Ctenophore research points to earlier origins of brain-like structures
New 3D reconstructions of a key sensory organ in ctenophores reveal an unexpected structural and functional complexity. The findings suggest that an elementary brain may have already appeared in our most ancient relatives, reshaping our understanding of nervous system evolution in animals.
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Ctenophores – or comb jellies – are gelatinous animals that appeared in the ocean an estimated 550 million years ago. The delicate animals possess a specialized sensory structure called the aboral organ (AO), which allows them to sense gravity, pressure and light. A new morphological study published in Science Advances reveals that this organ is far more complex than previously thought.
“We show that the AO is a complex and functionally unique sensory system” said Pawel Burkhardt, group leader at the Michael Sars Centre, University of Bergen. “Our study profoundly enhances our understanding of the evolution of behavioral coordination in animals”
Mapping ancient neurons
To uncover the internal organization of the aboral organ, the researchers teamed up with collaborator Maike Kittelmann at Oxford Brookes University to use state-of-the-art volume electron microscopy. The analysis of the high-resolution, three-dimensional AO reconstructions uncovered 17 distinct cell types, including 11 previously unknown secretory and ciliated cells. This extraordinary diversity firmly establishes the AO as a complex, multimodal sensory organ.
“I was amazed almost immediately by the morphological diversity of the aboral organ cells. Working with volume EM data feels like discovering new exciting things every day”, said Anna Ferraioli, a postdoctoral researcher at the Michael Sars Centre and first author of the study. “The AO has a striking complexity when compared to apical organs of cnidarian and bilaterian. It is so unique!”
A hybrid communication system
Beyond cellular diversity, data showed that the aboral organ is tightly integrated with the comb jelly’s nervous system - a continuous network of fused neurons. This nerve net forms direct synaptic contacts with aboral organ cells, defining a clear path for reciprocal communication between the two structures. Many AO cells also contain abundant vesicles suggesting that they release diffuse chemical signals, in a process called volume transmission. Together, these findings point to a hybrid signaling system combining synaptic and non-synaptic communication.
“I think our work provides an important perspective on how much we can learn from studying morphology”, Ferraioli explains. “I would say that the AO is definitely not like our brain, but it could be defined as the organ that ctenophores use as a brain.”
The researchers also examined how conserved developmental genes are expressed in ctenophores. Although many genes that define body organization in other animals are present, their expression patterns differ considerably. This could mean that the aboral organ is not directly homologous to brains in other animals. “In other words”, Burkhardt added, “evolution seems to have invented centralized nervous systems more than once.”
Linking structure and behavior
The findings are reinforced by complementary work led by Kei Jokura at the National Institute for Basic Biology, Japan, together with Prof. Gaspar Jekely from Heidelberg University. In a separate study to which Burkhardt also contributed, their team reconstructed the complete neural wiring of the comb jelly’s gravity-sensing organ.
By combining high-speed imaging with three-dimensional reconstructions of over 1,000 cells, they showed how a network of fused neurons coordinates ciliary beating on different sides of the body, allowing the animals to maintain their orientation in the water. “The similarities to neural circuits in other marine organisms suggest that comparable solutions to gravity sensing may have evolved independently in distant animal lineages”, Jokura said.
Rethinking the origins of brains
Taken together, the two studies provide new evidence that early nervous systems may have been more centralized than previously assumed. The next steps, Ferraioli says, will be to uncover the molecular identities of the newly discovered cell types and to further test the extent to which the aboral organ modulates behavior.