The fish with a transparent head!
Very little light reaches the dark depths of the ocean. Consequently we see many adaptions regarding light among deep-sea organisms, from extreme sensitivity to bio-luminescence. But as strange as they can seem, the barreleye fish blows them all away – it has evolved to see through its own head.
The green orbs you see are its eyes, which spend most of their time gazing upwards through its transparent “forehead” for prey. This “forehead” is actually a fluid-filled sack. When it spots something it likes, it rotates them forward so its field of vision and mouth are now aligned to hunt. Until 2009 it was thought that these eyes were fixed staring upwards, but finding a live specimen revealed their mobility.
The two spots you can see near the mouth are called nares, which the barreleye uses to filter the water for chemical traces. In addition to the stunning head adaptation, the yellow pigment of the eyes help the barreleye distinguish between sunlight from the surface and light coming from bio-luminiscent fish. A fascinating example of evolution indeed.