Sea pickles are not pickles at all, nor are they individual animals. They are free-floating colonies of tiny organisms called zooids, each one smaller than a grain of rice, fused together into a hollow, tube-shaped structure that drifts through the open ocean. Their formal name is Pyrosoma atlanticum, and they belong to a group called tunicates, which are more closely related to vertebrates than they are to jellyfish or coral. What earned them their nickname is partly their bumpy, translucent appearance and partly their habit of washing ashore in large numbers, baffling beachgoers who have no idea what they are looking at. But their most striking feature is the glow: sea pickles produce their own light, and the way they do it turns out to be a genuinely unusual story in bioluminescence research.
A Colony, Not a Single Animal
Each sea pickle is made up of hundreds to thousands of individual zooids embedded in a shared gelatinous tunic. The colony forms a hollow cylinder, open at one end and tapered at the other, and it moves by a surprisingly elegant method. Every zooid draws water in through its body to filter-feed, then pushes the filtered water into the colony’s hollow interior. All that expelled water flows out the open end, creating a gentle jet propulsion that nudges the whole colony forward. It is cooperative locomotion at a basic level: no single zooid is steering, but the combined effort moves the group.
Colonies you might encounter range from a few centimeters to well over half a meter long, though reports exist of much larger specimens in deep or offshore waters. The tunic itself is translucent and slightly firm to the touch, which is where the pickle comparison comes from. When they wash up on beaches, they tend to look like deflated, gelatinous tubes, and they lose most of their visual appeal quickly. In the water, though, they can be strikingly beautiful, especially at night.
How Sea Pickles Glow
Bioluminescence in the ocean is common, but the chemistry behind it varies enormously between species. In sea pickles, the glow comes from a chemical reaction between a light-producing molecule called coelenterazine and an enzyme the colony produces. Researchers sequenced the genetic material of Pyrosoma atlanticum and identified a novel luciferase enzyme, which they named PyroLuc, that matched sequences previously known only from a very different group of marine animals, the cnidarians (the group that includes jellyfish and corals). When the team combined PyroLuc with coelenterazine in the lab, the mixture produced light, confirming that this enzyme is the colony’s bioluminescent engine.1PubMed Central. A putative chordate luciferase from a cosmopolitan tunicate indicates convergent bioluminescence evolution across phyla
The interesting wrinkle is that tunicates and cnidarians are not closely related. Tunicates sit on the chordate branch of the tree of life, while cnidarians split off much earlier in evolutionary history. Finding a similar luciferase in both groups strongly suggests that bioluminescence evolved independently in these lineages, a textbook case of convergent evolution where unrelated organisms arrive at similar solutions to similar problems. The shared chemistry likely traces to coelenterazine being widely available in the ocean as a dietary molecule, so different animals independently evolved enzymes to exploit it for light production.1PubMed Central. A putative chordate luciferase from a cosmopolitan tunicate indicates convergent bioluminescence evolution across phyla
If you touch or disturb a sea pickle colony in the water at night, one zooid lights up, and then the glow propagates in a wave across the colony’s surface. Each zooid flashes in response to its neighbor’s flash, so a single poke can trigger a ripple of blue-green light across the whole tube. This chain-reaction signaling is part of what makes them so visually striking. The name Pyrosoma itself translates roughly to “fire body,” a label given to them in the early 1800s by naturalists who encountered their glow at sea.
What Sea Pickles Eat
Sea pickles are filter feeders. Each zooid draws seawater through a mucus net inside its body, trapping microscopic particles like phytoplankton and bacteria before pushing the filtered water into the colony’s shared interior. This means a large colony is essentially a floating filtration system, processing a continuous stream of seawater as it drifts.
