The reniform reflecting superposition compound eyes of Nephrops norvegicus: optics, susceptibility to light-induced damage, electrophysiology and a ray tracing model

The large reniform eyes of the reptant, tube-dwelling decapod Nephrops norvegicus are described in detail. Optically these reflecting superposition compound eyes are a little unusual in that they are laterally flattened, a feature that may enhance their sensitivity in that region, albeit at the expense of resolution. Electrophysiological and anatomical investigations suggest that the eyes are tuned to appropriate spectral and temporal sensitivities in the long and short term through movement of proximal pigments and possibly rhabdom adaptation. Although exposure to ambient surface light intensities is shown to cause damage to the retinal layer, especially in deeper living animals, there is no evidence yet that demonstrates an impact of eye damage on their survival. It is suggested that experimentation on marine decapods, with sensitive eyes, requires that particular attention is paid to their light environment

The Euphausia superba transcriptome database, SuperbaSE: An online, open resource for researchers

Antarctic krill (Euphausia superba) is a crucial component of the Southern Ocean ecosystem, acting as the major link between primary production and higher trophic levels with an annual predator demand of up to 470 million tonnes. It also acts as an ecosystem engineer, affecting carbon sequestration and recycling iron and nitrogen, and has increasing importance as a commercial product in the aquaculture and health industries. Here we describe the creation of a de novo assembled head transcriptome for E. superba. As an example of its potential as a molecular resource, we relate its exploitation in identifying and characterizing numerous genes related to the circadian clock in E. superba, including the major components of the central feedback loop. We have made the transcriptome openly accessible for a wider audience of ecologists, molecular biologists, evolutionary geneticists, and others in a user-friendly format at SuperbaSE, hosted at www.krill.le.ac.uk

Apposition compound eyes of Spongicoloides koehleri (Crustacea: Spongicolidae) are derived by neoteny.

Wedding shrimps, Spongicoloides koehleri, spend the adult phase of their life cycle within the cavity of a hexactinellid sponge. Although there is little light at the depths at which the sponges are found, the shrimps do not use the highly sensitive reflecting superposition optics commonly found in other shrimp-like decapods. Instead they have apposition eyes which are virtually free of shielding pigment. It is proposed that this is due to the paedomorphic retention of the larval optics through the process of neoteny

The anatomy and physiology of selected reflecting superposition eyes.

Aspects of the morphology and optical physiology of reflecting superposition eyes have been investigated using species from various decapod crustacean taxa. The eyes all have the same basic structure of a distal dioptric layer and a proximal retinula layer, separated by an unpigmented clear zone. In the eyes of shrimps, lobsters and crayfish the clear zone is crossed by crystalline cone cell extensions. In squat lobsters this region is crossed by rhabdomeric lightguides. Porcelain crabs possess an eye intermediate in design. The superposition ray path, the action of corneal lenses and the presence of lightguides have been demonstrated and the refractive indices of several optical elements determined. These results have been used to produce ray tracing diagrams showing the optical pathways within these eyes. All of the eyes function by redirecting light across the clear zone by reflection within a mirrored crystalline cone. In squat lobsters a rhabdomeric lightguide transmits axial light to the rhabdoms. The eyes have a small f-number resulting in good light-gathering power and maximum sensitivity. However, intracellular electrophysiological determinations of retinula cell angular sensitivity show that these eyes have poor resolution. Variations in morphology and optics represent adaptations to the underwater light field, especially in the tapeta and in the light- sensitive rhabdoms. In oceanic species the tapetum varies in both structure and reflectivity along a dorso-ventral gradient within each eye and also interspecifically. It is proposed that the need to remain well-camouflaged in the low-contrast oceanic environment leads to tapetal modifications. Where sufficient light is available the rhabdoms are adapted to maximize resolution and permit sensitivity to polarized light. In deeper water, where little light remains, the rhabdoms are adapted to increase sensitivity at the expense of resolution. The dorsal region of the eye retains apposition optics for the purpose of detecting small objects in silhouette against the downwelling light

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

ABSTRACT Locusts demonstrate remarkable phenotypic plasticity driven by changes in population density. This density dependent phase polyphenism is associated with many physiological, behavioural 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 24-36h 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 manoeuvres. The strength of DCMD's response to looming stimuli, characterised by the number of evoked spikes and peak firing rate, varies approximately sinusoidally with a period close to 24h under constant light in solitarious locusts. In gregarious individuals the 24h pattern is more complex, being modified by secondary ultradian rhythms. DCMD's strongest responses occur around expected dusk in solitarious locusts, but up to 6h 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 behaviour 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

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

Effects of simulated light regimes on gene expression in Antarctic krill (Euphausia superba Dana)

A change in photoperiod has been implicated in triggering a transition from an active to a quiescent state in Antarctic krill. We examined this process at the molecular level, to identify processes that are affected when passing a photoperiodic threshold. Antarctic krill captured in the austral autumn were divided into two groups and immediately incubated either under a photoperiod of 12 h light:12 h darkness (LD), simulating the natural light cycle, or in continuous darkness (DD), simulating winter. All other conditions were kept identical between incubations. After 7 days of adaptation, krill were sampled every 4 h over a 24 h period and frozen. Total RNA was extracted from the heads and pooled to construct a suppression subtractive hybridisation library. Differentially expressed sequences were identified and annotated into functional categories through database sequence matching. We found a difference in gene expression between LD and DD krill, with LD krill expressing more genes involved in functions such as metabolism, motor activity, protein binding and various other cellular activities. Eleven of these genes were examined further with quantitative polymerase chain reaction analyses, which revealed that expression levels were significantly higher in LD krill. The genes affected by simulated photoperiodic change are consistent with known features of quiescence, such as a slowing of moult rate, a lowering of activity levels and a reduction in metabolic rate. The expression of proteases involved in apolysis, where the old cuticle separates from the epidermis, showed particular sensitivity to photoperiod and point to the mechanism by which moult rate is adjusted seasonally. Our results show that key processes are already responding at the molecular level after just 7 days of exposure to a changed photoperiodic cycle. We propose that krill switch rapidly between active and quiescent states and that the photoperiodic cycle plays a key role in this process

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

Background: All 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. Results: A 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 beta-N-acetylglucosaminidase (beta-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 beta-NAGase precursors into the exuvial cleft. Trypsin, known to activate the beta-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. Conclusion: We 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