This paper addresses the use of eye structure and optics in the construction of crustacean phylogenies and presents an hypothesis for the evolution of superposition eyes in the Decapoda, based on the distribution of eye types in extant decapod families. It is suggested that reflecting superposition optics are symplesiomorphic for the Decapoda, having evolved only once, probably in the Devonian. Subsequent loss of reflecting superposition optics has occurred following the adoption of a new habitat (e.g. Aristeidae, Aeglidae) or by progenetic paedomorphosis (Paguroidea, Eubrachyura).
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.
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.[Gaten E., Tarling G., Dowse H., Kysiacou C. and Rosato E. 2008 Is vertical mignation in Antarctic krill (Euphausia superba) influenced by an underlying circadian rhythm? J. Genet. 87,[473][474][475][476][477][478][479][480][481][482][483]
In this study, lipofuscin was examined in the eyestalk ganglia of tagged European lobsters Homarus gammarus released into the wild at Scapa Flow, Orkney. Scotland, at approximately 3 mo of age and recaptured at ages between 5.4 and 9.6 yr. Lipofuscin deposits were often most abundant in cell cluster A of the medulla terminalis (MT-A), where they exhibited typical autofluorescence, histochemical, distributional and structural properties. Confocal fluorescence microscopy and image a'nalysis were used to quantify the deposits The study demonstrated that lipofuscin accumulation in the MT-A is age-dependent (r = 0 640, p -0.0002). For the available sample range, no other statistically significant relationships were found (carapace length vs age: r = 0.147, p = 0.359, body weight vs age: r = -0.054, p = 0.738; carapace length vs lipofuscin concentration: r = 0.331, p = 0.0849; body weight vs lipofuscin concentration: r = 0.181, p = 0.358). Body size had no age discriminating power. There was no difference in lipofuscin accumulation rate between males and females. When placed in perspective against the highest lipofuscin concentrations so far measured for wild individuals of this species, it was apparent that the available sample of tagged lobsters, spanning an age interval of about 4 yr, represented only a very small window of the total lifespan. The results indicate that measurement of MT-A lipofusc~n concentration will provide considerably more accurate age determination of fished lobsters than the current body-size-based approach. MT-A lipofuscin concentration correctly ages approximately 4 3 % of lobsters in a sample to within 1 yr or less of their true age, and 95 % of lobsters to within 3.5 yr Carapace length correctly ages only 3% of individuals correctly to withi.n 1 yr The width of the 95% confidence intervals for carapace-length-based age estimates are so large as to render these estimates meaningless. Size-at-age and number-at-age data obtained using lipofuscin will prove useful for estimating growth and mortality in wild lobster populations and provide insight into potential biases in the current, conventionally derived estimates of these parameters. From a practical perspective, it is easier to sample nerve tissue containing lipofuscin from the eyestalk than the brain and has the advantage of leaving the lobsters in marketable condition.
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 (Crangonallmani, Pandalus montagui and Nephrops norvegicus) are maximally sensitive to light of longer wavelengths (510 to 525 nm).
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