumstances, oxygenic photosynthesis and N₂ fixation coexist. Under these conditions, respiratory consumption of photosynthetically generated O₂ may have an important role in minimizing O₂-damage to nitrogenase.     

The ontogeny of facultative superposition optics in a shrimp eye: hatching through metamorphosis

Summary Compound eyes of larval and first postlarval grass shrimp (Palaemonetes pugio Holthuis) were studied with light and electron microscopy following adaptation to darkness or bright light. Larvae have well-developed apposition eyes, including 3 main types of accessory screening and reflecting pigments and a fourth class of putatively reflective granules recently described in adult shrimps. Rhabdoms contain orthogonally layered microvilli, and by the last larval stage, 8 retinular cells. Ocular accessory pigments in both light- and dark-adapted larvae are distributed much like those of light-adapted adults, but the distal mass of reflecting pigment is concentrated dorsally in larvae and ventrally in adults. Since larvae swim upside-down, reflecting pigment is oriented downward in all developmental stages and may function for countershading. Light and dark adaptational migrations of all 3 major accessory pigments commence abruptly at metamorphosis to the first postlarva. Upon dark adaptation in postlarvae, superposition optics remain impossible because (1) distal screening pigment migrates only slightly, (2) no clear zone has developed, and (3) the crystalline cones remain circular in cross section. Nevertheless, a slight improvement in photon catch is expected due to extensive redistributions of reflecting pigment and retinular cell screening pigment granules.

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Contamination detection by optical measurements in a real‐life environment: A hospital case study

Organic dirt on touch surfaces can be biological contaminants (microbes) or nutrients for those but is often invisible by the human eye causing challenges for evaluating the need for cleaning. Using hyperspectral scanning algorithm, touch surface cleanliness monitoring by optical imaging was studied in a real‐life hospital environment. As the highlight, a human eye invisible stain from a dirty chair armrest was revealed manually with algorithms including threshold levels for intensity and clustering analysis with two excitation lights (green and red) and one bandpass filter (wavelength λ = 500 nm). The same result was confirmed by automatic k‐means clustering analysis from the entire dirty data of visible light (red, green and blue) and filters 420 to 720 nm with 20 nm increments. Overall, the collected touch surface samples (N = 156) indicated the need for cleaning in some locations by the high culturable bacteria and adenosine triphosphate counts despite the lack of visible dirt. Examples of such locations were toilet door lock knobs and busy registration desk armchairs. Thus, the studied optical imaging system utilizing the safe visible light area shows a promising method for touch surface cleanliness evaluation in real‐life environments.