Linda Crampton is a writer and former science teacher with an honors degree in biology. She enjoys writing about science and nature.
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 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. 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. Other fish with similar features belong to the animal’s family and are also known as barreleyes. 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 the siphonophores.
The colours seen in the video above are due to light emitted from a remotely operated vehicle that has been reflected by the fish. Some species in the barreleye fish family are weakly bioluminescent, however. The two dark, eye-like structures visible at the front of the animal’s head are its nares, or nostrils.
Barreleyes: Classification, Habitat, and Vision
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 (including the species in the illustration above) have a light organ on their undersurface. The organ contains bioluminescent bacteria. I’ve seen no reference indicating that Macropinna microstoma has the organ, but it’s possible that it hasn’t yet been discovered.
Habitat of the Fish
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.
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.
Eyes and Vision
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 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
Feeding, Respiration, and Reproduction
The fish spend much of their time in deep water where there is no or very little light. Their eyes are still useful, however. The fish is often filmed stationary in the water, with its big 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 and 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 nares or nostrils of the barreleye fish are used for detecting odours. They aren’t used for breathing. Like other fish, the barreleye fish 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 with 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 species and about other aspects of its life soon.
The round structure at the 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.
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.
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 zooids (individual polyps and medusae) 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.
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 makes them the longest “animals” on the planet.
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.
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)
More Facts About Siphonophore Colonies
The different pattern of cell division in a siphonophore and a human may help people see why scientists say that the first organism is a colony of organisms while we are a single one.
An egg and a sperm from a siphonophore join to make a single-called zygote. All of the cells in the colony are derived from genetically-identical buds from this zygote. Budding is a type of asexual reproduction. A bud uses the appropriate genes to produce suitable characteristics for its job. Once the zooids of a siphonophore have been produced, they remain attached to the other zooids and indirectly to the cell that produced them.
Construction of a Human
The pattern in humans and many other animals is different. An egg and sperm join to form a single-celled zygote, as in siphonophores. The zygote then divides into two cells. Unlike the case in siphonophores, each of these cells then divides. The division process of each cell repeats, and specialization of cells occurs as the body is created.
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, some siphonophores create new types of individuals in a reiterating sequence. The scientist has made the following statement.
“This pattern is usually reiterated along a linear stem, with the exact same sequence of specialized zooids occurring over and over.”
Professor Dunn also says that siphonophores are “a joy to work with” and that many questions about them are “wide open.“ They are an intriguing group of animals. Studying them might reveal significant information with respect to our cells.
The Portuguese man o’war (also called the Portuguese man of war and Physalia physalis) is a siphonophore colony and not a jellyfish. It contains zooids adapted for multiple functions. The colony is found at the water surface. Its stinging cells enable it to catch fish. They can also hurt humans, even as long as weeks after a dead man o’war has washed ashore.
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
- Spook fish information from the National Oceanography Centre in the United Kingdom (Scroll down the page to see the relevant information.)
- “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 cell.com
© 2021 Linda Crampton