Skate specimens. Fishing net holding skates (Raja erinacea, with long tails) caught for biological research at the Woods Hole Marine Biology Labora- tory (MBL) in Massachusetts, USA. The skate has eyes unlike those of any other vertebrate, in that its retinas (the light sensitive membrane at the back of the eyeball) contain only rod cells. These are the light-sensitive cells that detect low light levels. All 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.