A transparent chain hangs in the blue water, each link a living barrel with bands of muscle around it. This is a salp colony. Its members remain attached while moving and feeding, so the chain has many pumping units. The surprising feature is that smooth travel need not require every unit to fire at once. Differences in their timing can help the whole chain move more steadily.
Look for the individual inside the chain
A salp draws water in through one opening and expels it through another as its muscle bands contract. The expelled water contributes to forward thrust. Salps are tunicates, and their transparent bodies should not be confused with jellyfish merely because both appear gelatinous. A study of swimming and feeding performance describes how differences in body form and movement influence this water-pumping system.
Each unit in an aggregate is called a zooid. It has its own body and contributes a jet, but its attachments constrain where it can move relative to its neighbors. That makes a chain different from a school of unattached fish. The members do not simply happen to travel in the same direction. Their physical arrangement is part of the structure that transmits their combined forces.
The chain also belongs to a life cycle. A solitary, asexually reproducing stage buds an aggregate of genetically identical zooids; the aggregate stage reproduces sexually and gives rise to the solitary stage. A 2024 study of colonial architecture follows how connected units develop into different arrangements. Some salp colonies form streamlined chains, while others have quite different shapes. “Salp chain” is a useful image, but it is not the only possible architecture.
Why imperfect synchrony can help
Consider a single pumping body. Its forward speed can rise during a jet and fall while it refills. If many attached units pulse together, the colony may experience pronounced surges. If their pulses occur at different times, the contributions overlap: one zooid can provide thrust while another is between pulses. That is a simple reason to suspect that a collection of intermittent engines could produce a steadier total push.
Sutherland and Weihs tested that idea with theory, high-speed laboratory recordings, and flow observations in the sea. Their 2017 study focused on the linear colonies of Salpa fusiformis and Weelia cylindrica. Asynchronous pulsing produced smoother swimming, and the measured timing was consistent with individual zooids operating without a common cruising beat. Steadier speed can reduce the hydrodynamic penalty associated with repeated acceleration and deceleration.
The result is narrower and more interesting than “salps always cooperate perfectly.” A beneficial group-level pattern can emerge from differences among the members’ pulse timing. It does not require a central individual to order each contraction. Nor does it mean the zooids are unable to communicate. The research distinguishes routine asynchronous swimming from coordinated responses possible through connections between the units.
“No captain” is therefore a metaphor for the absence of a single cruising conductor, not a claim that the colony lacks sensing or physiological integration. A good account needs both levels: each zooid is a functioning animal, and attachment creates a larger moving system. Treating the chain as one giant salp loses the contributions of its members; treating it as unrelated passengers loses the mechanics of their connection.
The same water carries food
Pumping also moves food toward an internal mucus filter. The animal’s travel and feeding are coupled, rather than requiring entirely separate machinery. In Woods Hole research on salp filters, experiments with particles helped explain how these animals capture extremely small food. The mesh and its interaction with flowing water matter, not just the intuitive idea of a net with fixed holes.
That coupling creates a useful question: when the body form or pumping pattern changes, what happens to feeding as well as speed? A movement that looks efficient for travel might change the flow through a filter. Examining both tasks keeps us from evaluating a living animal as if it were a vehicle carrying an unrelated lunchbox. Its propulsion system and its means of obtaining food share the same stream of water.
To see the forms in motion, the CNRS Plankton Chronicles salp episode offers underwater footage and still images. If the film is unavailable, the accompanying photographs show chains, pumping bodies, and developing forms. Use the sequence to locate one zooid first, then follow its neighbors. An image of a long colony becomes easier to read when you know what one complete unit looks like.
A tabletop timing exercise
Draw eight columns on a page and give each five short marks representing pulses. First align all the marks into the same rows. Next shift the marks in each column by a different amount. Count how many occur in each row across the page. Which arrangement has the larger gaps? This is a conceptual comparison of overlapping inputs, not a numerical model of salp forces or swimming efficiency.
Now add a second question: what if the units are turned at different angles? Timing alone no longer tells you the direction of the combined push. That is why colony architecture, individual anatomy, and pulse phase all belong in the explanation. A chain of nearly invisible animals becomes an elegant example of collective mechanics: the motion of the whole depends both on what each member does and on how the members are joined.
Sources and further exploration
- A comparison of filtration rates among pelagic tunicates using kinematic measurements — Primary 2010 study, hosted by the author’s laboratory.
- A Developmental Ontology for the Colonial Architecture of Salps — Primary 2024 account of colony development and geometry.
- Hydrodynamic advantages of swimming by salp chains — Theory, video, and in situ flow measurements; focuses on two linear forms.
- WHOI: Salps Catch the Ocean’s Tiniest Organisms — Research institution’s account of feeding experiments.
- CNRS Plankton Chronicles: Salps — Underwater film and photographs for examining individual zooids.
