Sea lamprey
With their teeth-filled circular sucking mouths, it’s no surprise these brown-grey fish have been given the nickname “vampire fish”. As parasites, they sucker themselves to other living creatures and feed on their host’s body tissues.
Shaped like eels but with no fins or jaws, they breathe through holes located behind their eyes. Their larvae are spawned in the mud of large rivers, but then migrate to the sea where they spend most of their lives until, as older adults, the sea lamprey return to the rivers to spawn. They then die.
They’re an ancient species too. Incredibly, sea lamprey fossils have been discovered that date back to around 450 million years ago.