Why Does This Fish Have a See Through Head?
The short answer
Because its real eyes are inside it, looking out. The barreleye's eyes are two glowing green barrels under a clear, fluid filled shield on top of its head. They look up through it for the shadows of small animals, 600 to 800 meters down. Scientists think the shield may also protect those eyes from stings.
The two dark spots on its face that look like eyes are its nostrils.
Looking up from the dark
The barreleye (Macropinna microstoma) is a fish no longer than your hand. It lives about 600 to 800 meters down in the North Pacific, where almost no sunlight is left. Its tube shaped eyes usually point straight up through the shield. They look for the faint shadows of small animals against the last light from above.
Its big flat fins let it hang almost motionless. When it spots food, it tilts up and its eyes rotate forward inside the shield, so it can watch what it is eating.
Why a shield at all? Scientists think the barreleye may steal food caught by siphonophores. These are long jelly colonies that trail stinging tentacles. The idea is that the fish picks off the catch and the shield keeps the stings out of its eyes. It is a hypothesis, not a settled fact.
Why the old drawings left it out
The fish was named in 1939 by Chapman, from specimens caught in nets. Chapman saw that the front of the head was transparent. What went undescribed was the fluid filled shield over the eyes. It was fragile and was probably destroyed on the way up. So for about 70 years, drawings showed the fish without it.
That changed when scientists at MBARI filmed living barreleyes with robot subs. Bruce Robison and Kim Reisenbichler described the shield and the rotating eyes in 2008. MBARI also kept a net caught barreleye alive in a tank on its ship for several hours and watched its eyes rotate.
It is still rarely seen. By 2021, MBARI's robots had filmed it only 9 times in more than 27,600 hours of video. That works out to roughly one sighting for every 3,000 hours.
Key numbers
| What | Number | Source |
|---|---|---|
| Depth where the barreleye lives | about 600 to 800 m | MBARI, 2009 |
| Times MBARI's robot subs had seen it, as of 2021 | 9 | MBARI, barreleye sighting video, 2021 |
| Hours of MBARI robot video those sightings came from | more than 27,600 hours | MBARI, barreleye sighting video, 2021 |
| Video per sighting | about 3,000 hours | Our arithmetic from MBARI, barreleye sighting video, 2021 |
| Year the barreleye was named | 1939 | Chapman, Proceedings of the U.S. National Museum, 1939 |
| Year the shield and rotating eyes were described | 2008 | Robison and Reisenbichler, Copeia, 2008 |
| Years drawings showed the fish without its shield | about 70 (1939 to 2008) | Our arithmetic from Chapman, Proceedings of the U.S. National Museum, 1939 and Robison and Reisenbichler, Copeia, 2008 |
| How long a net caught barreleye was kept alive in a ship's tank | several hours | MBARI, 2009 |
What the film simplifies
- The film says the barreleye lives 600 meters down. That is the shallow end of its depth range, which runs from about 600 to 800 meters.
- The 9 sightings in 27,600 hours is MBARI's own count from 2021, from one institute's robot subs. The film says it as if it were today's count.
- Stealing from siphonophores is an idea scientists have proposed, not a proven fact. The heist and the siphonophore's complaints are cartoon.
- The film says science drew the fish without its shield. Chapman did note in 1939 that the front of the head was transparent. What went undescribed was the fluid filled shield over the eyes.
- The barreleye's eyelids, voice and attitude are drawn for the story, and so are the siphonophore's face and eye.
At a glance
Runtime 1 minute 5 seconds · Topics: Deep sea animals, Eyes and vision, How science gets corrected · Captions on YouTube · Full script below
Test yourself
The barreleye seems to have two dark eyes on its face. What are those spots really, and where are its real eyes?
Answer
The spots are its nostrils. Its real eyes are the two green barrels inside the clear shield on top of its head, usually pointing up.
MBARI's robots saw the barreleye 9 times in more than 27,600 hours of video. About how many hours of video is that per sighting?
Answer
About 3,000 hours, since 27,600 divided by 9 is about 3,067.
The barreleye was named in 1939, but its shield was not described until 2008. Why did it take so long?
Answer
The first specimens came up in nets, which probably destroyed the fragile shield. Only when robot subs filmed living barreleyes could scientists see the shield whole.
The script
Host: Those aren't its eyes.
Barreleye: Up here.
Host: Those are its nostrils! Its real eyes are those green barrels, under a see through shield.
Host: Meet the barreleye. One lab's robots spotted it just 9 times in 27,600 hours!
Host: It lives 600 meters down, where the light is almost gone, so its eyes point up...
Host: ...to spot the shadows of tiny animals.
Barreleye: Snack. Snack. Not a snack.
Host: When it spots dinner, it tips up and its eyes roll forward to watch.
Host: But why the shield? Scientists think it may be for stealing.
Host: From that: a siphonophore, a jelly colony with thousands of stinging tentacles.
Siphonophore: Hey! Those are ours!
Host: The idea: it picks off the catch, and the shield keeps stings out of its eyes.
Barreleye: Safety glasses.
Siphonophore: Every single time.
Host: For about 70 years, science drew this fish without its shield.
Host: Barreleyes came up in nets that probably wrecked it, so the drawings looked like this.
Barreleye: I look like a tugboat.
Host: Then robot subs filmed it alive, shield and all!
Barreleye: Finally. My good side.
Barreleye: Now quit staring at my nose.
Sources
- The eyes sit in a transparent, fluid filled shield and rotate forward to look at food; barreleyes hover nearly motionless at 600 to 800 m; the shield was missing from earlier descriptions, likely destroyed by nets; the siphonophore food theft idea; a net caught fish kept alive for several hours in a ship's tank.MBARI, 2009
- The paper that described the shield and the rotating tubular eyes.Robison and Reisenbichler, Copeia, 2008
- MBARI's robots had seen the barreleye only 9 times in more than 27,600 hours of video, as of 2021.MBARI, barreleye sighting video, 2021
- The original 1939 description and drawing of Macropinna microstoma, which notes that the front of the head is transparent.Chapman, Proceedings of the U.S. National Museum, 1939