The Sea Lamprey (Petromyzon Marinus), is the largest cyclostome in Europe. It can be distinguished from the other lampreys by its larger size, the marbling of the greyish-green back, and the two dorsal fins, which are widely separated. An alternative common name is 'stone sucker', which may have arisen from the habit of males during spawning, when they create a depression in the river bed by wriggling and removing stones with the mouth .
Lampreys are some of the most primitive vertebrates alive today, they are known as cyclostomes, which means 'round mouths' and refers to the fact that they are jawless, having instead a round sucker-like mouth. A further primitive characteristic is that the skeleton consists of cartilage and not bone. Lampreys are similar in shape to eels, and have a series of uncovered round gill openings (known as gill pores) on the sides of the head and a single nostril on the upper surface of the head. These fish can be seen attached to the fins of species such as the Basking Shark.