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Tunicate Facts: Ascidians and Bioluminescent Pyrosomes

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

A photo of a bioluminescent pyrosome taken off the coast of East Timor

A photo of a bioluminescent pyrosome taken off the coast of East Timor

What Are Ascidians and Pyrosomes?

Ascidians and pyrosomes are intriguing marine animals. Adult ascidians have a sac-like body and are sessile. Pyrosomes exist as an elongated colony of attached animals that is mobile and bioluminescent. Some colonies are very large, as shown in the video below. Both creatures have fascinated observers for a long time.

Ascidians and pyrosomes are often known as tunicates. Despite the relatively simple body of ascidians as adults, their mobile larvae have features which show that the animals are related to vertebrates. The interest in pyrosomes has increased due to a mysterious population explosion on the west coast of the United States and Canada. The unexplained pyrosome bloom reached its peak in the summer of 2017.

Encountering a Large Pyrosome

Biological Classification

Ascidians and pyrosomes are classified as follows.

Phylum Chordata

Subphylum Tunicata (also known as Urochordata)

Class Ascidiacea (solitary or colonial animals, sessile as adults)

Class Thaliacea (includes pyrosomes, which are made of individuals joined together and are able to move slowly through the water)

The golden or ink-spot sea squirt (Polycarpa aurata) is an ascidian.

The golden or ink-spot sea squirt (Polycarpa aurata) is an ascidian.

Classification of Tunicates

Chordates and Vertebrates

Like tunicates, we and other vertebrates belong to the phylum Chordata. Vertebrates belong to the subphylum Vertebrata, however, while tunicates belong to the subphylum Tunicata. It's interesting that at least some tunicates have features that show that they are vertebrate relatives, even though they don't look like vertebrates.

Class Ascidiacea

According to our current knowledge, most tunicates are ascidians, or members of the class Ascidiacea. They are sessile animals as adults (unable to move from one place to another) and live on their own or in a colony. They do have moving parts, even though their body doesn't move as a unit, as shown in some of the animals in the video below. Their larvae have some resemblance to tadpoles, which are vertebrates.

Class Thaliacea

Pyrosomes belong to the class Thaliacea and are related to ascidians. They are colonial animals, and the colony moves through the water. Pyrosomes are a relatively poorly studied group. Though they seem to have multiple features in common with members of the class Ascidiacea, they may have more differences than we realize.

Internal anatomy of an adult ascidian (simplified)

Internal anatomy of an adult ascidian (simplified)

Introduction to Ascidians

Ascidians are frequently known as sea squirts. When one of the adults is touched, it often contracts, squirting out seawater in the process. The sac-like body animal is covered by a firm but flexible layer known as a tunic. The tunic is unusual because it contains a chemical resembling cellulose, which is a molecule in plant cell walls.

Ascidians have an interesting anatomy, as shown in the illustration above. The animals are filter feeders. Seawater enters the adult tunicate body through a tube known as an incurrent or brachial siphon. The animals filter plankton out of the water, which also supplies them with oxygen. Water filled with waste leaves the animal via an excurrent or atrial siphon.

Unfortunately, the terminology with respect to the body of an ascidian can be confusing because there are multiple names for the body parts. The branchial siphon is also known as the oral, buccal, or incurrent siphon, for example. The pharyngeal basket is also known as the branchial basket. The slits on the basket are sometimes known as gill slits.

Physiology of an Ascidian

Ingestion and Digestion

An ascidian feeds on the tiny plants and animals found in seawater and collectively known as plankton. Seawater enters the branchial siphon of an ascidian and travels into the sieve-like pharyngeal basket, where food is trapped. The plankton is trapped by the mucus made by the endostyle in the pharyngeal basket. It's then transported to the stomach and moved from there to the intestine, where digestion occurs.

Elimination and Excretion

After digestion has been completed and nutrients extracted from the food, feces leaves the ascidian's body through the atrial or excurrent siphon.

Excretion is the release of waste substances made during the processing of food molecules produced by digestion. Ascidians seem to have no specialized excretory structure, though it may not have been recognized yet. Specific waste substances are made, however, and they are released from the body into the water.

