Adult European River Lampreys live in the sea near the shore. In autumn, adults usually undergo reproductive migration from the sea to the rivers giving them their Anglicized name. During reproductive migration and reproduction, adults do not feed, but instead use their lipid reserves. They spawn in pre-dug-out pits in river beds, after which they die.
A European River Lamprey Mouth
The blindlarvae are filter feeders and live mostly buried in river beds where they were conceived. When they reach around 13 cm., the larvae will metamorphose into juveniles and migrate back to the sea. They will live there until they are ready to reproduce and feed on fish by sucking out blood and organs by latching onto the fish with their sharp teeth. After all of the organisms’ tissues are eaten, the lampreys will detach.
They reside all around Europe in temperate coastal waters. They can grow up to 50 centimeters long and .15 kilograms. in the wild. They have characteristic yellow dorsal portion (back) and a pale white ventral portion (belly).
I found most of this information on a website begun by Dr. E. Wilson, a world renowned entomologist and ecologist called the Encyclopedia of Life. Check it out for yourself. http://www.eol.org/index