In 2009, Monterey Bay scientists used live underwater footage to expose a surprising secret of a deep-sea fish. 13 years later, their ROVs had logged 5,600 dives, helping reveal why the ocean's strangest creatures can look radically different after collection

In the darkness of the deep Pacific, life can look almost impossible. Some animals produce their own light, others have transparent bodies or unusual sensory systems, while some are so gelatinous that they are mostly water. In 2009, Monterey Bay scientists used live underwater footage to reveal a surprising secret about the barreleye fish. Years later, their remotely operated vehicles had logged thousands of dives, helping show why live observations can reveal details that preserved specimens simply cannot, as per a report by Eco Portal.

An estimated one million deep-sea species remain undiscovered, making the deep ocean one of the least explored environments on Earth.

What did scientists discover about the barreleye fish?

For years, researchers thought they understood the barreleye fish. That changed when the Monterey Bay Aquarium Research Institute observed one alive underwater.

The fish has an entirely transparent, fluid-filled head. Inside are two bright green eyes that can rotate within the skull. They point upward when the fish is watching for prey against the faint light filtering down from above, then turn forward when it feeds.

The surprise came because preserved specimens had given scientists an incomplete picture. Their transparent tissue disappeared during collection, leaving researchers with an altered version of the animal.

The live encounter showed just how easily studying deep-sea creatures outside their natural environment can lead to the wrong impression.

The barreleye is only one example of the extraordinary animals living in the darkness.

How do deep-sea creatures survive without sunlight?

For some animals, the answer is to create their own light.

The vampire squid has light-producing organs called photophores, especially around its arm tips. When threatened, it can release flashes of bioluminescence that may confuse or distract predators.

Despite its dramatic name, the vampire squid does not hunt for blood. Instead, it remains suspended in the water, matching the density of seawater around it, and gathers drifting organic particles known as “marine snow” with long, filament-like appendages.

The anglerfish takes a completely different approach. It carries a glowing lure above its mouth, using light to draw prey close enough to capture with its needle-like teeth.

In a world without sunlight, producing light can make the difference between finding food and going hungry.

Why do some deep-sea animals look so strange?

The goblin shark is another example of how extreme environments can produce unusual anatomy.

Its skin is semi-translucent, allowing blood vessels to influence its pale white or pink appearance. It also has a long, flattened snout and jaws that can rapidly shoot forward to seize prey before retracting.

The giant isopod looks even more unusual. Resembling an enormous roly-poly, it can live as deep as 7,000 feet and survive for years without food. It scavenges whatever reaches the ocean floor and has a heavily segmented, armored body.

Its basic body design has changed remarkably little since appearing in the fossil record more than 300 million years ago. Rather than suggesting that evolution stopped, this stability shows how well a particular design can work in an environment.

The dumbo octopus has adapted in its own distinctive way. Living between 1,000 and 13,000 feet below the surface, it moves through the darkness by flapping ear-like fins. Unlike shallower octopuses, it has no ink sac, since predator pressure at those depths is low enough that the defense became unnecessary.

Could the simplest animals be the toughest?

Some of the strangest deep-sea creatures barely resemble conventional animals at all.

Gelatinous organisms, including sea jellies, can consist of at least 95 percent water. They have no blood, heart or brain, yet some produce neon-colored bioluminescence underwater. Others trail extraordinarily long tentacles through the darkness.

Sea jellies have existed for at least 500 million years, predating much of complex animal life. Their simplicity has not prevented them from surviving. Instead, it has proven to be an enduring strategy.

The deep ocean is filled with such examples of life being shaped by its surroundings.

Why is the deep ocean still so mysterious?

The deep ocean is Earth's largest habitat, but much of it remains unexplored. An estimated one million species living there have yet to be discovered, while many known animals have been observed only a handful of times.

The region also plays an important role in regulating climate, storing carbon and supporting biodiversity.

Ocean Conservancy argues that protecting the deep sea begins with recognizing how much remains unknown.

The barreleye fish makes that uncertainty especially clear. Scientists had already catalogued the species, yet a live underwater encounter revealed that their understanding of its appearance and anatomy was incomplete.

The same problem can apply to other deep-sea animals. What researchers see after an animal has been collected may not always be what the creature actually looks like in the darkness where it evolved.

The 2009 Monterey Bay observation therefore offered more than a remarkable glimpse of a strange fish. It showed how much can change when deep-sea life is observed where it actually belongs.

With ROVs logging thousands of dives and so many species still undiscovered, the darkness beneath the Pacific continues to hold plenty of surprises.

FAQs

Why is the barreleye fish unusual?
Its transparent head contains rotating green eyes.

How much of the deep ocean remains unknown?
An estimated one million species remain undiscovered.