Circulatory and Respiratory Systems

The animal has a heart, which periodically reverses the direction in which it pumps blood. The circulatory system is made of the heart, blood vessels, and spaces called sinuses. Oxygen from the incoming seawater is absorbed by blood vessels in the branchial basket. Carbon dioxide waste made by the animal is released through the excurrent siphon.

Nervous and Reproductive Systems

A cerebral ganglion is located between the two siphons and plays the role of a very simple brain. Nerves are connected to the brain and conduct impulses to other parts of the body.

The animal has both male and female reproductive organs (a testis or testicle and an ovary) and is therefore a hermaphrodite. A sperm of one individual and an egg from another one join in sea water and become a larva.

The Ascidian Larva

The larva of an ascidian looks somewhat like a tadpole. It's sometimes referred to as the "ascidian tadpole larva", though it's not an amphibian like true tadpoles. It has features identical or similar to those of vertebrates, however, including:

  • a dorsal nerve cord along its back
  • a flexible rod under the nerve cord called a notochord (which is present in human embryos but is soon replaced by the spine)
  • a cerebral vesicle, which resembles the area where the vertebrate brain develops
  • an eyespot or ocellus in the cerebral vesicle, which detects light and has similarities to the vertebrate eye
  • a statocyst in the cerebral vesicle, which is used for balance and orientation with respect to gravity; vertebrates have a similar structure called an otolith in their inner ear

The ascidian larva maintains its form for a maximum of only a few days. It has no mouth and doesn't feed. Its purpose seems to be to find a suitable habitat for the adult form. The larva sticks to a rock, shell, or other solid surface head first. It then digests its tail and other structures (including the ones that are similar to those of vertebrates) and makes new structures to form the adult body.

The regeneration abilities of the larva are very impressive. They may help researchers to understand and even improve regeneration in the human body.

Pyrosoma atlanticum in a tidal pool in California

Pyrosoma atlanticum in a tidal pool in California

Pyrosomes are sometimes called salps or comb jellies, but these terms are incorrect. Salps are gelatinous tunicates in the same class as pyrosomes. They start their lives as separate organisms but later link together to form chains. Comb jellies are gelatinous animals that belong to the phylum Ctenophora. They have rows of cilia that are used for swimming.

The Pyrosome Colony

Pyrosomes are still mysterious entities. There is a lot that is unknown and puzzling about their biology. Some facts have been discovered, however.

The individual animals in a pyrosome are known as zooids. They are tunicates but are very small in size. The colony generally resembles a thimble in shape. The one in the photo at the start of this article is about a centimetre long. Some colonies are much longer than an adult human and have an opening large enough for a person to enter. There may be hundreds, thousands, or even hundreds of thousands of zooids in a particular colony.

The zooids are connected by tissue. Some form of communication between them exists because they can coordinate their behaviour. When one zooid emits light in bioluminescence, they all do, for example.

Though pyrosomes are sometimes said to drift through the ocean and to be carried by currents, they do have a weak power of propulsion. The incurrent opening of the zooids faces the ocean, but the excurrent opening faces the cavity inside the "thimble". When the zooids release water after extracting food and oxygen, it flows out of the opening of the pyrosome. This produces a slow form of jet propulsion.

The zooids reproduce asexually to produce identical zooids that enlarge the colony. They reproduce sexually to produce a group of cells that gives rise to a new colony.

Pyrosome is derived from the Greek pyro for “fire” and soma for “body.”

— Newsweek

Bioluminescence in Pyrosomes

The bioluminescence of pyrosomes is unusual compared to that shown by other animals. The blue-green light is often sustained instead of being emitted in pulses. Due to the small amount of research about pyrosomes, the scientific paper that is often quoted in reference to their bioluminescence was published long ago in 1990. The authors reference even older research in their paper. A more recent publication is shown in the "References" section below. Still, more discoveries are needed. The information obtained so far may well be correct, but it would be nice to have additional studies to confirm this.

According to some research, the zooid has two light organs, one on either side of the incurrent siphon. The organs are reportedly triggered by touch or—unusually for bioluminescent animals—by light.

