Lamprey

Petromyzon marinus, a very primitive fish, of the family Petromyzonidae. It is adapted to living as a parasite on larger fish, to the undersides of which it attaches itself by means of a suctorial toothed pad, through which it can suck the blood of its victim. This unattractive lifestyle is matched by an unappetizing appearance: slimy, jawless, a single nostril on top, and seven little gill openings on each side.

Although it counts as a sea fish, the lamprey goes up rivers to spawn and is indeed most often met in estuaries or the lower reaches of rivers. It reaches a maximum length of 120 cm (48"), but is commonly half that size. The river lamprey or lampern, Lampetra fluviatilis, is a smaller fish, and so is the Arctic lamprey, L. japonica.