Barreleye Fish: Transparent Head Discovery

Why is the barreleye fish's head transparent?

The barreleye fish (Macropinna microstoma) has a completely transparent, dome-shaped head that allows its tube-shaped eyes to rotate inside its skull. The transparency protects the extremely sensitive eyes while allowing them freedom to point in different directions. Through the transparent dome, researchers can see the green barrel-shaped eyes pointing either upward (to detect prey silhouetted against the dim upper light).

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Eyes That Rotate Inside a Transparent Skull

A small fish hovers in the dim twilight of deep ocean 700 meters below the Pacific surface. Its body is normal-fish-shaped. But its entire head is transparent, a clear dome through which you can see two large green barrel-shaped eyes. As you watch, the eyes rotate inside the dome. First they point straight up toward whatever faint light remains above.

Then they rotate forward to focus on something ahead. The entire process is visible because there is no skull in the way, just a clear fluid-filled chamber.

This is the barreleye fish ( Macropinna microstoma ). Its rotating-eye mechanism was unknown to science until a 2008 study, which was widely publicized in 2009.

The Animal

Barreleye fish are small deep-sea fish with remarkable anatomy.

Physical features:

• Length: 15 cm typical
• Body: small, flattened, normal fish shape
• Head: completely transparent dome
• Eyes: barrel-shaped, green, rotate within head
• Mouth: tiny (hence scientific name "microstoma" = small mouth)
• Fins: standard fish arrangement

The transparency:

The dome that forms the head is:

• Clear and see-through
• Fluid-filled inside
• Flexible
• Protective of internal structures

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The Rotating Eyes

Barreleye eyes are among nature's strangest designs.

Eye structure:

• Large, barrel or tube-shaped
• Green colored (from pigments for detecting specific light)
• Significantly bigger than typical fish eyes
• Highly light-sensitive

The rotation:

• Eyes can rotate within the transparent dome
• Point upward: spot prey against surface light
• Point forward: focus on prey ahead during attack
• Various positions between
• Happens smoothly and rapidly

Why this matters:

For hunting, barreleye fish need:

• Detect distant prey (eyes up)
• Focus close for capture (eyes forward)
• React quickly (rapid rotation)
• Avoid moving the whole body (energy efficient)

The rotating eyes solve all these challenges with one mechanism.

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The 2009 Discovery

For close to 70 years, how barreleye eyes functioned remained mysterious.

Scientific knowledge before 2009:

Barreleye fish had been:

• First described in 1939
• Known from dead specimens
• Textbook subjects

But something was missing:

Researchers could not determine:

• Did the eyes rotate?
• How did they see prey at close range?
• Why the transparent head?
• Complete picture of hunting

The 2009 breakthrough:

Researchers Bruce Robison and Kim Reisenbichler at Monterey Bay Aquarium Research Institute (MBARI):

1. Used remote-operated vehicles (ROVs) at barreleye depths
2. Observed live specimens in natural habitat
3. Filmed eye rotation for first time
4. Published results revealing the rotating eyes

Why it took so long:

• Barreleye fish die quickly at surface (pressure change)
• Dead specimens cannot show living eye movement
• Previous photographs showed still images only
• Behavioral observation required advanced equipment

The 2009 observations changed textbook understanding of the species.

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The Transparent Dome

Barreleye heads solve multiple biological problems.

Why transparent:

1. Protects eyes: from damage while allowing light
2. Reduces weight: compared to solid skull
3. Provides space: for rotating eyes
4. Maintains shape: flexibility allows rotation

What's inside:

• Fluid-filled chamber
• Two large rotating eyes
• Light-sensitive tissue
• Optical nerves to brain
• Support structures

The olfactory organs:

What appear to be "eyes" in photos of barreleye fish are actually:

• Nostrils/olfactory organs
• On the outside of the dome
• Used for smell
• Completely separate from the actual eyes

Many early photos mistook these for eyes, the real eyes are inside the dome.

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Hunting Strategy

Barreleye fish have unusual feeding behaviors.

