The sea lamprey, shown here attached to an angler's wader, is the largest of the three lamprey species found in Welsh rivers.
Lampreys have sucker mouths with rasp-like teeth and seven circular gill openings on each side.
They attach themselves to the flank of a passing fish and feed upon the blood of their victim. Sea lampreys migrate up the rivers of Wales in the spring, and in May and June they cut redds in the gravel in much the same way as salmon and trout do. Once in fresh water it is rare for sea lampreys to feed. In recent times lampreys have become scarce and are now protected by wildlife legislation.