The visual system of the longest-living vertebrate, the Greenland shark (Somniosus microcephalus)
The Greenland shark (Somniosus microcephalus) is the longest-living vertebrate and inhabits the extremely dim and cold waters of the Arctic deep sea, leading to speculations that it has lost the ability to see. In this work, we provide genomic, transcriptomic, histological and functional evidence that the Greenland shark has an intact visual system well-adapted for life in dim light. Histology and in vitro opsin expression revealed visual adaptations typical of deep-sea species, including densely packed, elongated rods and a short-wavelength shift in rod visual pigment sensitivity. RNAscope confirmed the presence of essential visual cell types, such as rods, Muller glia, and bipolar, amacrine, and ganglion cells. Despite being centuries old, the examined specimens showed no signs of retinal degeneration. Using whole genome and retinal RNA-sequencing, we further show that dim-light (rod-based) vision genes are intact and robustly expressed, while many bright-light (cone-based) vision genes have been pseudogenized and/or are no longer expressed. Finally, our data suggest that efficient DNA repair mechanisms may contribute to the preservation of a functional visual system over centuries in Greenland sharks.