The simenchal or monkey eel (Simenchelys parasitica) is a cross between a predator and a parasite.

Simenchelys parasitica is a fascinating species of eel that has adapted to live in the bodies of certain deep-sea fish, such as grenadiers. It's known for its parasitic lifestyle, where it resides within the body cavity of its host, feeding on the host's blood and bodily fluids. This unique adaptation allows it to thrive in the harsh conditions of the deep ocean.
This 30-centimeter creature is armed with short and strong jaws with many close-set teeth. They are necessary to quickly cut through the wall of the prey's skin, and then gradually, biting off piece by piece, burrow into the victim's body and continue the feast inside.
There are cases when live monkey eels were found in the stomachs of captured and already dead sharks.