Sea lampreys. Sea lampreys (Petromyzon marinus) in a tank, with a view of the toothed, sucker-like mouth. These jawless fish are parasites of many other types of fish. A lamprey will attach itself by its mouth to the skin of its prey and rasp through it with its rough tongue. It then sucks blood and flesh from its victim. An anticoagulant chemical in its saliva prevents the wound healing. The lamprey drops off its host when it is full or its host has died. It spends much of its life in fresh water. It returns to a river to breed and spends the first few years of its life as a filter-feeding larva in slow-flowing parts of a river.