Welcome to the next episode of the WOrM Podcast 🪱

Today we're talking about a worm with teeth.

And a nervous system that has been rewired — by evolution — to become aggressive.

⸻

🧬 The central idea

Pristionchus pacificus is a predatory nematode.

It kills C. elegans larvae.

Sometimes for food.

Sometimes just to remove a competitor.

But how does its brain decide to attack?

⸻

🔬 What's actually going on?

This is not just predation.

It is a distinct behavioural state — aggression —

driven by a specific neurochemical system.

The researchers used machine learning to identify six distinct behavioural states:

• roaming and dwelling — shared with C. elegans
• predatory search, predatory biting, predatory feeding — unique to a predatory context

The worm doesn't attack randomly.

It switches modes.

⸻

⚡ Two chemicals. Opposite effects.

The key twist is this:

• Octopamine pushes the worm into aggressive, predatory states
• Tyramine pulls it back into passive, docile states

They act antagonistically — like a switch.

Remove octopamine → the worm stops attacking.

Remove tyramine as well → aggression returns.

⸻

🧠 The receptors tell the story

Two octopamine receptors are required: Ppa-ser-3 and Ppa-ser-6.

One tyramine receptor mediates the passive state: Ppa-lgc-55.

Crucially — these receptors are expressed in sensory neurons at the worm's nose.

Specifically, the IL2 neurons.

These are the first point of contact between predator and prey.

Silence the IL2 neurons → aggression drops.

⸻

🧠 A rewired circuit

In C. elegans, octopamine and tyramine do completely different things — fasting signals, escape responses.

In P. pacificus, evolution has repurposed these same molecules to regulate aggression.

The neurons producing them are conserved.

The function has diverged.

This is circuit-level evolutionary innovation.

⸻

🧠 Ancient and widespread

The same octopamine-aggression link was found in Allodiplogaster sudhausi —

a distant relative in the Diplogastridae family.

So this adaptation is not unique to P. pacificus.

It emerged early, in the predatory lineage — and stuck.

⸻

🌍 The bigger picture

This paper shows that:

• new behaviours can evolve through repurposing of existing neurochemical systems
• the same molecules can serve completely different functions in closely related species
• sensory neurons are a key site of neuromodulatory innovation

Evolution doesn't always build from scratch.

Sometimes it just rewires what's already there.

⸻

🧠 The take-home message

A predatory worm evolved aggression

not through new neurons,

but through new ways of using old chemistry.

Octopamine and tyramine — present across invertebrates —

were redeployed to gate an entirely new behavioural state.

That is elegant. And slightly terrifying.

⸻

📄 Paper discussed

Eren, G. G.; Böger, L.; Roca, M.; Hiramatsu, F.; Liu, J.; Alvarez, L.; Goetting, D. L.; Cockram, L. A.; Zorn, N.; Han, Z.; Okumura, M.; Scholz, M.; Lightfoot, J. W. (2026)Predatory aggression evolved through adaptations to noradrenergic circuitsNature, Vol 651https://doi.org/10.1038/s41586-025-10009-x

If you enjoyed this episode, please like, follow, and subscribe wherever you listen to the WOrM Podcast ⭐🎧 It really helps others in the community find the show.

This podcast is generated with artificial intelligence and curated by Veeren. If you'd like your publication or product featured on the show, please get in touch.

📩 More info:🔗 www.veerenchauhan.com📧 [email protected]