Pyrosomes: Strange and Bioluminescent Creatures in the Ocean
What Is a Pyrosome?
A pyrosome is a strange, gelatinous, and bioluminescent creature that is found in the ocean. It's actually a colony of marine animals known as tunicates. Pyrosomes have fascinated observers for a long time. The interest in the creatures has recently 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.
Tunicates are sac-like marine invertebrates. In free-living tunicates, the sac has two tubes at the top through which water enters and leaves the animal. The animal filters plankton out of the water, which also supplies it with oxygen.
Despite their relatively simple body as adults, tunicates have features which show that they are related to vertebrates. The individual tunicates in a pyrosome can be seen in the photo above. A pyrosome colony ranges from around a centimetre in length to ten metres long.
Pyrosomes are sometimes referred to as comb jellies or salps, but neither of these terms is correct. Comb jellies are gelatinous animals which have rows of cilia (the "combs") that are used for swimming. Salps are gelatinous tunicates that start their lives as free living organisms but later link together to form chains.
A Beautiful Example of a Pyrosome
The free-living tunicates discussed in this article are ascidians (members of the class Ascidiacea). They are the most common type of tunicates. Pyrosomes are thought to be related to ascidians.
What Are Tunicates?
The tunicates that make up a pyrosome colony belong to the phylum Chordata, just as vertebrates do. Vertebrates belong to the subphylum Vertebrata, however, while tunicates belong to the subphylum Tunicata (or Urochordata).
Tunicates are frequently known as sea squirts. When a tunicate is touched, it often contracts, squirting out seawater in the process. The sac-like body of the animal is covered by a firm but flexible layer known as a tunic. The tunic is unusual because it contains cellulose, which is a molecule in plant cell walls. Tunicates are sessile, or attached to a surface and unable to move from place to place.
Internal Anatomy and Physiology
Tunicates are filter feeders. Seawater enters the branchial siphon of an ascidian tunicate and travels into the sieve-like branchial basket, where food is trapped. The terminology can be confusing because there are multiple names for the body parts. The branchial siphon is also known as the oral, bucal, or incurrent siphon. The branchial basket is also known as the pharyngeal basket. The slits on the basket are sometimes known as gill slits.
The tunicate feeds on the tiny plants and animals found in seawater and collectively known as plankton. The plankton is trapped by the mucus made by the endostyle in the branchial basket. It's then transported to the stomach and moved from there to the intestine. After digestion has been completed and nutrients extracted from the food, feces leaves the tunicate's body through the atrial or excurrent siphon.
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.
A cerebral ganglion is located between the siphons and plays the role of a very simple brain. The animal has a heart, which periodically reverses the direction in which it pumps blood. It also has both male and female reproductive organs and is therefore a hermaphrodite.
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 eventually 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 animal are impressive. They may help researchers to understand and even improve regeneration in the human body.
Although the video below uses the word "worm" in its title, it's actually showing pyrosomes.
The Pyrosome Colony
Pyrosomes are still mysterious creatures. 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, 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
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 lack 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. The information may well be correct, but it would be nice to have additional and more recent studies to confirm it.
According to the 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 some light organs 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, however.
The woman in the video below talks about the "mouth" of the creature that she's found. She later discovered that the creature was a pyrosome and that the opening wasn't a mouth.
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 creatures 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 Pyrosomes
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 creatures may change.
Pyrosomes are fascinating and intriguing creatures. 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.
We need to learn more about the biology and ecology of the creatures in case another bloom occurs. Solving the mysteries of their existence would probably be a great addition to our knowledge about life on Earth.
- Tunicate and pyrosome information from ScienceDirect
- Similarity of the ascidian tadpole larva ocellus or eyespot to the vertebrate eye from the NIH (National Institute of Health)
- Pyrosome bloom facts and photos from National Geographic
- Millions of pyrosomes appear on the coast of British Columbia—an article from the CBC (Canadian Broadcasting Corporation)
© 2017 Linda Crampton