In many other bioluminescent animals, the light is known to be emitted when an enzyme called luciferase acts on a protein named luciferin. Bacteria live in the light organs of some species and are responsible for this reaction. Bacteria have been found in the light organs of pyrosome zooids, and luciferase has been found in their bodies. It hasn't yet been proven that the bacteria are making the luciferase or are responsible for the light production, though this is a strong possibility.

The woman in the video below talks about the "mouth" of the creature that she's found. She later discovered that the entity was a pyrosome and that the opening wasn't a mouth.

A Pyrosome Bloom

There were reports of some pyrosomes in 2014, and a few more in 2015 but this year there has been an unprecedented, insane amount.

— Olivia Blondheim, University of Oregon, in 2017

A Population Explosion

The unexplained population explosion of pyrosomes off the west coast of North America in 2017 is puzzling. The entities were discovered in California, Oregon, Washington, British Columbia, and even Alaska. Their population was sometimes so dense that commercial fishing wasn't possible.

The bloom primarily consisted of a species known as Pyrosoma atlanticum. (The pyrosome is given a scientific name as though it were an individual even though it's really a colony of animals.) The physical appearance of the pyrosome is shown in the video above and the photo below. Its length ranges from 5 cm all the way up to 60 cm. Its body is pale orange, pink, or blue-pink in colour. It has been described as having a "pimply" appearance. It dries out and becomes flat if it's out of the water for too long. The species is sometimes known as a sea pickle.

Pyrosoma atlanticum is usually found in warmer water than that found off the British Columbia coast. A scientist at the Institute of Ocean Sciences in Sidney, British Columbia, suspects that the creatures became stuck in an unusually warm current that developed in the eastern Pacific between 2014 and 2016. In May 2017, a research team from Oregon collected 60,000 pyrosomes after only five minutes of trawling with a net. The pyrosomes filled fishing nets, stopping other creatures from being caught.

A major concern related to pyrosome blooms is that the zooids eat the zooplankton (tiny animals) that are eaten by other creatures. These creatures include shrimp, crabs, and the crustaceans that are an important food source for fish and seabirds. Yet another potential problem is that if an environmental change causes all of the pyrosomes created in a bloom to die at around the same time, their decomposing bodies might create serious effects for the ecosystem.

Pyrosomes are generally found in the upper layers of the water column but may occasionally be found lower down.

Learning More About Both Creatures

At the moment, pyrosomes aren't considered to be an invasive species in British Columbia. The bloom that peaked in 2017 seems to be over. If another one occurs, however, the status of the entities may change.

Pyrosomes are fascinating and intriguing. It would be very interesting to know more about how the zooids in a colony communicate with each other and about how they coordinate their behaviour. It would also be interesting to know exactly why their population explodes and what the consequences of this explosion might be. Learning more about the biology and ecology of pyrosomes could be useful if another bloom occurs.

Ascidians are also interesting creatures that are worth exploring further. I find it fascinating that animals that are so different from us in their adult form and that seem to be comparatively primitive have larval features that are similar to ours. Solving the mysteries of tunicate existence would probably be a great addition to our knowledge about life on Earth.


  • Tunicate information from Smithsonian Ocean
  • Tunicate and pyrosome facts from ScienceDirect
  • Similarity of the ascidian tadpole larva ocellus or eyespot to the vertebrate eye: An abstract from the NIH (National Institute of Health)
  • Facts about pyrosome bioluminescence from the Nature journal
  • Millions of pyrosomes appear on the coast of British Columbia—an article from the CBC (Canadian Broadcasting Corporation)
  • "Mysterious Sea Pickles Invading West Coast in Bizarre Bloom" from Newsweek

© 2017 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on April 25, 2020:

I agree, Peggy. There are probably undiscovered facts about marine life that are more significant than what we already know. The ocean is fascinating.

Peggy Woods from Houston, Texas on April 25, 2020:

It is amazing what we keep learning about marine life, and yet, what we do not yet know is probably even more significant. Thanks for the education about pyrosomes. I hope that they do not become overpopulated and interrupt the natural order of things along the west coast. Those huge long tubes of them were so fascinating to view in the videos.

