Salp

Lore & Background

Salps are barrel-shaped, gelatinous tunicates that move by contracting their bodies to pump water through them, a method considered one of the most efficient forms of jet propulsion in the animal kingdom. They feed by straining phytoplankton through internal feeding filters. These animals are common in equatorial, temperate, and cold seas, appearing at the surface either singly or in long, stringy colonies. The highest concentrations are found in the Southern Ocean near Antarctica, where they sometimes form enormous swarms in deep water, occasionally outnumbering krill. Since 1910, salp populations in the Southern Ocean have increased while krill have declined, and they have also been seen in growing numbers off the coast of Washington, United States. Salps are closely related to other pelagic tunicates like Doliolida and Pyrosoma, as well as to bottom-dwelling tunicates. Despite resembling jellyfish, they are chordates with dorsal nerve cords, linking them to vertebrates. Small fish sometimes swim inside salps for protection. A notable historical event occurred in 1920 when a large incursion of *Salpa fusiformis* into the North Sea caused the failure of the Scottish herring fishery.

Reader's Guide

Salps are significant for their rapid response to phytoplankton blooms. When food is plentiful, they can bud off clones at a rate probably faster than any other multicellular animal, quickly stripping phytoplankton from the sea. Their blooms can cause beaches to become slimy with mats of salp bodies and lead to fluctuations in other planktonic species due to competition. Sinking fecal pellets and bodies of salps carry carbon to the seafloor, making them abundant enough to affect the ocean's biological pump. Large changes in their abundance or distribution may alter the ocean's carbon cycle and potentially play a role in climate change.

Did You Know?

• Salps are chordates, related to vertebrates, despite resembling jellyfish.
• Small fish swim inside salps as protection from predators.
• Mushroom corals and those of the genus Heteropsammia are known to feed on salps during blooms.
• Salps can grow at a rate probably faster than that of any other multicellular animal when food is plentiful.

Life as a Planktonic Drifter

Salps occupy a distinctive niche within the water column as metazoan zooplankton, a category often described as drifters. Unlike nektons—those powerful swimmers whose active propulsion lets them overcome the pull of currents—salps possess only limited motility. This means they remain suspended in the water column, largely at the mercy of surrounding water movements, drifting along with currents rather than swimming against them. They sit between the truly passive zooplankton, such as most aquatic larvae which have no motility at all, and the actively swimming nektons, representing an intermediate mode of aquatic locomotion. Their ecological positioning is purely environmental: they are grouped with other planktonic organisms not by shared ancestry but by the habitat they inhabit, making them part of a polyphyletic assemblage. Any morphological or behavioral resemblances they share with fellow drifters, such as jellyfish or larvaceans, are products of convergent evolution rather than close kinship. This drift-based existence shapes every aspect of their biology, from feeding strategies to reproductive dispersal.

Respiration and Aquatic Physiology

As a fully aquatic organism, a salp spends its entire existence immersed in a body of water, whether that is a saltwater ocean, a brackish estuary, or a freshwater system. Like the vast majority of animals that never leave the water, it relies on aquatic respiration to extract dissolved oxygen from its surroundings. This can occur through specialized gill structures, across the surface of the skin, or via the mucosal linings of the digestive tract. This stands in sharp contrast to secondarily aquatic creatures—marine mammals and reptiles that evolved from land ancestors and must surface to take a breath of air using lungs, effectively holding their breath during submersion. A salp has no such option. Remove it from the water for any extended period and it faces rapid death through dehydration or hypoxia, as its respiratory surfaces collapse or it suffocates under its own body weight. Its physiology is therefore entirely dependent on the chemical and physical properties of the water it inhabits, tying its survival to the health of its aquatic environment.

Reproduction and Life Cycle

Like nearly all aquatic animals, salps carry out their reproductive processes within the water itself, whether they release eggs into the current in an oviparous strategy or give birth to live young through viviparous reproduction. Their life cycle is tightly bound to the aquatic medium, and many species in the broader planktonic community pass through fully aquatic stages—typically as eggs and larvae—before any later developmental changes. Some aquatic organisms, such as certain amphibians and flying insects, eventually metamorphose into terrestrial or semi-aquatic adults, but planktonic drifters like salps remain suspended in the water column throughout their existence. Many aquatic species also undertake migrations between different water bodies at various points in their life cycle, a pattern that helps disperse populations and maintain genetic diversity. Because their reproductive success depends on water quality, temperature, and current patterns, any disruption to these environmental factors can directly impact their ability to reproduce and sustain populations across generations.

Ecological Role and Conservation Pressures

Salps, alongside aquatic plants, algae, and microbial communities, form essential links in the food webs of marine, brackish, and freshwater ecosystems. As members of the planktonic community, they serve as a vital trophic link, transferring energy between primary producers and larger predators. Their ecological importance extends beyond simple predation dynamics; they contribute to nutrient cycling and the overall productivity of the water bodies they inhabit. However, the very environments that sustain them are under mounting pressure. Aquatic animals face threats from overfishing and destructive harvesting practices, water pollution, ocean acidification, climate change, and competition from invasive species. Habitat destruction and fragmentation of aquatic ecosystems compound these risks. Conservationists pay particular attention to aquatic biodiversity because of the inherent fragility of these environments. The loss of species like salps would ripple through entire food webs, undermining the food, energy, and economic resources that aquatic ecosystems provide to the broader world.