Just like the river lamprey, the FFH fish species sea lamprey also does not belong to fishes, but to "round mouths" (cyclostomata) and hence to the jawless vertebrates. Sea lampreys have an eellike body form without pectoral and ventral fin and a length of up to 90 cm. On their head, there is a suckerlike mouth and 7 branchial gills, positioned behind the eye. As opposed to the other lamprey species, the mouth of the sea lamprey, equipped with horny teeth, is wider than its body.

Similarly to the river lamprey, the sea lamprey is also an andromous migratory fish, although it doesn't swim the water course so far upwards like the first one. Sea lampreys live in coastal areas of Scandinavia, over the North and Baltic Sea down to the west Mediterranean. Other large populations can be found by the East Coast of the North America. Unlike the endangered European populations, which are protected by the FFH Directive, the North-American populations are severely suppressed due to their massive expansion and threat to other fish species of economic significance in terms of fishery.

Sea lampreys spawn in small groups in pebbly areas, whereby the parents die off after the reproduction. After the larvae are hatched in a couple of weeks, they remain for approximately 6 to 7 years in fresh water and migrate after the metamorphosis to an adult animal away to the sea. The way of feeding is similar to that of other lamprey species.