Image: Glasshead Barreleye, Rhynchohyalus natalensis
A Glasshead Barreleye trawled from a depth of around 1000 m on the abyssal plain in the southern Tasman Sea, between Tasmania and New Zealand.
- Adrian Flynn
- © Adrian Flynn
The image shows the side of the fish. The grey, translucent, round structure near the top margin of the head is the primary eye. The fish also has a second set of silvery 'lower eyes' that are clearly visible. These ancillary mirror-organs collect light from the side and below. The opening through which light passes is clearly visible as a dark opening. The mirror-organs have guanine crystals and a 'retina' of sorts that bounce the light they detect back into the main eye or it may be detected by the mirror organ itself. The fish essentially has 4 'eyes'. View an excellent diagram on Wikipedia of the workings of the eyes of Dolichopteryx longipes, another species of barreleye.
The barreleyes are an unusual family of deepsea fishes, most of which have upwardly-directed eyes. The eyes detect the silhouettes of prey swimming above. In Australia, the Glasshead Barreleye has been collected from bathypelagic and mesopelagic depths off New South Wales.