Skate specimen. Researcher's gloved hands holding a skate (Raja erinacea) caught for use in biologi- cal research at the Woods Hole Marine Biology Laboratory (MBL) in Massachusetts, USA. The skate has eyes (not seen) unlike those of any other vertebrate, in that its retinas (light-sensitive membranes at the back of the eyeball) contain only rod cells. These are the light-sensitive cells that detect low light levels. Other vertebrates have both rods and colour-detecting cone cells in their retinas. Studying rods in isolation provides information on light/dark adaptation, and could lead to new ways of treating adaptation disorders such as night blindness and retinitis pigmentosa.