by MBARI https://www.youtube.com/watch?v=RM9o4VnfHJU

The Barreleye Fish: The Fish with The See-Through Head

With a transparent, dome-shaped head, nostrils that look like eyes, and eyes that look… nothing like eyes, this fish is definitely an oddball…

Looking more like an alien or a Disney character from a blockbuster animation.

These creatures remind us that truth is often much stranger than fiction…

Note the differences between the Brownsnout Spookfish, the pacific barreleye (today’s focus), and the two recently discovered species.

Interestingly, scientists differentiate their species because of differences in skin pigmentation.

These creatures’ weird appearance invites questions…

This fish sure ain’t your average tuna. However, far from being mother nature’s gimmick, it’s a beautiful illustration of evolution refining for purpose.

Its scientific name is Macropinna microstoma. Its class is Actinopterygii, from the order of Argentiniformes, and family Opisthoproctidae.

Got that?

Never mind. There’s no need to remember all of the Greek/Latin word-soup that defines its identity. 

Science calls it Macropinna, its friends call it the barreleye fish. The dumbfounded call it spook fish

We’re going with ‘barreleye’ – it fits, and you’ll see why later…

With the help of some great investigative science from dedicated researchers, we’re going to share the story of Macropinna microstoma. The Barreleye.

What’s in a name hey? Let’s find out…

It has nostril-like structures on the front of its face, that look like eyes, but are in fact olfactory organs akin to human nostrils. It’s small mouth looks more like a snout, and has no teeth.

In contrast to its head, its fins and body look reasonably normal and… fish-like. Yet it’s fins are pretty special—we’ll tell you why shortly…

The Barreleye Head

Their transparent heads serve as protection for their eyes while allowing them to see.

It’s often likened to the dome of glass that is seen on a fighter jet. It protects the pilots and allows them to see out.

evolution has given these creatures the ability to look straight up through their heads…

The tough transparent shield is filled with fluid that protects the green tubular eyes.

Importantly, evolution has given these creatures the ability to look straight up through their heads…

That is, it looks directly above itself without having to move.

Here is a cool short video showing them in action…

Macropinna microstoma: A deep-sea fish with a transparent head and tubular eyes

The Barreleye Mouth and Nose

You will notice two evenly spaced organs on its face that look a lot like eyes…

These are called nares and have nothing to do with vision.

They’re olfactory organs, that is, scent organs. They are very much like our nostrils. Loosely speaking, it’s a nose.

The mouth is small, perhaps disproportionately small

It’s pursed to a point where it looks more like a snout.

It has no teeth at all, hence no dentists bills, no drills, and no fillings. Woohoo.

DID YOU KNOW?

Biologists speculate that its small mouth indicates small prey. These fish may well be quite selective when it comes to dinner.

The Barreleye Eyes

…the barreleye is one of the few vertebrates that uses a mirror to collect light

The brilliant green eyes point in the same direction, therefore, giving the fish binocular vision

That is, the two eyes create one picture. Interestingly, the barreleye is one of the few vertebrates that uses a mirror to collect light.

Barreleye fish with see-through skull

The eyes sit in an upward portion looking directly above (through) their heads…

It’s this biological feature that makes these species unique.

DID YOU KNOW?

Contrary to what was previously believed by biologists, their eyes can actually rotate and look forwards, assisting them to catch their food… and not run into doors!

Seeing in the Dark

Biologists have always accepted that the tubular shaped eyes are brilliant at catching the light.

However…

There is also speculation that the green fluid is ultra-photosensitive.

This gives the fish the capability of capturing the smallest amounts of light, including the bioluminescence of prey.

The eyes lack cone cells in the retina. Cone cells work best in bright light and are responsible for color vision…

With barely any light in the twilight zone, evolution has deemed them unnecessary.

NOTE

In short, barreleyes look up to find food overhead (such as drifting jelly), then look forward to catch their prey.

The high number of rod cells in their eyes allows them to distinguish between the filtered light of the sun and the bioluminescence of their potential dinner.

Their green, barrel shaped eyes are perfectly adapted for hunting in a water zone with little to no light.

Let’s move on to the other features of the barreleye…

Barreleye Illustration

Barreleye Skin and Fins

Except for their head and fins, the body of the fish is a dark brown color. Their fins are silver, large and perfectly adapted to their behavior.

