Salps are barrel-shaped, gelatinous animals that drift through the open ocean, pumping water through their transparent bodies to feed and move. They look like jellyfish but are far more complex. Salps belong to the same broad group as humans (the phylum Chordata) and possess a brain, heart, intestines, and circulatory system. They play a surprisingly large role in moving carbon from the ocean surface to the deep sea.
What Salps Look Like
A single salp is a small, translucent tube, typically a few centimeters long, with visible internal organs and bands of muscle wrapped around its body. You can often see right through one. From a distance, or washed up on a beach, they’re easy to mistake for jellyfish. But salps have a firm, rubbery texture rather than the soft, amorphous feel of a true jellyfish. Their bodies contain complex organ systems, including a digestive tract that runs from one end to the other.
What really sets salps apart visually is their tendency to form chains. Individual salps link together into long, rope-like colonies that can stretch 6 to 7 meters. Beachgoers and fishermen sometimes encounter these chains floating at the surface or washed ashore, where the dead colonies break down into foam that can line kilometers of coastline.
Closer to Humans Than to Jellyfish
Despite their blob-like appearance, salps are tunicates, a subgroup of the chordates. That places them in the same phylum as fish, birds, and mammals. Jellyfish, by contrast, are cnidarians with no brain, no heart, and no centralized nervous system. Salps have all three. As the Woods Hole Oceanographic Institution puts it, these gelatinous drifters are more closely related to humans than to the brainless jellyfish they resemble.
How Salps Feed and Move
Salps do two things at once every time they contract their muscles: they eat and they swim. Rings of muscle squeeze the body in a rhythmic pumping motion, drawing water in through an opening at the front (the oral siphon), pushing it through an internal chamber, and jetting it out the back (the atrial siphon). That jet of water propels the salp forward.
Inside the body, a continuously produced mucus net strains food particles from the passing water. This mesh is astonishingly fine, with openings roughly 1.5 by 6 micrometers, far smaller than the width of a human hair. The mesh fibers themselves are only about 0.1 micrometers across. Because of the slow flow speed and tiny scale involved, salps can capture particles even smaller than the gaps in the net. Bacteria, single-celled algae, and other microscopic organisms stick to the mesh through direct contact and random Brownian motion, then get rolled into a food strand and carried to the esophagus.
Where Salps Live
Salps are found in every ocean, from tropical waters to the poles. One of the most studied species, Salpa thompsoni, is the most common tunicate in the Southern Ocean and was historically considered a polar-temperate species found between 40° and 60° south latitude. Over the past 50 years, though, it has been appearing more frequently in the coastal waters of the Antarctic Peninsula.
Most salps concentrate in the upper 170 meters of the water column, with the highest densities in the top 50 meters where phytoplankton is abundant. But they’ve been detected as deep as 990 meters during surveys, and their vertical range may extend to 2,000 meters. They tend to favor surface water masses and avoid colder, deeper layers. In Antarctic surveys, one cold intermediate water layer was completely devoid of salps.
A Two-Phase Life Cycle
Salps alternate between two distinct body forms across generations: a solitary phase and an aggregate (colonial) phase. The solitary form reproduces asexually, budding off chains of small clones called stolons. These clones are released as aggregates, the chain-forming colonial generation.
Aggregates reproduce sexually. Each one starts female, carrying one or two eggs that are fertilized by a larger male aggregate. The embryo develops inside the mother’s body, nourished through a structure that functions like a placenta. Once the embryo matures and is released, the mother aggregate transitions to male. The released offspring grow into the next generation of solitary salps, and the cycle repeats. In warm water, this whole process can unfold in days to weeks, allowing populations to explode rapidly when food is plentiful. In colder regions like the Southern Ocean, it takes weeks to months.
Why Salps Matter for Carbon
Salps punch well above their weight in the ocean’s carbon cycle. They graze on tiny surface organisms that have absorbed carbon dioxide from the atmosphere, then package the waste into dense fecal pellets that sink fast. Research in the subarctic northeast Pacific found that salp pellets sink at 400 to 1,200 meters per day, far faster than the waste produced by most other zooplankton. Bacteria on the pellets consume less than 1% of the carbon per day, meaning most of the carbon reaches the deep ocean intact rather than being recycled back into the water near the surface.
The scale is significant. In that same study, salp fecal pellets made up as much as 82% of all the particulate organic carbon produced as fecal matter by the entire community of surface-dwelling zooplankton. At the 100-meter depth line, salp pellets accounted for up to 48% of all sinking organic carbon. For an animal most people have never heard of, that is an outsized contribution to pulling carbon out of the atmosphere and locking it in the deep sea.
What Eats Salps
Despite being mostly water, salps are food for a range of ocean predators. Sea turtles, particularly leatherbacks, consume large quantities of salps and can mistake floating plastic bags for them. Various species of fish, seabirds, and other zooplankton also feed on salps or their chains. In the Southern Ocean, salps sometimes compete with krill for the same phytoplankton food supply, and shifts in the balance between salp and krill populations can ripple through the food web, affecting species from penguins to whales that depend heavily on krill.
Salp Blooms
Because of their rapid asexual reproduction, salps can form massive blooms when conditions are right. A bloom along the coast of southern Italy in 2013 produced chains 5 to 7 meters long, visible to hundreds of beachgoers and fishermen over a stretch of coastline lasting weeks. These blooms can clog fishing nets and make a mess of beaches when the colonies die and wash ashore, breaking down into thick foam.
Blooms tend to appear when phytoplankton is abundant and water temperatures are favorable. They can vanish almost as quickly as they arrive. The ecological consequences depend on location: in some regions, a massive salp bloom accelerates carbon export to the deep ocean, while in others it can temporarily reduce the food available to competing grazers like krill or copepods.