Subject: All fish breathe through gills. The water that contains the necessary oxygen is pumped over these gills thanks to the constant movement of the fish’s bony gill covers. Moray eels like this Yellowmargin Moray Eel (Gymnothorax flavimarginatus) lack these covers. To create the indispensable water flow over their internal gill chamber they need to orally pump the water by constantly opening and closing their mouths.
Technique: Get close to you subject, real close… I see way too often divers taking pictures or filming from too far away. Moray eels with their opening and closing mouth full of long teeth can sometimes be intimidating but unless you’re an octopus you’ll have nothing to fear.