The barreleye uses their large flat fins to suspend motionless in the deep water…

The size of the fins, while perfect for stability and keeping them stationary, deliver exceptional maneuverability for hunting.

The position of their eyes works perfectly in concert with their fins, making them an excellent ambush predator.

The Diminutive Barreleye

The barreleye is not particularly big…

At an average size of 6 inches, it’s actually pretty small. When fully mature, they can reach a length of 17 inches.

How did they even manage to find this small creature in the deep vast seas?

A Little Barreleye History

Presumably, barreleyes have been swimming our oceans for countless millennia

It’s a reasonable assumption because evolution commands it.

Considering our history on the ocean…

It seems surprising that these species were not discovered by science until 1939.

FUN FACT

When a marine biologist by the name of Chapman first observed the species, he couldn’t see that its head was transparent. In fact, science knew practically nothing.

The relatively late discovery of these creatures can be easily explained. It lives at 2,000 feet beneath the sea surface and has a limited global distribution.

Underwater View of the Sea
Underwater View of the Sea

Explorer’s Paradise

Scientists like to remind us, that even with our current technologies and research fervor, there is still much to discover in our oceans

The National Oceanic and Atmospheric Administration (NOAA) suggests that as of the year 2000, ninety-five percent of the ocean still remains unexplored

Recent Discoveries

We first discovered barreleyes as a result of bycatch in fishing nets. It wasn’t until 2004 that we discovered, and indeed photographed, this fish with a see through head.

Apparently, the netting process destroyed the transparency of their fragile dome-shaped heads.

Early Science Challenged by New Observation

Early researchers believed that the eyes of the barreleye remained fixed in place, looking up.

However…

Recent research by Bruce Robison and Kim Reisenbichler from the Monterey Bay Aquarium Research Institute has revealed their eyes not only move, but rotate inside in their transparent heads allowing them to look forward.

Bruce Robison and Kim Reisenbichler used video from MBARI’s remotely operated vehicles (ROVs) to study barreleyes in the deep waters just offshore of Central California.

biologists have observed that the barreleye has no lateral vision, that is, the ability to look sideways…

Interestingly, biologists have observed that the barreleye has no lateral vision, that is, the ability to look sideways.

It’s amazing what you can discover with remotely operated submersible vehicles!

Not so popular back in 1939 we guess, hence the mysteries.

Being able to observe creatures alive in their natural environments is, unsurprisingly, more revealing than staring at dead ones.

Speaking of mysteries…

Wanna know where barreleyes live?

The Deeper You Go, The Weirder it Gets

This section of the water column is often referred to as the twilight zone

While the name refers to the diminishing penetration of sunlight, we think it’s pretty descriptive of some of the local wildlife.

It’s the lower depths of this zone where critters start looking really weird.

Photo of a Zooplankton
Photo of a Zooplankton

A Private Life in the Deep Ocean

Barreleyes are found in the tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans.

The Pacific barreleye is known to populate North Pacific locations including the Bering Sea to Japan, and Baja California, Mexico.

They are also found in Bermuda, the Bahamas, the Greater Antillies and the Gulf of Mexico. New discoveries include locations such as American Samoa and New Zealand

DID YOU KNOW?

The Barreleye is a deep oceanic fish. You’re unlikely to stumble across one snorkeling at your local beach. Or even diving for that matter.

Recent observations in the wild are thanks entirely to submersibles technology.

Sneaky Tactics

As well as being consummate ambush predators, they’re clever little thieves. They steal the food from the tentacles of siphonophores before they get a chance to eat them.

Biologists suggest that they maneuver around the siphonophore stings to avoid being stung.

It is also believed that their transparent heads protect their eyes from stings.

Barreleye Predators

Marine biologists remain unaware of any specific barreleye predators. However, the absence of information or observations doesn’t mean they’re off the hook.

The word in the scientific world is, that further studies will expose the barreleye’s arch-nemesis. It’s pretty much a given that few if any aquatic creatures live free of predation.

A Misconception

When we think of a barreleye as prey, we tend to think first of these creatures when it’s fully grown.

the barreleye is vulnerable to countless creatures in the sea…

We need to remind ourselves that as eggs and fry, the barreleye is vulnerable to countless creatures in the sea…

From birds, small fish, big fish, turtles, whale sharks and, well…everything.

