Eels lack the large gill covers found in true fishes. Because of this, eels must use their mouth to pump a fresh supply of oxygen-filled water over their gills. That is why they constantly open and close their mouth.
Eels rely heavily on their sense of smell rather than sight to detect their prey.
The smell sensors protrude from the nose rather than being internal as ours are. Although the green moray looks quite fierce , they allow their teeth, mouths and gills to be cleaned by small neon gobies and shrimps. It is quite extraordinary to see the small fish swim into the mouth of the fearsome eel and come out again unscathed. Many fishes cannot swim backwards. Not so with the eel.