Kleptoparasitism:

Their most remarkable habit is believed to be stealing food from siphonophores:

• Approach siphonophore jellyfish
• Siphonophore has captured prey in tentacles
• Barreleye positions near the captured prey
• Steals the prey without being stung
• Transparent head protects eyes from stings

Why it works:

The transparent head provides:

• No traditional bony protection around eyes
• But complete physical protection
• Stinging cells cannot penetrate tough tissue
• Fish eats safely

Other hunting:

Barreleye fish also:

• Hunt small fish directly
• Eat small crustaceans and other zooplankton
• Detect prey silhouetted above
• Approach stealthily

Feeding mechanics:

• Small mouth limits prey size
• Suck prey in through specialized jaw
• Swallow whole

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Where They Live

Barreleye fish have a Pacific-restricted distribution.

Range:

• Pacific Coast of North America (most common observations)
• Monterey Bay, California (best-studied population)
• Alaskan waters
• Japanese Pacific coast
• Various other Pacific locations

Depth:

• 600-800 meters typical
• Mesopelagic zone
• Where faint sunlight still reaches
• Supports their upward-hunting strategy

Habitat preferences:

• Presence of siphonophore jellyfish (for food stealing)
• Stable temperature
• Appropriate depth light levels

Why mesopelagic:

Unlike many deep-sea fish that live in total darkness, barreleyes need:

• Some filtered sunlight from above
• Silhouette prey against this light
• Visible prey to hunt effectively
• Therefore shallower than abyss species

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Biology

Barreleye fish have specialized physiology.

Eyes:

• Extremely light-sensitive
• Green-colored photoreceptors (for specific wavelengths)
• Large rod cells
• Effective in dim conditions

Swimming:

• Slow and deliberate
• Energy-efficient
• Low metabolism
• Suits deep-sea lifestyle

Pressure adaptation:

• Soft tissues (including transparent head)
• No swim bladder
• Near-neutral buoyancy
• Standard deep-sea pressure tolerance

Body design:

• Normal fish body (compared to head)
• Standard fin arrangement
• Sensory organs beyond eyes (lateral line, etc.)
• Breathing and feeding systems typical

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Discovery History

Barreleye fish have been studied by scientists for 80+ years.

1939: First description:

• Dead specimen examined
• Morphology documented
• Species name assigned

1940s-1970s:

• Occasional specimens in deep-sea nets
• Photos of preserved specimens
• Eye structure studied
• Behavior unknown

1980s-2000s:

• Improved deep-sea research capabilities
• Occasional ROV observations
• Understanding slowly expanded
• Rotating eyes still unknown

2009:

• MBARI researchers finally observed rotating eyes
• First video footage
• Textbook understanding updated
• Viral social media sharing

2010s-present:

• Continued research
• More observations
• Behavior better understood

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Cultural Impact

Barreleye fish became internet-famous in 2009.

Viral 2009:

The first videos of living barreleyes with rotating eyes:

• Went viral on video platforms
• Changed public perception of the species
• Featured in nature documentaries
• Educational materials updated

Public perception:

• "That weird fish with the transparent head"
• Example of deep-sea strangeness
• Often cited in articles about unusual animals
• Symbol of ongoing scientific discovery

Recent observations:

Each new video of barreleye fish:

• Gets shared extensively online
• Raises public interest in deep-sea biology
• Supports funding for continued research
• Educates new generations

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Scientific Significance

Barreleye fish teach multiple biological lessons.

Evolution:

• Extreme adaptation to deep-sea life
• Solution to light-detection challenges
• Specialized hunting in specific niche
• Unique body plan development

Deep-sea biology:

• Different strategies for deep-sea life
• Mesopelagic zone adaptations
• Kleptoparasitic relationships
• Sensory specialization

Anatomy:

• Transparent tissue in vertebrates
• Specialized sensory organs
• Protective structures
• Internal eye rotation mechanisms

Research methods:

• Importance of ROV observation
• Limitations of dead specimens
• Need for non-invasive research
• Deep-sea exploration priorities

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Conservation

Barreleye fish status is largely unassessed.

Population:

• Exact numbers unknown
• Appear relatively common in suitable habitat
• Restricted geographic range (Pacific)
• No specific population data

Threats:

• Deep-sea fishing bycatch
• Climate change affecting deep oceans
• Potentially deep-sea mining
• Ocean acidification impacts

IUCN status:

No conservation assessment was found in the sources consulted.

Conservation concerns:

• Limited distribution (only Pacific)
• Specialized habitat requirements
• Dependence on siphonophore populations
• Vulnerable to disruption of mesopelagic zone

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Why They Matter

Barreleye fish represent the ongoing discovery of deep-sea biology.

