Some eels possess two sets of movable jaws, so are Xenomorphs.
The first set of jaw is regular one, namely oral jaw. The second set of jaw is called pharyngeal jaw. The usage of the jaw is best illustrated by this gif.
Not all eels have pharyngeal jaws. The eels with this feature belong to Muraenidae, i.e. Moray eels. And not only eels have pharyngeal jaws. Cichlids have that too. In fact, the pharyngeal jaw structure has made cichlids one of the most diverge species of fish.
As for moray eels, the pharyngeal jaw helps the eating process. Most bony fish eat by suction. However, the moray eels do not have strong muscles to create the low pressure inside their body to suck in their prey. Instead, they bite and swallow with the help of the second jaw. Maybe this is the result of evolution. The moray eels are concealed inside crevices and alcoves, which are narrow - No space for the large muscles needed for suction. The restriction of the environment induced the gill arches of the moray eels to develop into movable jaws.
The function of the pharyngeal jaws in moray eels are not (re)discovered until 2007, by the work conducted people from UC Davis. The researchers found that in 1960s, similar studies have been published (but did not got popular). Another example of Stigler’s law of eponymy.
Func facts: 1. Moray eels sometimes cooperate with groupers for hunting, which is probably the only known example interspecies cooperative hunting in fish. 2. Eel swims by generating a travelling waving along its body. This enables swimming in the reversed direction. It would be fun to observe the vortex-shedding from the eel body. 3. H. R. Giger indicated that he has not studied any animals when creating the Aliens in 1979. He and the director just wanted to make it frightening.
Weel, the nature always acts ahead.
Reference:
- Pharyngeal jaw
- Moray eel
- And the linkes included in the article.