De novo Transcriptome Analyses Provide Insights into Opsin-based Photoreception in the Lantern shark Etmopterus spinax

The velvet belly lantern shark (Etmopterus spinax) is a small deep-sea shark commonly found in the Easter Atlantic and the Mediterranean Sea. In this study, paired-end illumina HiSeqTM technology has been employed to analyse transcriptome data from eye and ventral skin tissues of the lantershark species. About 64 and 49 million Illumina reads were generated from skin and eyetissues respectively. The assembly allowed us to predict 119,749 total unigenes including 94,569 for the skin transcriptome and 94,365 for the eye transcriptome while 74,753 were commonly found in both transcriptomes. Among unigenes, 60,322 sequences were annotated using classical public databases. The assembled and annotated transcriptomes provide a valuable resource for further understanding of the shark biology. We identified potential \"light-interacting toolkit\" genes including multiple genes related to ocular and extraocular light perception processes such as opsins. In particular, a single rhodopsin gene mRNA and its potentially associated peropsin were only detected in the eye transcriptome confirming a monochromatic vision of the lantern-shark. Conversely, an encephalopsin mRNA was mainly detected in the skin transcriptome. The encephalopsin was immunolocalized in various shark tissues confirming its wide expression in the shark skin and pinpointing a possible functional relation with the photophore, i.e. epidermal light organs. We hypothesize that extraocular photoreception might be involved in the bioluminescence control possibly acting on the shutter opening and/or the photocyte activity itself.