P. M. J. Shelton

Are vent shrimps blinded by science

The exploration of deep-sea hydrothermal vents has depended on the use of manned submersibles, which are invariably equipped with high-intensity floodlights. But the eyes of many deep-sea crustaceans, which are exquisitely adapted for the dim conditions at such depths, can suffer permanent retinal damage as a result. We suggest that the use of floodlights has irretrievably damaged the eyes of many of the decapod shrimps (family Bresiliidae) that dominate the fauna at vents on the Mid-Atlantic Ridge.

Light and retinal damage in Nephrops norvegicus (L.) (Crustacea)

Nephrops norvegicus is a burrow-dwelling marine crustacean normally only active in dim light. The eye has typical crustacean rhabdoms each consisting of alternating layers of microvilli. On light adaptation, proximal shielding pigment moves up from the bases of retinula cells to surround the rhabdoms. In dark-adapted eyes the proximal pigment moves proximally to form a band just above the basement membrane. In this position the tapetum is unshielded and it reflects light back into the eye. The only other detectable difference between light- and dark-adapted eyes is a night-time increase in rhabdom volume. Creel-caught animals raised to the surface of Loch Torridon (NW Scotland) were exposed to ambient surface light for periods ranging from 9 min to 5 h. A short exposure (9 min, average intensity 380 μmol m–2 s–1) is sufficient to cause damage to the retinula cell layer. It is histologically detectable one month later. Animals fixed immediately after 15 min exposure show evidence of retinula cell breakdown with swelling of cell bodies and nuclei, escape of proximal shielding pigment from the retinula cells and vesiculation of the rhabdoms. After 2 h of illumination the microvilli of the rhabdom are completely disrupted with only membrane whorls remaining; proximal shielding pigment is found deep within the rhabdom. After 6 h of illumination the retinula cell body layer is absent and there is a massive invasion of the eye by haemocytes. By using animals acclimated to a 12 h light–12 h dark cycle (green light, 0.24 μmol m–2 s–1) we were able to test the effects of natural daylight (average intensity 180 μmol m–2 s–1) on dark- and light-adapted eyes of known physiological state. The animals were kept alive for two weeks after exposure and the percentage area of the retina destroyed was measured from serial wax sections. Dark-adapted eyes have substantial damage (76.74%) after only 15 s but light adaptation prevents damage with a similar exposure. After 5 min exposure, destruction is almost total (light-adapted, 97.16%; dark-adapted, 98.97%). Intensities of 1000 and 250 μmol m-2 s–1 with an artificial tungsten light source gave similar results. Light-adapted eyes are less sensitive than dark-adapted ones and longer exposures cause greater damage. At these relatively high intensities the damage caused by the two light levels is not very different. At lower intensities (10–250 μmol m–2 s–1) the amount of damage is proportional to the intensity. By using the tungsten source and 10 s exposures we found that dark-adapted eyes are damaged at 25 μmol m–2 s–1 and that light-adapted eyes are affected at 100 μmol m–2 s–1.

Spectral sensitivities of five marine decapod crustaceans and a review of spectral sensitivity variation in relation to habitat

The spectral sensitivities of five species of decapod crustaceans have been determined by electroretinogram measurements. Their spectral sensitivities conform to the general picture for marine crustacea with high sensitivity to blue-green wavelengths and some showing sensitivity to violet/near ultraviolet. Two deep-water species ( Paromola cuvieri and Chaceon ( Geryon ) affinis ) have spectral sensitivity maxima below 500 nm, whereas the three coastal species examined ( Crangon allmani , Pandalus montagui and Nephrops norvegicus ) are maximally sensitive to light of longer wavelengths (510 to 525 nm).

Depth related variation in the structure and functioning of the compound eye of the Norway lobster Nephrops norvegicus

The mobility and quantity of retinula cell proximal screening pigment, and the liability of the eyes to light-induced damage, were investigated in the Norway lobster, Nephrops norvegicus (L.), obtained from three separate populations from depths of 18, 75, and 135 m. During the morning after capture, the migration of the proximal pigment in response to the onset of illumination below the threshold for damage varied between the three populations. In the eyes of deep water N. norvegicus , the proximal screening pigment was located close to or below the basement membrane when dark-adapted and rose to a position midway up the rhabdoms when light-adapted. In the dark-adapted N. norvegicus from shallow water the proximal pigment was located more distally than in eyes of deep water animals. After the onset of illumination, the pigment migrated distally to completely cover the rhabdoms. The amount of retinula cell proximal screening pigment was found to decrease linearly with depth. When dark-adapted individuals from each depth were exposed to light a positive correlation was obtained between the photon fluence rate (PER) and the proportion of the retina damaged. For a given light exposure the amount of damage was highest in animals from deeper water. The PFR causing 25% damage was approximately 1 log unit higher in animals from 18 m compared to those from 135 m. The amount of damage varied with the delay between capture of the animals and exposure to light. When exposed 2 h after capture significant differences between depths were seen but the results were influenced by the incomplete dark adaptation of some specimens.

