Pyrosome

Quick Take

• Pyrosomes look like a single organism, but what's actually swimming through the ocean is far stranger than that. Discover their colonial nature →
• Their name literally means 'fire body,' and the reason why becomes one of the most spectacular sights in the open ocean. See their bioluminescence →
• A pyrosome can be small enough to overlook or large enough for a human to swim through, and that enormous size range changes everything about how scientists study them. Explore their size range →
• Pyrosomes reproduce in a way that blurs the line between individual and colony, which is also why catching one intact is nearly impossible. Understand their reproduction →

Pyrosomes are colonial tunicates comprising hundreds or thousands of zooids. Being pelagic, they float freely with the currents in the open ocean, occasionally influencing their movement through their water filtration system. They occur mainly in tropical waters around the world, though they sometimes also occur in colder regions like the Antarctic. They employ both sexual and asexual reproduction to grow their colonies and regenerate themselves. Because they are a network of many individual organisms, they are delicate and often break apart when caught in nets.

5 Pyrosome Facts

• Composed of many tiny organisms: Pyrosomes are not single organisms; rather, they comprise hundreds to thousands of tiny organisms called zooids.
• Bioluminescent: These fascinating organisms are bioluminescent, often putting on stunning light displays near the surface.
• Tubular: Pyrosomes resemble gelatinous tubes in the water. Because their sizes vary greatly, swimmers are often able to swim from one end of the tube to the other without harming the organism itself.
• Drift with the current: Pyrosomes are planktonic, which means they mainly drift with the current. However, they are occasionally able to influence their movement by means of their water filtration process.
• Filter-feeders: These tunicates rely on filter-feeding to survive. Using cilia, the individual zooids filter out plankton from the surrounding water, returning the stripped water to the environment.

Classification and Scientific Name

Pyrosomes are colonial tunicates belonging to the genus Pyrosoma. Their name comes from the Greek pyro (“fire”) and soma (“body”), which refers to their bioluminescence. Other common names include sea pickles, sea worms, sea squirts, fire salps, fire bodies, and cockroaches of the sea. They further belong to the family Pyrosomatidae, the order Pyrosomida, and the class Thaliacea (pyrosomes, dolioloids, and salps). As tunicates (marine invertebrates), they belong to the subphylum Tunicata within the phylum Chordata.

The World Register of Marine Species (WoRMS) lists several species within the genus Pyrosoma, with Pyrosoma atlanticum being the most widely recognized. Some species names are considered synonyms or have been reassigned.

Notably, scientists have reassigned the giant pyrosome (formerly Pyrosoma spinosum) to the genus Pyrostremma as the giant fire salp (Pyrostremma spinosum). A number of other former Pyrosoma species have undergone a similar process.

Appearance

Pyrosomes are invertebrate chordates with gelatinous, baglike bodies. They are tubular or conical in shape. Rather than being a single organism, they are composed of hundreds or thousands of zooids (multicellular animals that join together to form a larger organism). These zooids work together, often performing different functions to enable the larger organism to function. The outer surface is bumpy, slimy, and tough to the touch.

One of the most striking features of these organisms is their coloration. In sunlight, they are light pink, yellow, or blue. However, at night their bioluminescence manifests as colorful flashes of light triggered by other zooids or physical disturbances. These flashes or waves of light can be quite dramatic, particularly at night near the surface of the water.

Pyrosomes range greatly in size. The smallest are well under an inch while the largest grow up to 60 feet in length. Individual zooids are typically only a few millimeters (a fraction of an inch) in size.

Evolution and History

The evolutionary history of pyrosomes is difficult to determine due to the extremely sparse fossil record regarding tunicates. Tunicates have soft, gelatinous bodies that do not fossilize well aside from spicules (hard mineral particles in some tunics). Two of the three classes within the subphylum Tunicata likely derived from a single lineage within the third class, Ascidiacea. It is likely that pyrosomes branched off from within the class Thaliacea with a distant connection to dolioloids and salps. Their colonial nature shows some similarities to the coloniality of ascidians.

Behavior

Pyrosomes spend most of their lives drifting with the current. However, individual zooids influence their movement as they filter water through their bodies using cilia. At night, these organisms sink to depths of up to 2,300 feet, surfacing again during the day. They can migrate as far as 1,345 feet in a single day. They are often found in larger groups called blooms, which form a vital part of their ecosystems.

Habitat

Pyrosomes are pelagic and free-floating, preferring the open tropical waters of the Atlantic, Pacific, and Indian Oceans. However, they also occur occasionally in colder waters, including those of the Antarctic Ocean. They typically remain fairly close to the surface, though at night they descend to depths of up to 2,300 feet. It is possible to spot them off the coasts of every continent in the world.

Beyond inhabiting these environments, these organisms also contribute to the habitats of other species in the water column. They provide vital biological substrates for other species in need of shelter and food.

Diet, Predators, and Threats

Pyrosomes are omnivorous filter-feeders with an extremely limited diet. However, they themselves provide an important food source for a variety of predators.

What Do Pyrosomes Eat?

These unique organisms survive by filtering out plankton, phytoplankton, and possibly bacteria from the surrounding water. They themselves are planktonic.

What Eats Pyrosomes?

These organisms fall prey to a number of predators, particularly after sudden death when they descend rapidly to the ocean floor. These predators include various species of fish as well as sea turtles, sea urchins, crabs, eels, and seabirds. Observers have also seen some of these animals swimming through the tubes of pyrosomes.

Reproduction and Life Cycle

Pyrosomes employ both sexual and asexual reproduction. Being hermaphroditic, each zooid produces both eggs and sperm. They are thus able to fertilize their own eggs. These eggs develop into embryos that grow into four attached zooids. These zooids then use asexual reproduction to produce genetically identical copies of themselves. Combining sexual and asexual reproduction enables the colony to enlarge and regenerate itself.

Pyrosomes are notoriously delicate, which often frustrates scientists attempting to catch them in nets for study. Because they frequently break up on contact, it can be challenging to tell their original size. After they die, their bodies often descend to the ocean floor, where they provide vital nourishment for bottom-dwelling creatures.

Population

Because pyrosomes are elusive, scientists do not know how many currently exist in the wild. Additionally, the IUCN does not include them on its Red List of Threatened Species.