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The aim of this study was to examine the impact of dispersed crude oil on retinoid metabolism and signaling in developing haddock larvae (Melanogrammus aeglefinus). Retinoids and its active metabolites are involved in the regulation of several developmental processes as cell differentiation, growth and limb patterning. Thus, disruption of retinoid signaling, especially during early developmental stages, may have detrimental effects on the organism. In the present study, crude oil exposure during embryonic development and early larvae development disrupted vitamin A signaling by affecting retinoid levels and genes involved in retinoid metabolism. The disruption of retinoid signaling was also evident when looking at the gene expression pattern at several time-points during and after exposure, demonstrating the complexity and temporal factor of retinoid signaling disruption. Exposure during the embryonic period (pre-hatch) resulted in reduced eye size and increased incidence of abnormal eye morphology. The observed changes in expression of retinoid metabolic genes related to the visual cycle indicates that there is a link between the modulated retinoid levels (alltrans retinoic acid and retinol) and increased incidence of abnormal eye development as a result of crude oil exposure. This is the first study to demonstrate disrupted retinoid signaling in fish following crude oil exposure at environmental relevant levels, giving novel insight in to the mechanism of toxicity.

Arthropods comprise the most populous groups of animals and show an astonishing diversity. Aquatic arthropods belonging to the sub-class Copepoda encompass a range of ecological roles from endo- and ectoparasites to grazers of phytoplankton that link primary producers to higher trophic levels. Despite the important role of copepods in central ecosystem services and their impact as parasites, representative genomic data and genome assemblies are scarce. This limits our opportunities to understand both the specific biology of individual species and unifying copepod genomic features that may govern their capacities to, for example, adapt to a changing environment. Among the copepod parasites we find Lepeophtheirus salmonis, an important ectoparasite that represents a threat to wild salmonid stocks and causes large annual losses to the salmon farming industry. Here we present the salmon louse genome - the first genome of a parasitic copepod that is fully sequenced and annotated. The 695.4 Mbp assembly was validated by a genetic linkage map and comprises 13 autosomes that recombine almost exclusively in males, one autosome that is shielded from recombination in both sexes and a ZZ-ZW style sex chromosome system. The genome assembly contains approximate 60% repetitive regions and comprise 13081 annotated predicted protein-coding genes. The predicted gene set appears to be quite complete as 92.4% of the expected Arthropod genes were found by a BUSCO. The gene annotations were validated by transcriptome sequencing that corresponds to the expected function of selected tissues. Transcriptome sequencing further revealed a marked shift in the gene expression pattern at the transition from the planktonic dispersal phase to the parasitic lifestyle after host attachment. Among other features, genes related to circadian rhythm are downregulated upon attaching to a host - probably reflecting abandoning a planktonic life with diurnal migration. The genome shows several evolutionary signatures including a large expansion of FNII domains, commonly considered vertebrate specific, and an incomplete heme homeostasis pathway suggesting that heme proteins are obtained from the host. The salmon louse has repeatedly demonstrated a large capacity to develop resistance against chemical treatments. Nonetheless, it exhibits low reduced numbers of several genes commonly involved in detoxification; cytochrome P450, ATP-Binding Cassette type transporters and Glutathione S-transferases. Interestingly, only one gene family with a putative detoxification role was expanded: the major vault proteins. Finally, the salmon louse has lost the ability to sustain peroxisomes, a loss apparently shared in the Caligid family but not among copepods in general.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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