Imma Perfetto
Cosmos science journalist
When food runs out and competition heats up, nematodes assemble into living towers. They writhe and twist towards the sky with the goal of latching on to a passing animal to hitch a ride to more comfortable digs.
Scientists had hypothesised this for decades, but no one had seen these aggregations form outside of the laboratory. Now, researchers in Germany have recorded the first video footage of nematodes “towering” in the real world in decaying apples and pears.
Their combined fieldwork and laboratory experiments provide the first direct evidence that towering behaviour occurs naturally and is done for collective transport.
“A nematode tower is not just a pile of worms,” says Dr Daniela Perez, a postdoctoral researcher at the Max Planck Institute of Animal Behavior (MPI-AB) and first author of a study presenting the findings in the journal Current Biology.
“It’s a coordinated structure, a superorganism in motion.”
Co-author Ryan Greenway, a technical assistant at the MPI-AB, spent months with a digital microscope combing through rotting fruit in orchards near the University of Konstanz to record the behaviour of the worm towers.
He found that, as the fruits decomposed, crystalised sugars and protruding flesh served as bases for the towers to form on.
“I was ecstatic when I saw these natural towers for the first time,” says Dr Serena Ding, group leader at MPI-AB and senior author of the study.
“For so long natural worm towers existed only in our imaginations. But with the right equipment and lots of curiosity, we found them hiding in plain sight.”
Though the fruits were crawling with many species of nematodes, the team discovered that each tower was made up of individuals of same species. And, while adult nematodes were observed crawling and feeding nearby, all the individuals in the towers were at the stress-resistant larval stage known as a “dauer”.
These natural dauer towers waved about in unison, which was reminiscent of another nematode behaviour – “nictating”. Here, single nematodes stand on their tails to latch onto passing animals.
To investigate further, Perez built a tower using laboratory cultures of Caenorhabditis elegans – a 1mm-long species of nematode used extensively as a model organism in biological sciences.
She placed the little worms on an agar plate that did not contain food but was decorated with a small vertical post made of a single toothbrush bristle.
Within just 2 hours, the hungry worms formed living towers on it.
These wiggling structures remained stable for 12 hours and could extend exploratory “arms” into surrounding space, before collapsing and merging back into the main tower.
Entire worm towers could even respond to touch and collectively attach to insects such as fruit flies, to hitchhike to new environments.
“The towers are actively sensing and growing,” says Perez.
“When we touched them, they responded immediately, growing toward the stimulus and attaching to it.
“On one occasion,” the authors report, “the arm bridged [an approximately] 3mm gap beyond the tip of the pillar to the petri dish lid above, forming a smooth and stable bridge allowing the transport of worms from the agar surface at the bottom to the lid at the top.”
Nematodes, or roundworms, are among the most abundant animals on Earth. They live freely in soil, fresh water, and marine environments or as parasites in plants and animals.
Towering seems to be an important strategy for the many nematode species which live boom-and-bust lifestyles and depend on dispersal to find and colonise new resources.
“Our study opens up a whole new system for exploring how and why animals move together,” says Ding.
“By harnessing the genetic tools available for C. elegans, we now have a powerful model to study the ecology and evolution of collective dispersal.”
