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Facts About Bizarre Barreleye Fish and Strange Siphonophores

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

A species of barreleye fish (Opisthoproctus soleatus)

A species of barreleye fish (Opisthoproctus soleatus)

Intriguing Marine Animals

The barreleye fish is a bizarre animal that lives in the deep ocean. It has a transparent head. Its eyes are tubular and mobile, and they have bulbous green lenses. They are positioned on the top of the head under the transparent covering instead of on the sides of the body as in other fish. They can be seen in the preserved animal above and in the living animal in the video screen below.

A major component of the Californian barreleye fish’s diet is siphonophores, which are also strange and impressive creatures. It has been postulated that in addition to eating parts of the siphonophores, the fish sometimes picks small crustaceans off their bodies with its narrow and pointed mouth. Siphonophores are related to jellyfish but have some distinct features.

The scientific name of the barreleye fish that has been observed in California is Macropinna microstoma. In this article, I focus on the Californian species. I also discuss the features of siphonophores. Technically, they are colonies of individuals rather than a single animal. There is still much to discover about both the fish and their siphonophore prey.

The colours seen in the video above are due to light emitted from a remotely operated vehicle that has been reflected by the fish. The two dark, eye-like structures visible at the front of the animal’s head are actually its nares, or nostrils.

Californian Barreleye Classification and Habitat

Biological Classification

Macropinna microstoma belongs to the class Actinopteri (the ray-finned fish), the order Argentiniformes, and the family Opisthoproctidae. The family contains multiple barreleye species, which are also known as spook fish. Researchers have discovered that some members of the family are bioluminescent, though the mechanism by which they produce light hasn't been determined. I’ve seen no reference indicating that Macropinna microstoma is bioluminescent, but it’s possible that this feature hasn’t yet been discovered in the animal.

Habitat of the Californian Barreleye

The Californian barreleye fish lives at a water depth of 2,000 to 2,600 feet. It’s found on its own and appears to be a solitary animal. The close-up views of the fish are fascinating and can be seen in the video above. The animal is actually less than six inches in length but has relatively large fins. The species lives in the North Pacific Ocean and perhaps in other areas.

Interesting discoveries have been made about the fish by means of remotely operated vehicles (ROV). The vehicles were deployed by MBARI, or the Monterey Bay Research Institute. They enable investigators to explore areas that are too deep for divers.

When an animal that lives in deep water under high pressure is pulled out of the ocean, it may not be able to survive at the lower pressure and its appearance may be distorted. The most humane way to study the animals is in their natural habitat. At least one Californian barreleye fish has been brought to the surface after being caught in a net, however. Researchers were able to keep the animal alive in an aquarium for several hours and discovered the movement of the eyes mentioned below.

An illustration of Opisthoproctus grimaldii

An illustration of Opisthoproctus grimaldii

Eyes, Vision, and Catching Food

Despite the fact that the barreleye fish was discovered in 1939, there are still unknown facts about its life. The eyes are protected by the transparent cover around them and the fluid underneath the cover. It was once thought that they were fixed in place and were always directed upwards. Scientists have now discovered that the eyes can rotate within their chamber through an angle of around 75º. This action enables the fish to see what is above it and what is in front of it.

The fish spend much of their time in deep water where there is no or very little light. Their eyes are still useful, however. A barreleye is often filmed stationary in the water, with its big and flat fins helping to stabilize it. In this state, It’s thought to be scanning the water above to find silhouettes of potential prey. Once it finds prey, it moves towards it. The fish also moves its eyes forward so that it can see where to attack the prey. The siphonophores that it approaches for food contains stinging cells, but the transparent shield around the eyes is believed to protect the fish.

The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellies or other animals directly overhead.

— Monterey Bay Research Institute

Respiration and Reproduction

The nares or nostrils of the barreleye fish are used for detecting odours. They aren’t used for breathing. Like other fish, the barreleye breathes via gills, not lungs. The water enters the mouth, flows over the gills, and leaves the body via the opening at the back of the operculum (the bony covering over the gills). Blood vessels in the gills remove oxygen from the water and add carbon dioxide to it.

Not much is known about reproduction in the species. According to an entry in FishBase (a database containing fish information), the barreleye fish is oviparous, or one that lays eggs. After fertilization, an egg hatches into a larva, which eventually becomes an adult, assuming it survives the larval stage. Hopefully, researchers will learn more about the reproduction of the fish and about other aspects of its life soon. It's an intriguing animal.

A siphonophore named Marrus orthocanna

A siphonophore named Marrus orthocanna

The round structure at the left tip of the siphonophore above is a float. The big zooids (components of the siphonophore) next to it are the swimmers that propel the colony. The smaller zooids in the fuzzy area catch food and do other jobs for the colony.

Siphonophore Facts

Biological Classification

Siphonophores belong to the phylum Cnidaria, like jellyfish. They are classified in the class Hydrozoa within the phylum and the order Siphonophorae. Although they have a gelatinous body and are sometimes referred to as “jellies” like the rest of their phylum, they have different features from jellyfish. Like jellyfish, however, most siphonophores spend their entire lives swimming in the ocean. They are rarely found near the shore because the surf damages their delicate body. So far, around 175 species are known.

