The Barreleye (Macropinna microstoma) is a truly bizarre-looking fish with a transparent head and amazing eyes.
The common name comes from the fishes' barrel-shaped, tubular eyes which are very good at collecting light to enable it to see better in the dark ocean waters in which it lives.
The eyes were believed to be fixed, giving the Barreleye a very narrow "tunnel-vision" field of view covering only objects above the fish's head. However, research by Bruce Robinson and Kim Reisenbichler from the Monterey Bay Aquarium Research Institute provided evidence to suggest that the fish can rotate their eyes within a transparent shield which covers their head, allowing them to look upwards at potential prey above, or point their eyes forward to see what is ahead.
The fluid-filled shield gives the appearance that the fish has a see-through head. Check out the video:
The Barreleye normally hangs nearly motionless in the water, with the help of its large flat fins, at a depth of about 600-800m/2,000-2,600 ft, its eyes directed upward. In the low light conditions it is assumed the fish detects prey such as small fish and jellyfish by their silhouette.
The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping it spot the bioluminescent glow of jellyfish or other animals directly overhead.
It's also thought that Barreleyes steal the food of siphonophores (colonial jellies). These siphonophores grow to over 10m/33 ft and are equipped with thousands of stinging tentacles, which capture copepods and other small animals. The Barreleyes are thought to move among the tentacles, picking off captured organisms. The transparent shield would help to protect the fish's eyes from the siphonophore's stinging cells.
Macropinna microstoma has been known to science since 1939, but was not photographed alive until a few years ago. Until then scientists had no idea that the fish had a transparent dome, presumably because the fragile structure had always been destroyed when the fish was brought up from the depths in fishing nets.
Why not check out some of our other Weird fish of the week features?