This ancient sea creature may already have had a brain

Ctenophores (commonly known as comb jellies) are delicate gelatinous marine animals that appeared in Earth's oceans roughly 550 million years ago. These organisms contain a specialized sensory structure called the aboral organ (AO), which enables them to detect gravity, pressure, and light. A new morphological study published in Science Advances shows that this organ is significantly more complex than earlier studies suggested.

"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 the Cellular Architecture of an Ancient Organ

To understand how the aboral organ is organized internally, researchers collaborated with Maike Kittelmann at Oxford Brookes University and used advanced volume electron microscopy. This approach allowed them to create extremely detailed three dimensional reconstructions of the structure.

The analysis revealed 17 different cell types within the aboral organ, including 11 secretory and ciliated cell types that had never been identified before. This wide variety of cells confirms that the AO functions as a sophisticated 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 Neural Communication System

Beyond its cellular diversity, the aboral organ also appears to be closely connected to the comb jelly's nervous system. Ctenophores possess a nerve network made up of fused neurons that forms a continuous structure throughout the body.

Researchers found that this nerve net forms direct synaptic connections with cells in the aboral organ, creating a pathway for two way communication. At the same time, many cells within the AO contain numerous vesicles, indicating they may release widespread chemical signals through a process known as volume transmission. Together, these mechanisms suggest the organ relies on both synaptic and non synaptic forms of signaling.

"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."

Clues About the Evolution of Brains

The team also examined how certain developmental genes are expressed in ctenophores. Many genes involved in shaping body organization in other animals are present in these organisms, but their expression patterns differ substantially.

This difference suggests that the aboral organ may not be directly equivalent to brains found in other animal groups. "In other words," Burkhardt added, "evolution seems to have invented centralized nervous systems more than once."

Linking Neural Structure to Behavior

Additional support for these findings comes from related research led by Kei Jokura at the National Institute for Basic Biology in Japan, together with Prof. Gaspar Jekely from Heidelberg University. In a separate study that also included Burkhardt, scientists reconstructed the full neural wiring of the comb jelly's gravity sensing organ.

By combining high speed imaging with three dimensional reconstructions of more than 1,000 cells, the researchers showed how networks of fused neurons coordinate the beating of cilia on different parts of the animal's body. This coordination allows comb jellies to maintain their orientation as they move through 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 Nervous Systems

Together, these studies suggest that early nervous systems may have been more centralized than scientists previously believed. According to Ferraioli, the next phase of research will focus on identifying the molecular characteristics of the newly discovered cell types and exploring how strongly the aboral organ influences comb jelly behavior.