Lamprey
Lampreys, which are jawless fish with an eel-like appearance, can survive in both freshwater and saltwater habitats. They have circular mouths lined with teeth, and once they find a fish suitable for a meal, they use their mouths to latch onto the body of their prey, sucking out the innards. Lampreys are said to have the oldest living lineage of vertebrates, with more than 35 species in existence , according to National Geographic.
More information: http://science.nationalgeographic.com/wallpaper/science/photos/carboniferous-period/pacific-lamprey/