Research on sea pickles collected from the Northern California Current looked at the fatty acid profiles of colony tissue to figure out what they had been eating. The most abundant fatty acid was docosahexaenoic acid, a compound that serves as a biomarker for dinoflagellates, a type of single-celled alga. The study also found that the specific mix of fatty acid biomarkers shifted depending on latitude, suggesting that sea pickles are dietary generalists whose meals reflect whatever microscopic organisms dominate the local water. Nitrogen isotope data from the same work placed pyrosomes at a relatively low position in the food web compared to other zooplankton, consistent with a diet based mainly on tiny primary producers rather than on other animals.2Marine Ecology Progress Series. Abundance, distribution, and feeding ecology of Pyrosoma atlanticum in the Northern California Current
This low trophic position matters ecologically. Sea pickles are converting the energy locked in phytoplankton and bacteria into larger, denser packages of tissue and fecal pellets. That conversion has consequences for the ocean’s carbon cycle, which is worth its own discussion.
Their Role in Moving Carbon to the Deep Ocean
One of the more consequential things sea pickles do has nothing to do with their appearance or their glow. As filter feeders that produce dense fecal pellets and eventually die and sink, they help move carbon from surface waters down into the deep ocean, a process scientists call the biological carbon pump. Anything that speeds carbon’s downward trip matters for the ocean’s ability to sequester carbon dioxide.
A study off Cabo Verde in the northeastern Atlantic estimated that downward-migrating pyrosome colonies actively transported roughly 13% of their fecal pellets below the mixed layer, which is the upper portion of the ocean that interacts directly with the atmosphere. That translates to a carbon flux in the range of about 2 to 65 milligrams of carbon per square meter per day, a wide range that reflects variation in colony density and behavior. On top of that, dying colonies that sank to the island’s benthic slopes contributed an average of about 15 milligrams of carbon per square meter to the deep seafloor.3PubMed Central. Distribution, associations and role in the biological carbon pump of Pyrosoma atlanticum (Tunicata, Thaliacea) off Cabo Verde, NE Atlantic
The same research used environmental DNA to detect pyrosome-derived material at depths well beyond where living colonies had been observed, suggesting that their ecological footprint extends deeper than their migration range. When huge numbers of sea pickles bloom and then die, the pulse of sinking organic material can be a meaningful food source for deep-sea scavengers and a significant input to the carbon budget of the water column below.3PubMed Central. Distribution, associations and role in the biological carbon pump of Pyrosoma atlanticum (Tunicata, Thaliacea) off Cabo Verde, NE Atlantic
The Massive Blooms off the U.S. West Coast
Sea pickles were historically considered warm-water organisms, common in tropical and subtropical seas. So when unprecedented aggregations of Pyrosoma atlanticum appeared in the Northern California Current between 2016 and 2018, researchers took notice.2Marine Ecology Progress Series. Abundance, distribution, and feeding ecology of Pyrosoma atlanticum in the Northern California Current Fishers in Oregon and Washington were hauling up nets packed with the gelatinous tubes, sometimes catching more pyrosomes by weight than actual target species. Beaches along the Pacific Northwest were covered in them. For most people in the region, it was the first time they had ever seen anything like it.
The timing coincided with a period of anomalously warm ocean conditions in the northeastern Pacific, including the aftermath of a massive marine heatwave sometimes called “the Blob” that developed in 2014 and 2015. Warmer water pushed the range of many warm-water species northward, and sea pickles appear to have been among the beneficiaries. Whether these blooms will become a recurring feature of the Northern California Current as ocean temperatures continue to shift is still an open question, but the sheer scale of those 2016-2018 events made clear that pyrosome populations can expand rapidly when conditions are right.
These blooms posed practical problems. Large numbers of sea pickles clogged fishing nets and sampling gear. Their gelatinous bodies fouled equipment, made catches difficult to sort, and raised concerns about potential impacts on local food webs. When millions of filter feeders suddenly appear in a region, they compete with other zooplankton for the same pool of phytoplankton, and they reroute energy through the ecosystem in ways that are hard to predict without long-term monitoring.
How Colonies Reproduce and Grow
Sea pickle colonies reproduce through a combination of sexual and asexual processes. Individual zooids within a colony can produce eggs and sperm, and fertilized eggs develop into a larval form. But colony growth also relies heavily on budding, where a single zooid divides and produces new zooids that remain attached to the shared tunic. This asexual budding is what allows a colony to expand so quickly. A small founding group of zooids can multiply into a full-sized colony in a relatively short time, which helps explain how populations can explode when environmental conditions are favorable.
