Among the most primitive of all vertebrate species, the sea lamprey is a parasitic fish native to the northern and western Atlantic Ocean. Due to their similar body shapes, lampreys have sometimes inaccurately been called "lamprey eels," but they are actually more closely related to sharks!
Unlike "bony" fishes like trout, cod, and herring, lampreys lack scales, fins, and gill covers. Like sharks, their skeletons are made of cartilage. They breathe through a distinctive row of seven pairs of tiny gill openings located behind their mouths and eyes.
But the anatomical trait that makes the sea lamprey an efficient killer of lake trout and other bony fishes is its disc-shaped, suction-cup mouth, ringed with sharp, horny teeth, with which it latches on to an unfortunate fish. The lamprey then uses its rough tongue to rasp away the fish's flesh so it can feed on its host's blood and body fluids. One lamprey kills about 40 pounds of fish every year.
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Original article: What is a sea lamprey?