How does a tiny worm decide when to bite?

New research from the Salk Institute in the US has investigated how tiny nematode worms with only 302 neurons navigate complex decision-making.

The researchers focused on two nematode species: Caenorhabditis elegans, a popular model organism for biological experiments, and its lesser-known relative Pristionchus pacificus.

In ecological terms, C. elegans is both competitor and prey for P. pacificus. Both species primarily feed on bacteria, but P. pacificus is also known to attack and eat C. elegans on occasion, especially when the latter species is in its tiny, immature larval stage. P. pacificus will also bite adult C. elegans, but is less likely to successfully kill and eat the adult worm.

Nematode behaviour may be more complicated than once thought

What the researchers uncovered was that, rather than representing failed hunting, incidents where P. pacificus bit an adult C. elegans were actually part of a distinct strategy to defend its territory and bacterial prey from a competitor.

“Scientists have always assumed that worms were simple – when P. pacificus bites, we thought that was always for a singular predatory purpose,” says first author Kathleen Quach, a postdoctoral fellow at the Salk Institute.

“Actually, P. pacificus is versatile and can use the same action, biting C. elegans, to achieve different long-term goals.”

The researchers studied both species’ behaviour under several different parameters, such as whether C. elegans was larval or adult, whether bacterial food sources were present and how plentiful they were, and where food sources were located.

They found that the likelihood of P. pacificus biting larval C. elegans was highest when bacterial abundance was low and decreased as bacterial abundance increased. By contrast, the likelihood of biting adult C. elegans was low when bacteria were absent, highest when bacteria were present in low abundance, and then decreased with increasing bacterial abundance.

The researchers believe that this is illustrative of the two separate biting strategies used for different purposes. P. pacificus prefers bacterial prey, but bites C. elegans larvae to feed on them more often when bacterial food sources are low. On the other hand, biting adult C. elegans is favoured most when competition is high for their shared bacterial food source, but of negligible use to P. pacificus if there are no bacteria to compete over.

Finally, the team examined possible neural signalling mechanisms involved in the regulation of biting behaviour. They found that treating P. pacificus with a drug that blocks the dopamine D2 receptor enhanced territorial biting, while blocking octopamine receptors caused the worms to switch from a territorial to a predatory biting strategy.  

Why study nematode decision-making?

“Our study shows you can use a simple system such as the worm to study something complex, like goal-directed decision-making,” says senior author Sreekanth Chalasani, an associate professor in the Salk Institute’s Molecular Neurobiology Laboratory.

“We also demonstrated that behaviour can tell us a lot about how the brain works.”

“Even simple systems like worms have different strategies, and they can choose between those strategies, deciding which one works well for them in a given situation. That provides a framework for understanding how these decisions are made in more complex systems, such as humans.”

Further research on decision-making in these tiny worms may also enhance our ability to encode complex decision-making capability in small artificial neural networks.