Reversal of a hallmark of brain ageing: lipofuscin accumulation

The prospect of removing cellular deposits of lipofuscin is of considerable interest because they may contribute to age related functional decline and disease. Here, we use a decapod crustacean model to circumvent a number of problems inherent in previous studies on lipofuscin loss. We employ (a) validated lipofuscin quantification methods, (b) an in vivo context, (c) essentially natural environmental conditions and (d) a situation without accelerated production of residual material or (e) application of pharmacological compounds. We use a novel CNS biopsy technique that produces both an anti-ageing effect and also permits longitudinal sampling of individuals, thus (f) avoiding conventional purely cross-sectional population data that may suffer from selective mortality biases. We quantitatively demonstrate that lipofuscin, accrued through normal ageing, can be lost from neural tissue. The mechanism of loss probably involves exocytosis and possibly blood transport. If non-disruptive ways to accelerate lipofuscin removal can be found, our results suggest that therapeutic reversal of this most universal manifestation of cellular ageing may be possible.

Death rates reflect accumulating brain damage in arthropods

We present the results of the first quantitative, whole-lifespan study of the relationship between age-specific neurolipofuscin concentration and natural mortality rate in any organism. In a convenient laboratory animal, the African migratory locust, Locusta migratoria, we find an unusual delayed-onset neurolipofuscin accumulation pattern that is highly correlated with exponentially accelerating age-specific Gompertz–Makeham death rates in both males (r=0.93, p=0.0064) and females (r=0.97, p=0.0052). We then test the conservation of this association by aggregating the locust results with available population-specific data for a range of other terrestrial, freshwater, marine, tropical and temperate arthropods whose longevities span three orders of magnitude. This synthesis shows that the strong association between neurolipofuscin deposition and natural mortality is a phylogenetically and environmentally widespread phenomenon (r=0.96, p<0.0001). These results highlight neurolipofuscin as a unique and outstanding integral biomarker of ageing. They also offer compelling evidence for the proposal that, in vital organs like the brain, either the accumulation of toxic garbage in the form of lipofuscin itself, or the particular molecular reactions underlying lipofuscinogenesis, including free-radical damage, are the primary events in senescence.

A Golgi-electron-microscopical study of the structure and development of the lamina ganglionaris of the locust optic lobe

SummaryThe gross structure as well as the neuronal and non-neuronal components of the lamina ganglionaris of the locust Schistocerca gregaria are described on the basis of light- and electron-microscopical preparations of Golgj (selective silver) and ordinary histological preparations. The array of optic cartridges within the lamina neuropile — their order and arrangement — and the composition of the cartridges are described. There are six types of monopolar neurons: three whose branches reach to other cartridges and three whose branches are confined to their own cartridges. Retinula axons terminate either in the lamina or the medulla neuropiles. There are three types of centrifugal neurons, two types of horizontal neuron, as well as glia and trachea in the lamina neuropile. The development of the lamina neuropile is described in terms of developing monopolar and centrifugal axons, growing retinula fibres, and composition of the developing optic cartridges.

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

Abstract 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.

Cuticular Photophores of Two Decapod Crustaceans, Oplophorus spinosus and Systellaspis debilis

The organization, ultrastructure, growth, and development of two types of cuticular photophore in oplophorid shrimps (Oplophorus spinosus and Systellaspis debilis) are described. Photophores located in the third maxilliped consist of a unit structure comprising a single photocyte and associated pigment cells. Reflecting pigment cells contain white pigment and form an apical cap above the photocyte; sheath cells contain red carotenoid pigment and form a light-absorbing layer around the photophore. Photophores located on the pleopods are compound structures comprising many photocytes. They also contain the same types of pigment cell that are found in the unit photophores of the maxilliped. Paracrystalline bodies at the apical ends of the photocytes in both types of photophore are thought to be associated with light generation. Both types of photophore have mechanisms for tilting in the pitch plane. In the maxilliped, the apices of the photophores are connected to a ligament that has its origin in the propodus. Flexion or extension of the dactylus displaces the ligament, which tilts the photophores synchronously. The cuticular window beneath each photophore remains stationary. The tilt mechanism of the pleopod photophores is quite different, and depends upon muscular contraction. A main and an accessory longitudinal muscle cause backwards rotation of the photophore by deforming the cuticle surface. A loop muscle that passes around the anterior face of the photophore causes forward rotation. The two mechanisms optimize the use of the photophores in ventral camouflage. They allow photophore rotation to compensate for changes in the shrimps orientation in the plane of pitch and thus maintain the ventral direction of the luminescence.

Relationship of Dorsoventral Eyeshine Distributions to Habitat Depth and Animal Size in Mesopelagic Decapods

Eyeshine distribution patterns recorded from the eyes of 19 mesopelagic decapod species were examined and related to the depths at which the species are found. For most species examined, eyeshine was found to be brighter ventrally than dorsally. Deep-water decapod species that do not undergo diel vertical migrations had brighter dorsal eyeshine than migratory species. Eyeshine intensity increased with body size in five of the species examined and decreased in two. These changes in eyeshine intensity may be an adaptation to variations in depth distributions that occur with increasing body size. It is suggested that the depth and size-related changes reflect the importance of remaining camouflaged in the mesopelagic realm and are an example of ecologically functional development.