Moray eels are the closest earthly thing you will get to the xenomorphs in the Alien series. Like the Giger beasts, this ambush predator has developed a terrifyingly efficient set of secondary grasping jaws to aid in yanking the eel's meals down its gullet. This video from UC Davis show it in action.
Known as the Pharyngeal jaw, this evolutionary adaptation is surprisingly common among fish with more than 30,000 known species employing a set. However, none are as mobile (or frightening) as the Moray's. Since these eels can't suck in and gulp down their prey as other predatory fish do, they instead evolved this swing-out set of small, grabbing mandibles. The eel first captures its prey with its primary jaws, then swings out its secondary set to grab the fish and pull it down the eel's throat while the first set of jaws chomp down on the next length of flesh.