Sea lamprey (Petromyzon marinus) building a nest. When it is time to reproduce sea lampreys migrate to freshwater streams to create nests called 'redds'. They use their mouths to suction rocks and form nests around 1 metre in diameter and 15 centimetres deep. The female then lays tens of thousands of eggs and the male fertilises them. Threats to their survival include reduced water flow, poor water quality and obstacles to spawning areas. Photographed in the River Teifi, Wales, UK.