Brain Pooling: How Comb Jellies Turn Two Nervous Systems Into One

What if two strangers could become one organism - brain, gut, and all, overnight?

For most animals, this is biological fiction. For Mnemiopsis leidyi, the sea walnut, it is Tuesday.

In a 2024 study published in Current Biology researchers at the University of Exeter made an accidental yet extraordinary observation. While studying a tank of comb jellies, they noticed one individual was missing, only to realise that a larger jelly was actually two individuals fused together. The team replicated the experiment deliberately: they injured 20 comb jellies by removing small sections of their bodies and placed them near each other. Nine pairs fused completely, typically within 24 hours.

What happened next defies everything we understand about animal individuality.

The fused individuals synchronized their muscle contractions and shared a digestive tract, with food and nutrients passing freely between what were once two separate animals. More remarkably, the data imply that two separate individuals rapidly merged their nervous systems and began sharing action potentials - the electrical signals that drive all animal sensation and movement. When poked, the fused organism reacted as a single entity, indicating full neural integration.

This is physiologically staggering. In every other known animal, neurons communicate across synapses ,the discrete chemical junctions. But comb jellies are ancient outliers. Their nerve net lacks distinct synaptic connection points entirely; instead, neurons share a continuous fused cell membrane, making cross-individual merging physically conceivable in a way it simply isn't for vertebrates or even insects.

The mechanism behind the fusion points to a profound immunological gap. Ctenophores appear to lack allorecognition which is the biological ability to distinguish "self" from "other" meaning their immune system raises no alarm when foreign tissue arrives with no rejection or attack.

This rewrites our understanding of animal selfhood, immunity, and the deep evolutionary origins of the nervous system -from a creature that has been drifting through Earth's oceans for 600 million years. Sometimes, the oldest animals on the planet have the most to teach us about what it means to be alive.

References:

Jokura, K., Clarkin, C., & Bhatt, D. (2024). Rapid physiological integration of fused ctenophores. Current Biology, 34(19), R889–R890.