Overfeed This Flatworm and It Turns Into a Chain of Clones

How abundant food turns Stenostomum flatworms into chains

When a tiny flatworm has access to plentiful food, its otherwise solitary body quickly stretches out as fresh heads appear in sequence.

In only a few days, one worm can turn into a temporary, tethered chain of four or five connected clones.

This remarkable change is not caused by a new genetic mutation. Instead, it happens when the worm's normal reproductive cycles begin to overlap.

Put simply, during a feeding bonanza the worm outpaces itself: it starts producing its next clone before its previous one has broken away.

Researchers at the University of Warsaw in Poland found that a rich food supply can produce this dramatic shift in four microscopic, primarily freshwater flatworm species in the genus Stenostomum.

The research is available as a bioRxiv preprint and has not yet undergone peer review.

Flatworms are soft-bodied animals without backbones, and many can reproduce without mating, creating offspring that are genetically identical to themselves.

Under usual conditions, a flatworm forms a new head and the corresponding body segment within its existing body. After this section has fully developed, it generally detaches to live as an independent animal.

This mode of asexual reproduction is called paratomy. In Stenostomum, it involves a burst of gene activity, producing rapid growth and the essential organisation of the body axis.

Flatworms are also renowned for their extraordinary developmental adaptability. In laboratory settings, some species can be induced to regrow heads shaped like those of different species without altering their DNA, while others can naturally develop a head at either end.

Reproductive cycles begin to overlap

In the latest experiments, plentiful food changed the schedule of reproduction. The worms expanded so rapidly that a further reproductive cycle started before the preceding cycle had finished.

As a result, multiple developing clone segments stayed connected, producing a short-lived, multi-headed chain.

"We suspected from the beginning that the food could be responsible for triggering chain formation," University of Warsaw zoologist Ludwik Gąsiorowski told ScienceAlert.

"However, I personally was expecting that rather food quality (i.e., prey species) and not food quantity would trigger the process."

At first, the researchers presented six Stenostomum species with seven types of microscopic prey.

Four species repeatedly grew and reproduced when they were fed a single-celled freshwater organism that contains green algae.

The team then adjusted the quantity of that prey, finding that chain formation rose substantially as food availability increased.

"What surprised me the most is how reliable and reproducible this effect is," Gąsiorowski said.

"If we want to obtain worms in chains, it's just enough to put them in a known high concentration of specific prey for three days, and we will always see the chains."

Abundant food caused *Stenostomum worms to shift from their normal asexual reproduction (left) to making chains of connected clones (right). (Gąsiorowski et al., bioRxiv, 2026)*

After consuming the green prey cells, the worms' digestive systems became bright green, while their bodies swiftly grew longer.

"Within roughly two days, the first of the new heads becomes visible," Gąsiorowski explained.

Initially, the developing head is visible as two clear spots halfway along the worm, because the emerging brain pushes the darker gut tissue inwards.

More heads subsequently appear along the elongating body, as shown in the image below.

By roughly four days after feeding starts, the longest chains consist of four or five zooids joined tail-to-head.

Confocal microscopy image of a chain of Stenostomum brevipharyngium, revealing varying stages of development among individual heads (arrows). Scale bar: 100 µm (Comparative Invertebrate Zoology Group, University of Warsaw/CC BY-NC 4.0)

Why the chains break apart

These chains do not last. When the oldest developing head is finished, the worms split into shorter chains or separate individuals. Where food continues to be plentiful, they grow again, form chains again, and divide again.

Additional food speeds up the worm's growth in length, but a new head still needs about four days to develop. Consequently, the worm reaches a size that lets it begin another reproductive cycle before the first one is complete.

The researchers describe the chain-like formations as "paracolonies", as they resemble animals that establish clonal colonies but ultimately separate.

The researchers also investigated whether staying linked in a chain offered any immediate benefit. They found no sign that connected segments divided up the task of digestion.

In experiments involving one Stenostomum species, however, chains were less likely than individual worms to be chosen by a predator. This effect was not seen in another species.

That does not necessarily mean Stenostomum chains possess extra brainpower that somehow enables them to outsmart predators.

Gąsiorowski noted that any benefit could arise from their larger body size - and thus a greater number of cilia to drive the worms through water - rather than from having several heads joined together.

"We still know very little about the ecology of these microscopic invertebrates – we don't even know what their main predators are in the natural environment, which limits our capacity for testing some ecological mechanisms," he said.

"Even if the worms are selected for the ability to increase their body size, the formation of chains remains an accidental developmental byproduct."

Permanent chains in related flatworms

For Stenostomum, chain formation is just a passing stage. Yet certain related flatworms are known to create permanent chains, implying that this body arrangement may be beneficial to them.

For example, Catenula lemnae can form chains of more than 15 connected segments, and another related flatworm species has been recorded with up to 50.

A temporary chain in *Stenostomum brevipharyngium (top) and a permanent chain in Catenula lemnae (bottom). Arrows mark the heads of successive zooids. Scale bars: 100 µm (Comparative Invertebrate Zoology Group, University of Warsaw/CC BY-NC 4.0)*

However, Gąsiorowski's group has not been able to maintain C. lemnae in the laboratory. As a result, they cannot determine whether the species' reproductive approach represents a more established form of Stenostomum's food-triggered chains.

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"We cannot directly test if alternation between asexual reproduction and chain formation follows similar logic as in Stenostomum," Gąsiorowski said, although given how closely related they are, "it seems plausible that chains in Catenula evolved from conditions similar to what we observe for Stenostomum."

So little is known about the watery environments inhabited by these worms that, for the moment, these remain intriguing questions to explore.

The study is available on the bioRxiv preprint server.

This article was fact-checked by Rachel Garner and edited by Clare Watson. Although we take pride in our process, we are only human. If you notice an error, please tell us.