Deep-sea creatures are often perceived as bizarre， dwelling in perpetual darkness and seemingly unconcerned with their appearance， often exhibiting haphazard growth patterns.
△ A typical deep-sea animal， indeed peculiar.
However， the subject of today’s discussion—the Pacific barreleye fish (Macropinna microstoma)—stands out as one of the most striking among these oddities.
It possesses a transparent， fluid-filled shield-like head， within which its two large， green， barrel-shaped eyes are housed， appearing particularly prominent. Its brain and all the nerve endings that form its head are also clearly visible.
This is what they look like:
Upon seeing such an image， what might come to mind?
For many， it likely evokes the image of an “alien submarine，” with its two eyes resembling “aliens” peering out through a skylight， while the rest of its head appears to be composed of mechanical components.
Adult Pacific barreleye fish measure around 15 centimeters in length and inhabit the deep sea at depths ranging from 600 to 800 meters. They are widely distributed in the northern Pacific， meaning humans have known of their existence for a considerable time， with the earliest description dating back to 1939.
However， early descriptions did not mention their transparent heads. This is because the transparent shield of barreleye fish caught in nets was often damaged during the capture process.
△ A barreleye fish that has been caught.
It wasn’t until 2004 that live specimens were photographed， revealing the remarkable nature of their transparent heads.
This species has large， flattened fins， allowing it to hover almost motionless in the deep sea. Its two eyes then stare intently upwards through the clear “skylight，” never missing any potential prey above them.
It is believed that the green pigment in their eyes helps filter out sunlight directly from the surface， aiding them in detecting the bioluminescence of prey located overhead.
The barreleye fish possesses a small mouth and feeds on small fish and jellyfish. It also snatches tiny crustaceans and other small organisms from the tentacles of larger jellyfish.
Jellyfish tentacles are equipped with stinging cells (nematocysts). The barreleye fish’s fluid-filled transparent head likely serves as a protective mechanism， shielding its eyes from these stinging cells.
Prevailing theories suggested its eyes were always directed upwards. However， in 2009， researchers from the Monterey Bay Aquarium Research Institute (MBARI) captured a live specimen that survived for several hours in a shipboard aquarium.
This research confirmed that the fish can rotate its tubular eyeballs forward to focus on prey in front of it. Subsequent footage captured by remotely operated vehicles also documented its eyes moving in the deep sea， further substantiating their mobility.
The ocean is home to many entirely transparent creatures， which use this trait for camouflage. However， having only parts of the body transparent， as seen in the barreleye fish， is relatively rare.
Yet， they are not unique in this regard. At least one other species exhibits a similar characteristic， as seen below:
This is the glass frog， found in the tropical regions of Central and South America. Its underside is transparent， allowing a clear view of its heart， liver， and other internal organs through its translucent skin.
The transparency of glass frogs also serves a camouflage purpose. Their clear coloration helps them blend more effectively with foliage， making them less visible to predators.
Interestingly， glass frogs have a unique way of concealing their blood. When at rest， they draw their red blood cells into their liver， thus hiding the blood-filled red vessels beneath their transparent belly.
However， when they are active， their blood vessels become clearly visible beneath their transparent skin.
