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Transparent Headed Fish (Macropinna microstoma)
There are strange things in the deep sea.
One of the more bizzare creatures to inhabit the deep is Macropinna microstoma, which belongs to the barreleye family Opisthoproctidae.
It's main claim to fame is the transparent fluid-filled dome that covers most of its head and the strange anatomy of its eyes. Barreleyes are so named because their upwards pointing, tubular, barrel-shaped eyes.
This fish was first described by science in 1939, however until 2004 it was only known from specimens that were dragged up by deep sea fishing. The nets always ended up destroying the fragile fluid-filled domes of these fish. As a result for many years it was a mystery to scientists as to why these fish had such strange, upwards-facing, barrel-shaped eyes.
Old drawings of this fish in scientific papers never showed the fluid-filled dome. It was thought that their eyes always faced up and didn't have the ability to rotate, however this would make feeding very difficult. The relatively small size of the mouth of this species would further compound this problem.
The external, lumpy structures above the mouth that appear whitish in this photo are not eyes, these are organs that help the fish smell. The actual eyes are completely enclosed within the transparent fluid-filled dome.
Their remarkable location within this transparent dome is thought to protect the eyes from the stinging cells of siphonophores (jellyfish-like creatures), while still allowing clear vision. They are thought to use their good vision to navigate through the tentacles of these siphonophores to scavenge off their captured prey.
When not scavenging food of siphonophores, it is throught that their upwards turned eyes help Macropinna microstoma watch for the faint shilouette of prey (tiny fish and jellyfishes) above it in the low light conditions of the deep sea.
Normally this fish likes to stay nearly motionless in waters between 600 m (2000 ft) and 800 m (2600 ft) deep with its eyes facing upwards watching for prey. When the fish sees something it wishes to feast upon in begins to swim upwards towards it, turning its barrel eyes inside its head so that they are now facing towards its front where its prey is.
Each eye is capped by a large, bright green lens. Due to the presence of specialised pigments in these lenses it is thought that these fish have the ability to filter out faint natural sunlight and pick up on the bio-luminescence that some creatures use to mimic it to avoid appearing as a shilouette to predators below. The eyes of this fish are also highly sensitive, enhancing their ability to spot prey in their low-light environment.
The photo above may give the impression that this fish is of a substantial size, but in actual fact it is only several inches long.