Salps

Salps are semi-transparent barrel-shaped zooplankton resembling jellyfish. However, despite the resemblance, salps are in a separate phylum from jellyfish, their ancestries having diverged some 800 million years ago. Jellyfish are in Phylum Cnidaria, along with corals. Salps are in Phylum Chordata, same as us, making them taxonomically closer to humans than jellyfish. In fact, they are related to all the animals with backbones. Larval salps have a notochord running down their back – a tough, flexible rod which protects the central nerve cord and provides an attachment point for muscles, functionally similar to our backbone. Adults lose their notochord as they grow, but the larval notochord is one of the ways we know that salps are one of our closest living invertebrate relatives.

By contracting bands of muscles that ring the body, salps draw water in from the front and pump it out the rear, which pushes the animal along through the water, like a very slow jet engine. When a salp pushes water in one direction, its body moves in the opposite direction. Individual salps vary from about 4 to 12 inches in length, but chains of them can be much longer. They are often overlooked, but are sometimes so numerous that they can even challenge krill populations by sheer numbers. Tunicates (salps and their relatives) are the second most abundant class of zooplankton (the first being copepods).

Salps are found worldwide in equatorial, temperate, and cold waters. They are found at depths from the surface to nearly 5000 feet. The most abundant concentrations are found in the Antarctic Ocean. Salps are filter feeders that eat everything they trap in their feeding net of mucus that hangs inside their bodies. Although the mesh of their feeding net is efficient enough to catch a variety of different sizes of particles, their main food is phytoplankton. They pump water through their hollow bodies and sieve out the phytoplankton. The feeding net is like a conveyer belt, always moving the bits of food into the animal’s stomach. Because of the net’s sticky design, particles frequently build up, which effectively creates a net with increasingly smaller holes, thus allowing the salp to consume phytoplankton in quite a wide range of sizes. This may help explain how salps can survive in the open ocean where the supply of larger food particles is low – where not many other grazers survive. Also, it highlights the importance of the salp’s role in cycling carbon from the atmosphere into the deep ocean. As they eat, they consume a very broad range of carbon-containing particles, which are then concentrated into fecal pellets that sink rapidly to the ocean depths, where that carbon won’t see the light of day for years, or even centuries. This reduces the amount of carbon in the surface waters, letting more carbon dioxide enter the ocean from the atmosphere. Even though they are much larger than the microscopic phytoplankton they consume, salps are also considered plankton, as they are carried by currents stronger than their own propulsion. The word plankton comes from the Greek planktos, meaning wandering.

Because of their very effective feeding nets, when salps are in very dense populations of phytoplankton, they can actually become clogged with their food source and sink. When this happens, it is not uncommon for the beaches to be full of salp bodies that have washed up due to food comas, especially during phytoplankton blooms. Salps were once thought to be “trophic dead ends,” meaning they have little caloric value as food for other species. However, it seems they are more nutritious than previously thought and provide a gelatinous feast for fish, seabirds, sea turtles, shellfish, whales, other plankton, and siphonophores, such as the Portuguese man o’ war.

Salps have a complex life cycle alternating between a sexual and asexual form. A solitary salp reproduces asexually by budding a chain of clones that bioluminesce. A string of cells comes out of its body and each divides itself up, resulting in many tiny salps. The clones are all identical and all connected together. As the string gets longer and the babies grow larger, they break away as a long, independent chain. Another string forms after the first one, and then another, until many chains of babies have been released into the ocean. These strands can reach more than fifty feet, and the chains of some species form complex shapes, such as giant wheels or double helixes. Salps that are linked together communicate through electrical signals to synchronize their movements, which can produce a chain of harmonized light pulses that snakes or spins its way through the water. Each member of a salp chain is a sequential hermaphrodite, starting life as a female with one or two eggs, and then turning into a male. Mating occurs with chains of older salps that have already transformed into males. A fertilized salp female broods her embryo in a placenta-like membrane until the young salp can survive on its own. Released embryos grow into solitary creatures that asexually bud their own chains of clones, which will then reproduce sexually. By the time she releases her embryo, the mother salp is already turning into a male.

Asexual cloning is an extremely fast form of reproduction but can lead to an evolutionary dead end because it doesn’t result in the genetic diversity needed to adapt to changing conditions in the environment. Sexual reproduction creates genetic diversity within a species, but it’s a comparatively slow way to reproduce. Salps have somehow arrived at a strategy that combines the best of both reproductive methods. When food is abundant, salps clone themselves extraordinarily fast, reaching maturity in only 48 hours. This quick turnaround time lets them take advantage of algal blooms, increasing their population size rapidly – though as mentioned previously, this gluttony can lead to massive die-offs of overstuffed salps.

A swarm of salps can emerge in just a few weeks, and can cover hundreds, even thousands, of square miles, quickly taking advantage of sudden phytoplankton blooms. The effect salps have on the ocean’s carbon cycle wouldn’t be a big deal if there were just a few salps, but in large swarms, in particular, there is enough of both the fecal matter and the bodies sinking to the ocean floor that they can have a huge impact on the biological pump of the ocean, not only affecting the ocean’s carbon cycle, but even contributing to climate changes in the area. Scientists from the Woods Hole Oceanographic Institution, in expeditions conducted in the Mid-Atlantic Bight region (between Cape Hatteras and Georges Bank in the North Atlantic), estimated that a swarm of salps consumes up to 74 percent of phytoplankton from the surface water each night, and their sinking fecal pellets can transport up to 4,000 tons of carbon a day to deep water. (Salps feed in surface waters at night and retreat several hundred meters down during the day, possibly to avoid predators.) Their bodies sink even faster than pellets, so the ones that don’t wash up on the beach are sending even more carbon to the deep. Though they look like lumps of limp gelatin when they’re stranded on the beach, apparently swimming with salps has been described as a pleasant experience, like swimming with marshmallows.

Where I learned about salps, and you can too!

World Register of Marine Species
www.marinespecies.org/aphia.php?p=taxdetails&id=137217#distributions

Australian Museum
australian.museum/learn/animals/sea-squirts/what-is-a-salp/

Marine Education Society of Australasia
www.mesa.edu.au/tunicates/tunicates01.asp

Woods Hole Oceanographic Institution
www.whoi.edu/oceanus/feature/salps-catch-the-oceans-tiniest-organisms/
www.whoi.edu/know-your-ocean/ocean-topics/ocean-life/ecosystems/polar-life/the-watery-world-of-salps/

It’s Nature
itsnature.org/sea/other/salps/

Coastal Interpretive Center
interpretivecenter.org/salps-a-mystery-on-the-beach/

Mexican Fish
mexican-fish.com/salps-of-the-salpidae-family/

Dive and Discover
divediscover.whoi.edu/archives/expedition10/

Earth Life
earthlife.net/inverts/thaliacea

Science Daily

www.sciencedaily.com/releases/2006/07/060702085004.htm

Live Science
www.livescience.com/4181-ocean-gummy-bears-fight-global-warming.html

Oregon Marine Reserves
oregonmarinereserves.com/2018/02/15/salps/

The Nippon Foundation Nereus Program
nereusprogram.org/works/our-jelly-like-relatives-common-misconceptions-about-salps/

Kidadl
kidadl.com/facts/animals/salp-facts

Evolution News & Science Today
evolutionnews.org/2023/02/meet-the-ghostly-organisms-that-rescue-the-planet/

The Weather Channel
weather.com/science/nature/news/salp-antarctic-oceans-krill-decrease-warmer-waters

Wikipedia
en.wikipedia.org/wiki/Salp