A Colony and Zooids

To a casual observer, a siphonophore may appear to be a single organism with multiple cell types. Researchers say that a siphonophore is actually a colony containing multiple individuals that are specialized for specific jobs. It’s made of units called zooids joined together. The zooids communicate and support each another. “Zooid” is the spelling used in most of the scientific articles that I’ve read. Some reports spell the word “zoid,“ but others use that spelling to refer to a different structure in biology.

Different members of the siphonophore colony can have different functions. For example, some zooids catch prey, some digest the prey and then share nutrients with the other members of the colony, some are responsible for motion of the colony, and some are specialized for reproduction. Other specialties exist. The division of labour can be impressive. The zooids that catch prey contain specialized stinging capsules called nematocysts. These inject a harpoon-like structure into the prey and either kill it or trap it.

Beautiful and Filamentous Creatures

Many siphonophores are filamentous or at least have filamentous sections, like the ones in the video below. The zooids are attached to a stalk-like structure in the filament. Some siphonophores are longer than 130 feet, which is longer than a blue whale and is sometimes said to make them the longest “animals” on the planet. The comparison isn't really fair, however, since the whale is a single individual and siphonophores are colonies.

Siphonophores can be beautiful and often colourful entities. It’s hard for biologists to study them, however. Many of them live in deep water. In addition, many of the creatures disintegrate when they are pulled out of the water. One species that may be seen more easily is the Portuguese man o'war (Physalia physalis). It's a siphonophore colony and not a jellyfish. It's seen at the surface of the water.

More Facts About Siphonophore Colonies

The different pattern of cell division in a human and a siphonophore may help people see why scientists say that we are a single organism but a siphonophore is a colony of organisms.

Construction of a Human Body

In humans and many other animals, an egg and sperm join to form a single-celled zygote. The zygote then divides into two cells. Each of these cells then divides. The division process of each cell repeats, and specialization of cells occurs as the body is created. Once multipotential hematopoietic stem cells have formed, they produce blood cells and no other type of cell. Once mesenchymal stem cells have formed, they produce bone, cartilage, muscle, and fat cells. Neural stem cells produce cells of the nervous system, and so on.

Construction of a Siphonophore Colony

An egg and a sperm from a siphonophore join to make a single-called zygote, as in us. The cells in the colony are derived from buds from the mature zygote (also called a protozooid) instead of arising from a specialized cell. Budding is a type of asexual reproduction. Each bud uses the appropriate genes to produce suitable characteristics for its job and is then known as a zooid. Once the zooids of a siphonophore have been produced, they remain attached to the other zooids and indirectly to the cell that produced them. A variation of this process exists, as described below.

The complexity of how these bodies are specialized and organized in the colony far exceeds that of any other colonial animal.

— Dr Casey Dunn (with respect to siphonophores)

A Recent Discovery

Casey Dunn is a leading siphonophore researcher in the United States and (according to his LinkedIn profile) is currently a professor at Yale University and an assistant professor at Brown University. He says that a modification to the siphonophore colony production process has recently been discovered.

According to Professor Dunn, most siphonophores create new types of individuals in a reiterating sequence. A bud forms on the mature zygote. This bud then produces a specific sequence of zoids adapted for specific functions. Another bud on the zygote forms an identical sequence of zoids, and so on.

Professor Dunn says that siphonophores are “a joy to work with” and that many questions about them are “wide open.“ Hopefully we will understand more about their creation and behaviour soon. They are an interesting and still somewhat mysterious group of animals.

Fascinating Life Forms in the Ocean

Life on Earth is often fascinating. Life in the deep ocean is still mysterious, though scientists are learning more about it with their remotely operated vehicles. It’s a slow process but a very interesting one. The adaptations of organisms for life and the variety in their characteristics are always impressive.

Studying nature is valuable in order to improve our understanding of living things. It sometimes has an additional benefit. When we study other organisms, we sometimes discover facts related to the human body or facts that can help us in some way. This might be the case in the investigation of the strange deep-sea barreleye and the unusual siphonophore group. I’m looking forward to the discovery of more information about them.


  • Deep-sea fish with tubular eyes from MBARI
  • Barreleye fish observation by MBARI’s ROV Ventana from Live Science
  • Information about Micropinna microstoma by FishBase
  • Light organs in members of the family Opisthoproctidae from EurekAlert, AAAS (American Association for the Advancement of Science)
  • Spook fish information from the National Oceanography Centre in the United Kingdom (Scroll down the page to see the relevant information about Opisthoproctus soleatus.)
  • “Meet the Siphonophore“ from Nautilus Live, Ocean Exploration Trust
  • ”Creature Report: Siphonophores“ from the Schmidt Ocean Institute
  • Information about the giant siphonophore from the Monterey Bay Aquarium
  • A quick guide to siphonophores from Casey Dunn and
  • "What is Portuguese Man o'War?" from NOAA (National Oceanic and Atmospheric Administration)

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Linda Crampton