Edward Gaten

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.

Is vertical migration in Antarctic krill (Euphausia superba) influenced by an underlying circadian rhythm

Antarctic krill (Euphausia superba) is a keystone species in the southern ocean ecosystem where it is the main consumer of phytoplankton and constitutes the main food item of many higher predators. Both food and predators are most abundant at the surface, thus krill hide in the depth of the ocean during the day and migrate to the upper layers at night, to feed at a time when the predatory risk is lowest. Although the functional significance of this diel vertical migration (DVM) is clear and its modulation by environmental factors has been described, the involvement of an endogenous circadian clock in this behaviour is as yet not fully resolved. We have analysed the circadian behaviour of Euphausia superba in a laboratory setting and here we present the first description of locomotor activity rhythms for this species. Our results are in agreement with the hypothesis that the circadian clock plays a key role in DVM. They also suggest that the interplay between food availability, social cues and the light:dark cycle acts as the predominant Zeitgeber for DVM in this species.

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.

Differential gene expression during the moult cycle of Antarctic krill (Euphausia superba).

BackgroundAll crustaceans periodically moult to renew their exoskeleton. In krill this involves partial digestion and resorption of the old exoskeleton and synthesis of new cuticle. Molecular events that underlie the moult cycle are poorly understood in calcifying crustaceans and even less so in non-calcifying organisms such as krill. To address this we constructed an Antarctic krill cDNA microarray in order to generate gene expression profiles across the moult cycle and identify possible activation pathways.ResultsA total of 26 different cuticle genes were identified that showed differential gene expression across the moult cycle. Almost all cuticle genes were up regulated during premoult and down regulated during late intermoult. There were a number of transcripts with significant sequence homology to genes potentially involved in the synthesis, breakdown and resorption of chitin. During early premoult glutamine synthetase, a gene involved in generating an amino acid used in the synthesis of glucosamine, a constituent of chitin, was up regulated more than twofold. Mannosyltransferase 1, a member of the glycosyltransferase family of enzymes that includes chitin synthase was also up regulated during early premoult. Transcripts homologous to a β-N-acetylglucosaminidase (β-NAGase) precursor were expressed at a higher level during late intermoult (prior to apolysis) than during premoult. This observation coincided with the up regulation during late intermoult, of a coatomer subunit epsilon involved in the production of vesicles that maybe used to transport the β-NAGase precursors into the exuvial cleft. Trypsin, known to activate the β-NAGase precursor, was up regulated more than fourfold during premoult. The up regulation of a predicted oligopeptide transporter during premoult may allow the transport of chitin breakdown products across the newly synthesised epi- and exocuticle layers.ConclusionWe have identified many genes differentially expressed across the moult cycle of krill that correspond with known phenotypic structural changes. This study has provided a better understanding of the processes involved in krill moulting and how they may be controlled at the gene expression level.

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.

Solitary and Gregarious Locusts Differ in Circadian Rhythmicity of a Visual Output Neuron

Locusts demonstrate remarkable phenotypic plasticity driven by changes in population density. This density dependent phase polyphenism is associated with many physiological, behavioral, and morphological changes, including observations that cryptic solitarious (solitary-reared) individuals start to fly at dusk, whereas gregarious (crowd-reared) individuals are day-active. We have recorded for 24-36 h, from an identified visual output neuron, the descending contralateral movement detector (DCMD) of Schistocerca gregaria in solitarious and gregarious animals. DCMD signals impending collision and participates in flight avoidance maneuvers. The strength of DCMD’s response to looming stimuli, characterized by the number of evoked spikes and peak firing rate, varies approximately sinusoidally with a period close to 24 h under constant light in solitarious locusts. In gregarious individuals the 24-h pattern is more complex, being modified by secondary ultradian rhythms. DCMD’s strongest responses occur around expected dusk in solitarious locusts but up to 6 h earlier in gregarious locusts, matching the times of day at which locusts of each type are most active. We thus demonstrate a neuronal correlate of a temporal shift in behavior that is observed in gregarious locusts. Our ability to alter the nature of a circadian rhythm by manipulating the rearing density of locusts under identical light-dark cycles may provide important tools to investigate further the mechanisms underlying diurnal rhythmicity.

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.

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.