Linda Crampton (author) from British Columbia, Canada on February 01, 2018:

Hi, Val. Thanks for the comment. Nature can certainly be mysterious as well as impressive. It gives us a lot to think about.

ValKaras on February 01, 2018:

Linda---What a fascinating article! As a qigong practitioner and believer in energy medicine, I keep wondering about all strange aspects of bio-energy, and that started first with eels and their generating electricity.

Then I watched a TV documentary with exactly the same kind of mysterious marine creatures emitting light that you are describing.

I don't know about you, but every time I think of all these wonders of nature, I get this silly feeling that "something" is mocking the human intellect's limitations.

Thank you for sharing it, my friend.

Linda Crampton (author) from British Columbia, Canada on December 31, 2017:

Hi, Colin. I agree. It does make the mind boggle! Nature is fascinating.

colin powell from march on December 31, 2017:

It makes the mind boggle. How much we still need to learn about life in our seas. It seems that we are discovering new life forms on a regular basis.

Linda Crampton (author) from British Columbia, Canada on December 06, 2017:

That sounds like a beautiful experience, Dale. Thanks for sharing it.

Dale Anderson from The High Seas on December 06, 2017:

Hey interesting stuff. I kayak at night and often see twin vortexes of swirling, bioluminescent trails curling off of each paddle as I stroke my way over the water. Prettiest thing you can imagine.

Linda Crampton (author) from British Columbia, Canada on November 16, 2017:

Thanks, Paul. No, I don't contribute to Wikipedia.

Paul Venter on November 16, 2017:

Need your skills on Wikipedia.....or do you already contribute?

Lovely bit of reading.....

Linda Crampton (author) from British Columbia, Canada on October 18, 2017:

Thank you for the comment, Peg. I appreciate the share very much, too. Pyrosomes are certainly interesting. Their population explosion is puzzling.

Peg Cole from North Dallas, Texas on October 18, 2017:

This is fascinating, in particular, the sea pickles showing up on that Oregon beach. Wonder what's up with that? And the photos and diagrams are amazing as well. These pyrosome creatures are strange but also fascinating. The videos were incredible; that weird tube thing, wow. Sharing this.

Linda Crampton (author) from British Columbia, Canada on September 13, 2017:

Thank you very much for the kind and lovely comment, Rachel. I'm looking forward to reading more of your articles.

Blessings to you as well.

Rachel L Alba from Every Day Cooking and Baking on September 13, 2017:

Hello Linda friend, I haven't been visiting too much but missed your hubs so I checked them out. I am absolutely fascinated with sea creatures and was so happy to see your article on these above. I love the videos and I love seeing diagrams of the insides of them. Thank you for all your work on this hub.

Blessings to you.

Linda Crampton (author) from British Columbia, Canada on September 09, 2017:

I appreciate all your comments, Devika. Thank you very much for the visits.

DDE on September 09, 2017:

Informative and you definitely share well-presented titles and with great research.

Linda Crampton (author) from British Columbia, Canada on August 22, 2017:

Thank you very much for the kind comment, Alun. I appreciate your visit a great deal.

Greensleeves Hubs from Essex, UK on August 22, 2017:

I vaguely remember Tunicates from school or university biology lectures, but the detail learned about them was exactly that - vague. They were obscure sea creatures I was never likely to encounter, and so were rapidly forgotten after finishing college.

I can't however say that I have ever heard the name 'pyrosome' but they do seem an interesting and extraordinary looking group, and I'm sure the bioluminescence can make them spectacularly beautiful - the one in the opening photo looks like a glittering jewel.

Just googled pyrosomes - unsurprisingly Linda, your article is better than any on the first page of the Google Search. Alun

Linda Crampton (author) from British Columbia, Canada on July 28, 2017:

Hi, Jackie. The colours of some of the pyrosomes are lovely, though their local population explosion isn't! Ocean life is very interesting. I appreciate your visit.

Jackie Lynnley from the beautiful south on July 28, 2017:

So interesting and loved the Pyrosomes. Pink and blue, wow. Scary places with such beautiful sea life. Thank you for sharing this!

Linda Crampton (author) from British Columbia, Canada on July 10, 2017:

Hi, Cynthia. Synchronicity is always an interesting experience! Thank you for commenting.

