2. Macropinna Microstoma
Macropinna and Opisthoproctus genera of the Opisthoproctidae family. These three particular species of barreleye fish, while not technically deep sea, all have one common biological trait in common which make them easily some of the craziest fish in existence. You know, the transparent head thing. Not even slightly kidding about this. Their heads are see through, protecting their eyes but at the same time still letting them see. Due to the low light conditions, fish which rely on sight for hunting and navigation often have vastly different eyes to surface dwelling species. Some are just very large, but the barrel-eyed variety focusses all the available light streaming down from above through the unusual shape of their eyes. Its also been suggested that the transparent dome may help in collecting light. Macropinna microstoma is the best documented species and from where most of the information about these remarkable fish has come from. It has been known about since 1939 but because the transparent dome was always destroyed during capture, it wasnt until it was first photographed alive in 2004 that this fascinating and unique feature could be seen and better understood. The eyes point directly upwards scanning for prey above the fish, which uses wide fins to remain relatively motionless in the open water. When they lock onto prey they then tilt their bodies vertically, rotating their eyes in their heads as they move. This allows them keep the prey in view while silently moving in to strike.