The river lamprey (lampetra fluviatilis) belongs to the cyclostomata, and lacks paired fins. The fish grow to a length of 30-40 cm, and then migrate upriver from the sea to spawn in the upper reaches (generally in the grayling to trout regions) on gravel beds. The larvae grow in the river sediment as filter-feeders, and after some years migrate to the sea, where they prey parasitically on other fish. With the present-day improvement of water quality, it is likely that river lampreys are again migrating up the Rhine in tens of thousands and spawning in numerous Rhine tributaries in NRW.
