Many eels are catadromous, meaning they migrate from freshwater river systems to the ocean in order to breed. This is due to the ocean being much more productive for feeding. To undergo this move from freshwater to saltwater, the hormone cortisol causes chloride cells in the gills to pump out excess salt that the fish takes in. Moray eels have what's called a "pharyngeal jaw", which is a second set of jaws in its throat. The first set of jaws captures the prey and the second set comes forward to grip onto it and pull it down the throat. Many eels are catadromous, meaning they migrate from freshwater river systems to the ocean in order to breed. This is due to the ocean being much more productive for feeding. To undergo this move from freshwater to saltwater, the hormone cortisol causes chloride cells in the gills to pump out excess salt that the fish takes in. Moray eels have what's called a "pharyngeal jaw", which is a second set of jaws in its throat. The first set of jaws captures the prey and the second set comes forward to grip onto it and pull it down the throat.