What they teach us:

• Deep sea still surprises: 70 years after discovery, we learned something new
• Direct observation matters: dead specimens can't tell complete stories
• Unique solutions exist: nature produces unexpected answers to challenges
• Conservation applies broadly: even small, obscure species deserve protection

Biological significance:

• Evolution of transparent tissue in vertebrates
• Kleptoparasitic relationships
• Mesopelagic zone ecology
• Specialized visual systems

Public engagement:

• Helps people appreciate deep-sea biology
• Educational value in their unusual features
• Gateway to interest in marine conservation
• Inspirational research successes

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The Transparent Hunter

Every barreleye fish in Pacific deep waters is a living example of evolution's creativity.

When you see footage of a barreleye rotating its green eyes inside its transparent dome, you are watching an animal using features few other fish share. The combination of transparent head + rotating eyes + small mouth + kleptoparasitic feeding + mesopelagic hunting represents a unique evolutionary package.

The species will probably continue to thrive as long as their Pacific deep waters stay healthy.

Each barreleye fish is hunting right now, hundreds of meters deep in Pacific waters. Its transparent head is letting light reach eyes that rotate to track potential meals. Its small mouth is ready to steal food from nearby siphonophores. And almost no one will ever see it doing this, except when research submersibles happen by.

In 2009, we finally saw them properly. What else in the deep sea remains hidden, waiting for the next scientific discovery to reveal behaviors that have existed for millions of years?

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The 2009 Discovery in Context

The publication that transformed barreleye biology came from Bruce Robison and Kim Reisenbichler of the Monterey Bay Aquarium Research Institute, who combined ROV video of living Macropinna microstoma with lab observations in 2008.

Their 2008 paper in Copeia documented what no previous specimen had revealed: the fish's eyes are not fixed looking upward as had been assumed from dead specimens [1]. Instead, the eyes rotate forward inside the transparent head to look at prey the fish is about to consume.

The misunderstanding had persisted for nearly 70 years. Barreleye fish had been collected from net trawls since the 1930s, but the transparent head dome collapses when the fish is removed from deep-water pressure. What scientists saw in preserved specimens was a partially-damaged fish with rigid tubular eyes stuck pointing upward.

Deep-Sea Fish Specimen Recovery Challenges

Structure	At depth	After trawl recovery
Transparent head dome	Intact, clear	Collapsed, damaged
Tubular eye position	Rotates freely	Stuck in fixed position
Soft tissues	Normal	Damaged by decompression

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Barreleye Family Diversity

Researchers notes that Macropinna microstoma is the most famous barreleye but only one of the species in the family Opisthoproctidae. All have a small mouth and large tubular eyes. Different species have adapted the basic plan to different depths and feeding strategies.

Barreleye Species Summary

Species	Habitat depth	Special feature	Notes
Macropinna microstoma	600-800 m	Transparent head dome	Most famous, filmed in 2008
Opisthoproctus soleatus	Below about 400 m	Sole-like flat belly	Has species-specific luminous organ
Dolichopteryx longipes	400-2,500 m	Eyes with mirrors (not just lenses)	First vertebrate with mirror eyes

The 2009 discovery of reflective mirror-based vision in Dolichopteryx longipes by Hans-Joachim Wagner and colleagues was another major revision of vertebrate visual biology [2]. Mirror-based image formation had previously been considered a feature of invertebrates only. Dolichopteryx uses both traditional lens-based vision and specialized diverticula with a reflective crystal layer to form a second, complementary image, effectively giving the fish multiple simultaneous visual channels.

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Notable Research Findings

• Barreleye fish are believed to steal food from siphonophore colonies.
• Multiple functions serve the barreleye's transparent head: it shields the tubular eyes from siphonophore stinging cells that would otherwise touch directly, and it permits the rotated eyes to view in multiple directions unimpeded by external obstruction.
• Barreleye fish have two small pigmented spots on the front of the head that look exactly like eyes but are actually olfactory nostrils.
• Barreleye fish reach only about 15 cm in length as adults. Despite their small size, their tubular eyes are proportionally enormous, each taking up a significant fraction of the head cavity.
• The tubular eye design of barreleyes is optimized for detecting faint silhouettes against the dim downwelling light from above.
• MBARI's continuing ROV work in Monterey Bay has produced additional rare barreleye sightings since the 2008 discovery, confirming that the species is widespread across the North Pacific at mesopelagic depths.

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Illustration.