Cynthia Zirkwitz from Vancouver Island, Canada on July 10, 2017:

Wow, Linda, a little synchronicity is going on here-- we were driving home the other night alongside the Comox Harbour, admiring the full moon, when the topic of bioluminescence came up. I can now discuss this a little more knowledgeably, thanks to your well-written article.

Linda Crampton (author) from British Columbia, Canada on July 10, 2017:

Thank you for such a nice comment, Chitrangada Sharan. I appreciate your visit.

Chitrangada Sharan from New Delhi, India on July 10, 2017:

Such an informative, educational and interesting article! I thoroughly enjoyed going through the amazing information you provided about Pyrosomes.

There is so much to know and wonder about in the ocean World. Most of us can only read about it .

Thanks for sharing!

Linda Crampton (author) from British Columbia, Canada on July 08, 2017:

Hi, Peggy. I hope that the pyrosome population explosion is an aberration, too. It could be a serious problem if it isn't! Thanks for the visit.

Peggy Woods on July 08, 2017:

How interesting! I had not previously known anything about Pyrosomes. Hopefully it is an aberration that they are clogging up fishing nets and interfering with the fishing industry right now. Our world is truly an amazing place!

Linda Crampton (author) from British Columbia, Canada on July 08, 2017:

Hi, Martie. Yes, some very unusual creatures share the Earth with us! They are fascinating to study. Thanks for the comment.

Martie Coetser from South Africa on July 08, 2017:

How fascinating! These mysterious bioluminescent marine animals are beyond my comprehension. They all seem to be alien, and yet we're sharing a planet with them.

Thanks, Linda, for researching and sharing these amazing hubs about weird creatures :)

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Thank you so much, John. I appreciate your kindness.

John Hansen from Australia (Gondwana Land) on July 06, 2017:

This was a totally fascinating hub and as always informs and educates. Your articles never disappoint, Linda.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Hi, Flourish. Yes, I agree—there is a lot that other organisms can teach us. A balance between different species is important, as you say. Thanks for the visit.

FlourishAnyway from USA on July 06, 2017:

We have so much to learn from the creatures around us! They have amped up survival of the fittest on so many levels. We should be careful of their ability to rapidly populate waters and crush out competing organisms.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Thanks for the interesting comment, Dugitron9001.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Hi, Heidi. I think you've raised an important point. It is fascinating to learn more about space, but we do need to learn more about our own planet as well. Cheers to you as well!

Dugitron9001 on July 06, 2017:

Puzzling to human understanding only.

Heidi Thorne from Chicago Area on July 06, 2017:

While I'm a strong proponent of space exploration, we still need to know so much more about the planet we inhabit! This is certainly an example of why. Thanks so much for always piquing our curiosity with your science posts! Cheers!

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Thank you very much for the comment, Dora. There's a lot to explore in the ocean. It's an amazing place.

Dora Weithers from The Caribbean on July 06, 2017:

Fascinating indeed! A heart that pumps blood in the reverse and having two genders? Where have they been? Glad they are not harmful to people, although they're still a nuisance. Linda, you do have some interesting stories to tell.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Thank you for the kind comment, Bill. I appreciate your visit very much.

Bill Holland from Olympia, WA on July 06, 2017:

Such a fascinating world we live in. This was such an enjoyable article for a science geek like me. Thanks for the great information, Linda!

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Hi, Jill. Yes, there's lots to admire in tunicates! I appreciate your visit and comment.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

Thank you very much, Hari.

Linda Crampton (author) from British Columbia, Canada on July 06, 2017:

I agree, Louise. There are so many wonders in the ocean. It's a fascinating place to explore.

Jill Spencer from United States on July 06, 2017:

A fascinating article! Tunicates were designed for survival, huh? A reversible heart AND both sets of reproductive organs. You have to admire that.

Hari Prasad S from Bangalore on July 06, 2017:

Comprehensive and very informative. Thanks Linda.

- Hari

Louise Powles from Norfolk, England on July 06, 2017:

There certainly is a lot of wonders in the ocean. The videos of the pyrosome is amazing. There is so much in the ocean we don't know about.