Gulper eel, Eurpharynx pelecanoides, or the pelican eel, also called the umbrella mouth gulper. I think these pictures, especially of the gills, are unique. This one is from a cod-end trawl off California at a depth of 600 metres but apparently found much deeper too. These fish do not survive long after being taken from so deep in a big net, the temperature at the surface being much higher than where they live. The recently dead specimen was filmed in a kreisel aquarium that had a regular water flow and its skin texture and details are probably identical to a living fish. It is about half a metre long but can swell up with water to 11 times its volume and perhaps take quite large prey in its expandable jaws. However, it has been found with small crustaceans and plankton in its gut so perhaps filters the water like some species of whales, sieving out the plankton. In this case it would need to expel large amounts of water and you can see that it has some strange gill slits situated about a third of the way down the body, after the mouth, which might help in getting rid of huge mouthfuls of water. The mouth itself is super matt black, suggesting it holds it open for animals to blunder into in the dark. There was a bioluminescent organ on the tail but this is not seen here. Not a true eel - just looks like one the internal anatomy is very different.