Spirostomum ambiguum is a tiny, single-celled organism that lives at the bottom of water bodies. Known for being very contractile, it can contract to one quarter of its body length in less than five milliseconds, hundreds of times faster than a human can blink.
- It looks like spaghetti, doesn’t have a gut or mouth – and has been feeding on bioplastics for millions of years
- When marine scientists found a plastic bottle floating in the ocean, they were shocked to see this large creature living inside, unable to escape…
A team of researchers from various universities in the US recently discovered just how this tiny organism achieves this amazing feat. The findings were published in the journal Proceedings of the National Academy of Sciences in May 2026.
S. ambiguum is able to contract at a rate of around 100 body lengths per second and quickly repeat the motion. By comparison, human muscle fibres can shorten by similar fractions, but it takes about 10 times as long.
The mechanical differences underlying this ability are of particular interest to scientists, because they have implications for designing faster artificial muscles and synthetic cellular machinery.
“The difference between what Spirostomum can do and what we can do comes down to what is powering the contraction, and what the machinery behind it looks like,” said Mary Elting, associate professor of biophysics at North Carolina State University and co-corresponding author of the study.
The researchers used two techniques to study S. ambiguum – electron microscopy, which uses a focused beam of electrons instead of light to view specimens at the nanoscale, and immunofluorescence microscopy, which uses fluorescently labelled antibodies to locate and visualise specific proteins or biomolecules within cells and tissues.
Single-celled organisms such as S. ambiguum don’t have muscle fibres like we do – instead, they have myonemes: fibrous structures inside the cell composed of the calcium-binding proteins centrin and Sfi1. In S. ambiguum, these myonemes form a fishnet-shaped web across the exterior of the organism.
The study revealed that the organism utilises calcium ions to trigger contraction and the fishnet-like structure to complete the movement. When the contraction is triggered, the net shrinks into itself – and then springs back.
“The fishnet geometry is unique because it lets Spirostomum contract uniformly, which protects its internal organelles (single cells’ versions of organs) while it moves so quickly,” said Elting.
“It works because the Sfi1 protein in the myoneme can shift from stiff to flexible. In the presence of calcium ions Sfi1 loses its stiffness and clumps up like a ball of wet spaghetti, which causes the fishnet to pull tight, shrinking the organism.”
- It has an eerie transparent skull that looks like a diving helmet and is the only animal on Earth to have eyes under its skin inside its head
- Strange organism that looks like a tongue found in English woodland. It has never been seen before in the UK
Further applications
In humans, energy is stored and released to muscle fibres by a molecule called adenosine triphosphate (ATP), which triggers contraction.
“Comparing the way our muscles contract to the way Spirostomum works is like comparing gas to electric power,” explained Elting.
“ATP undergoes a chemical change and gets ‘burned up’, like gasoline, whereas calcium ions act like an electrical current, although we still don’t know what produces the voltage that starts the current, or how it gets ‘reset’ so contraction can happen again.”
Further research into the calcium trigger in S. ambiguum and how it’s able to reset it after each contraction is the next step for the scientists.
“We would expect calcium-triggered reactions to be ‘one shot’, but Spirostomum can do it repeatedly,” said Elting. “Understanding those aspects of its motion are the keys to building a fast-moving, ATP-independent artificial muscle.”
Read the full findings here.
Top image: Photomicrograph of a live Spirostomum ambiguum species. Fimon lake, Vicenza, Italy. Credit: Exa-Volt, CC BY-SA 4.0 via Wikimedia Commons