Surviving from egg to mature adult is a tough road for thousands of species. There’s no reason to assume it’s any different for the barreleye.

Threats from Human Activity

The greatest threat to many creatures of the marine environment is humans. Fortunately, the barreleye has no commercial value in terms of human consumption.

Fish Caught In a Net
Fish Caught In a Net

However, we know they are getting killed by accident as a result of commercial fishing bycatch. But, records of the actual numbers are not available.

They’re no threat to humans. So, there are no threats from human protective measures, such as beach netting for sharks.

DID YOU KNOW?

Because of the water depth at which they live, they are not at risk from tourist activities like species such as the whale shark

Under normal circumstances, divers can’t reach the depths of the barreleye habitation zone.

Pollution a Threat to All Marine Life

One can assume, however, that like all marine species, the barreleye is subject to significant habitat degradation as a result of human-induced pollution.

They are not registered as vulnerable or endangered. The International Union for Conservation of Nature (IUCN) has no designation for barreleyes…

This is positive news; the IUCN’s red list is not a place you want to see your species listed.

Love And Breeding

It’s believed that barreleyes are loners…

Being anti-social and living a solitary life as they do, makes starting relationships pretty tough, yes? 

Bars, social groups and internet dating are certainly out of the question for these creatures.

So how do they breed? How do boys meet girls?

No Need for Contact 

While scientific information is limited, it is believed the barreleye are pelagic spawners

This means that females will lay eggs in water and males will release sperm in the water. With fertilization taking place in the water, it would appear there’s no barreleye romance

The fertilized eggs float. They move with the flow of the current remaining in the shallows until hatching. They move progressively deeper in the water column as they mature.

Limited Technology

In the past, exploration using SOV’s required not only the ridiculously expensive submersibles but the expertise and infrastructure to deploy and operate them.

Large exploration ships, staffed by scientists, submariners and technicians from countless disciplines were required to man an expedition.

It would seem unlikely that sufficient funds would have been raised to ever explore a creature such as barreleyes.

The Future of Exploration is Small and Remote

The technology is now so advanced and sophisticated that SOV’s can be operated by just about anybody.

Frequently called underwater drones, very small SOV’s mitigate against the prohibitive costs that underwater exploration traditionally incurred.

Now, with a set of instructions, a modest boat, and some willing scientists, small underwater drones can be deployed to film the mysteries of the deep.

Catching a Barrel Eye

Catching a barreleye has certainly been achieved. Unfortunately, we’ve caught them accidentally in fishing nets and killed them.

Studying a live specimen requires less clunky methods. But how does one catch these creatures? The answer…It’s tricky.

Even the most clever of biologists using sophisticated devices struggle to catch these creatures. Fancy trying to catch fish that can see through the top of their heads. It’s a tough job, as this video illustrates.

Footage of Barreleyes

Wrapping up The Barreleye

The Barreleye is not an alien. Nor is it a fantasy creature from the latest Disney production.

It’s got some awesome hunting skills, but as far as we know…

It has no mystical powers. And it’s not just weird-looking, it’s flat-out bizarre.

One thing is certain. These creatures are definitely one of the most fascinating members of the earth’s diverse aquatic community.

FUN FACT

Science is yet to discover anything quite like the barreleye fish. And it makes you wonder if it’s not one of mother nature’s tricks to confuse the curious.

Product of Evolution

Truth is, recent studies demonstrate there are no tricks here. In an evolutionary sense, barreleyes are a prime example of adaptation.

With tubular eyes on the top of its transparent head, large fins that allow for deft maneuvers in the deep ocean, these creatures are a masterpiece of evolutionary fitness.

There is More to Learn

Given that research breakthroughs are only recent, we can assume there’s at least a handful of marine biologists bursting at the test tube to learn more.

There’s plenty more to learn about the strange and wonderful barreleye fish, and no doubt, it’ll be as interesting as their transparent heads.

So stay tuned to the science journals for their regular fishy dispatches

If you enjoyed the article, your friends might be curious too… feel free to share this and sate their curious minds as well!

Image Reference

Featured Image – Screenshot from Macropinna microstoma: A deep-sea fish with a transparent head and tubular eyes (00:32) uploaded by Monterey Bay Aquarium Research Institute Feb 23,2009 https://youtu.be/RM9o4VnfHJU?t=32

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