This Elusive Fish Has Only Been Seen 9 Times!

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Can you imagine how cool it would be to see out of your forehead? That is what the incredible barreleye fish (Macropinna microstoma) can do. Moreover, a transparent head is not the only impressive adaptation that this fish has to help it thrive in our oceans. As this video by the Monterey Bay Aquarium Research Institute shows, these unique creatures would not look out of place in outer space. Here we will examine exactly why they have evolved with a transparent face and head. We will also consider how that helps them to stay alive in a challenging environment.

What Are Barreleye Fish?

Barreleye fish belong to the Opisthoproctidae family of fish and were not described in the scientific literature until 1939. They are a relative of smelts and can reach around six inches in length.

As you can see from the above clip, most of their body is a grey/brown color. They have fairly powerful, flat fins that allow them to hover in the water almost motionless and maneuver their bodies very precisely. The stand-out feature is the domed, transparent cowl-like membrane covering their eyes.

Where Do Barreleye Fish Live?

These fish live in the North Pacific Ocean, from the Bering Sea to Japan and Baja California, Mexico. They inhabit the ocean layer called the mesopelagic zone, which is also sometimes called the twilight zone or the midwater zone. This zone extends from a depth of 660 feet down to 3,300 feet. It is the layer below the top layer (epipelagic zone). The barreleye is most often found between 2,000-2,600 feet. Currently, they are not classed as endangered.

In the mesopelagic zone, there is some sunlight, but it is very faint. The area can be subject to quite extreme temperature changes, and the water temperature decreases quite rapidly with increasing depth. Due to the lack of light, the creatures that live there have adapted to make the most of the meagre illumination.

Unusual Head Anatomy

There are two striking features that make the barreleye fish unmistakable. The first is the transparent shield or ‘forehead’ that protects the eyes. This is a tough outer protective layer attached to the dorsal and medial scales behind the head. It is also attached to the subocular bones (also known as the subocular shelf) that extend inwards from the suborbital bones (above the eye socket) of some fish. In barreleye fish, these bones are also transparent. The transparent shield encases a fluid-filled chamber that also provides protection for the eyes.

The eyes themselves are the second striking feature. In the above video, you can see them as green circles beneath the transparent shield. The eyes are tubular and can look forward and upward – they have a total arc of rotation of around 75 degrees. The vivid green discs that you can see in the clip are actually the eye lenses. You have probably mistaken the two dark circles as eyes but these are their olfactory organs that function a bit like a nose. Their barreleyes glow green, also known as bioluminescence, which helps them see in the low-light conditions.

Why the Transparent Forehead?

To figure out why these fish have evolved to have a transparent forehead, we need to look at what they eat and how they catch their food. A lot of the time, they feed on zooplankton – both gelatinous and crustacean. However, scientists now believe that they are stealthy fish and that one of their feeding methods is to steal prey from siphonophores. These are long invertebrates and include jellyfish. Analysis of their stomach contents has revealed siphonophore tentacles, which supports this.

The problem with stealing food from jellyfish is that they often have stinging tentacles, which causes damage to their eyes. To get around this, the barreleye hovers in the water, directing its barreleyes upwards so that it can spot a potential meal illuminated by the lighter waters above. The green pigments in the lenses filter out sunlight and help them spot the bioluminescence of creatures that are potential prey.

New Discoveries

Scientists have also now discovered that once the barreleye fish spots a potential snack, it rotates its eyes forward and swims upwards to eat the meal. If there are any stinging tentacles to deal with, the transparent shield fully protects the delicate eyes.

Experts now think that they use their deft maneuvering skills to move among the trailing tentacles. They pick off captured prey and claim it as their own.

These discoveries have only been possible because of modern underwater robots such as those deployed by Monterey Bay Aquarium Research Institute in the clip above. Yet, despite launching more than 5,600 successful dives, this elusive fish has only been seen nine times!

The post This Elusive Fish Has Only Been Seen 9 Times! appeared first on A-Z Animals.