The lamprey’s bite—a blend of small punctures and suction—may never spark a thriller, but it’s plenty lethal. Looking like worms on steroids, these primitive fish snare victims with an inescapable grip, then emit an anticoagulant that turns the host’s circulatory system into a parasite’s pantry. Often the host dies of blood loss or infection. The lamprey simply finds new prey. AVERAGE SIZE: 30 inches (76 centimeters).