Each zooid in the colony is a complete, functioning organism with its own feeding apparatus, reproductive organs, and light-producing cells, but it cannot survive independently. The zooids are physically embedded in the colony’s tunic and depend on the collective for locomotion and the flow of water through the shared interior. In that sense, a sea pickle colony is something between a single organism and a community: the individuals are genetically identical clones produced by budding, and they function as a unit, but each one retains its own anatomy.
Where You Can Find Them
Pyrosoma atlanticum is found in tropical and subtropical oceans worldwide, typically in the upper few hundred meters of the water column. They are pelagic, meaning they spend their lives drifting in open water rather than attached to the seafloor or near the coast. They do, however, undergo vertical migration, rising closer to the surface at night and descending to deeper water during the day. This daily commute is a behavior shared by many zooplankton groups and is thought to reduce predation risk from visual predators that hunt in sunlit water.
Despite being widespread, sea pickles were not heavily studied for most of the twentieth century. They are fragile, difficult to keep alive in laboratory settings, and tend to occur in offshore waters where routine sampling is sparse. Much of the recent surge in scientific interest was driven by those Pacific Northwest bloom events, which put pyrosomes literally on the doorstep of marine research institutions and commercial fisheries. Before 2016, many marine biologists working in temperate waters had never encountered one in the field.
Predators and the Broader Food Web
Sea pickles sit near the bottom of the food chain, but they are not without predators. Sea turtles, some fish species, and certain seabirds have been documented feeding on pyrosomes. Their gelatinous bodies are low in caloric density compared to crustaceans or fish, which means a predator needs to eat a lot of them to get a meaningful energy return. Still, when colonies bloom in huge numbers, the sheer volume of available biomass can make them a worthwhile target.
There is also an indirect food web role. Sea pickles serve as habitat for small crustaceans and other invertebrates that shelter inside or on the colony’s tunic. Some species of amphipods and isopods have been found living inside pyrosome colonies, using them as floating refuges in the open ocean where hard surfaces and shelter are scarce. A large pyrosome bloom therefore creates a temporary micro-ecosystem that supports a community beyond just the pyrosomes themselves.
Why They Are Related to You
One of the more surprising facts about sea pickles is their evolutionary position. Tunicates, the group that includes pyrosomes as well as more familiar animals like sea squirts, are chordates. That places them in the same phylum as fish, amphibians, birds, and mammals. The connection is not obvious from looking at an adult sea pickle, but tunicate larvae have a notochord, a flexible rod that is the defining feature of the chordate body plan and the evolutionary precursor to the vertebral column. The notochord is lost or absorbed as the animal matures, so the adult form looks nothing like a vertebrate, but the developmental evidence is clear.
This placement makes tunicates the closest living invertebrate relatives of vertebrates, which is why they attract interest from evolutionary biologists studying the origins of backboned animals. Genomic studies over the past two decades have confirmed the relationship and refined where exactly tunicates sit on the family tree. For the casual beachgoer poking at a gelatinous tube on the sand, it is a strange thought: that bumpy, translucent blob is a closer relative of yours than any insect, octopus, or crab on the beach.
When Sea Pickles Wash Ashore
If you find a sea pickle on the beach, you are almost certainly looking at a dead or dying colony. Out of water, they lose their shape and structural integrity within hours. They are not dangerous to handle. They do not sting, bite, or produce any known toxins. The tunic feels firm and slightly rubbery when fresh, but it breaks down quickly in the sun. After a bloom event, beaches can be littered with thousands of them in varying states of decomposition, producing a mild but noticeable smell as bacteria go to work.
The most common reaction from people who find them is confusion. Sea pickles do not look like any familiar marine animal, and their uniform, bumpy texture makes them look almost manufactured. Social media has amplified this confusion considerably. Photographs of sea pickles on beaches go mildly viral with some regularity, usually accompanied by questions like “what is this thing?” The answer, once you know it, is more interesting than most people expect: a bioluminescent colonial tunicate that is more closely related to humans than to jellyfish, that helps move carbon into the deep ocean, and that has been expanding its range into waters where it was rarely seen before.