A Lesson from Jellies: Don’t Underestimate Small Nerve Systems

Far from being just drifting creatures, new insights from the University of Copenhagen illuminate the cognitive prowess of Caribbean box jellyfish. Despite only having about a thousand nerve cells and lacking a centralized brain, these marine animals demonstrate a capability for complex learning that challenges our foundational notions about neurology.

For millennia, jellyfish have thrived in our oceans, earning their evolutionary stamp of success. Yet, they were perceived as simple entities, with constrained learning capacities.

Traditionally, it was believed that a more intricate nervous system equated to advanced cognitive potential in creatures. Jellyfish and their kin, the cnidarians, are among the most ancient animals with nervous systems, albeit rudimentary and decentralized.

Neurobiologist Anders Garm has spent over a decade investigating box jellyfish, notorious for their potent venom. However, Garm’s research indicates that there’s more to these jellies than just their sting. This revelation challenges the prevailing idea about the cognitive reach of basic nerve structures.

Garm remarked, “It was once presumed that jellyfish can only manage the simplest forms of learning, including habituation – i.e., the ability to get used to a certain stimulation, such as a constant sound or constant touch. Now, we see that jellyfish have a much more refined ability to learn, and that they can actually learn from their mistakes. And in doing so, modify their behavior.”

Changing behaviors based on experiences – essentially, the act of learning – is a hallmark of complex nervous systems. Garm, alongside Jan Bielecki from Kiel University, probed this very trait in box jellyfish. Their research was recently revealed in the journal,  Current Biology.

The Complexity of Simple Neural Systems: New Findings on Box Jellyfish

Researchers delving into the Caribbean box jellyfish, Tripedalia cystophora, found surprising capabilities in these small, fingernail-sized creatures dwelling in Caribbean mangrove ecosystems. With their intricate visual network of 24 eyes, these jellyfish adeptly seek out minuscule copepods amid the mangrove roots. Though these roots provide fertile hunting grounds, they also pose dangers to these delicate beings.

Navigating this terrain, the box jellyfish have honed a precise timing mechanism. An early turn means missed prey, but a delayed one might result in a collision with the roots, jeopardizing their fragile frames. Mastering distance judgment is vital, and here, light contrast plays a pivotal role.

Anders Garm explained, “Our experiments show that contrast, i.e., how dark the root is in relation to the water, is used by the jellyfish to assess distances to roots, which allows them to swim away at just the right moment. Even more interesting is that the relationship between distance and contrast changes on a daily basis due to rainwater, algae, and wave action.”

He further observed, “We can see that as each new day of hunting begins, box jellyfish learn from the current contrasts by combining visual impressions and sensations during evasive maneuvers that fail. So, despite having a mere one thousand nerve cells – our brains have roughly 100 billion – they can connect temporal convergences of various impressions and learn a connection – or what we call associative learning. And they actually learn about as quickly as advanced animals like fruit flies and mice.”

These groundbreaking findings challenge long-standing beliefs in neuroscience.

“For fundamental neuroscience, this is pretty big news. It provides a new perspective on what can be done with a simple nervous system. This suggests that advanced learning may have been one of the most important evolutionary benefits of the nervous system from the very beginning,” added Garm.

Deciphering Memory’s Home within Neural Cells

The team identified the specific areas in box jellyfish where learning occurs. This discovery has opened a new avenue to examine the exact modifications that take place in a nerve cell during sophisticated learning processes.

Garm remarked, “We hope that this can become a supermodel system for looking at cellular processes in the advanced learning of all sorts of animals. We are now in the process of trying to pinpoint exactly which cells are involved in learning and memory formation. Upon doing so, we will be able to go in and look at what structural and physiological changes occur in the cells as learning takes place.”

Should the team successfully identify the specific learning mechanisms within jellyfish, their next ambition is to determine if this phenomenon is exclusive to jellyfish or ubiquitous among all creatures.

“Eventually, we will look for the same mechanisms in other animals, to see if this is how memory works in general,” Garm said.

For the author, “Understanding something as enigmatic and immensely complex as the brain is in itself an absolutely amazing thing. But there are unimaginably many useful possibilities. One major problem in the future will undoubtedly be various forms of dementia. I don’t claim that we are finding the cure for dementia, but if we can gain a better understanding of what memory is, which is a central problem in dementia, we may be able to lay a building block to better understand the disease and perhaps counteract it.”

THE METHODOLOGY

To emulate the conditions of a mangrove swamp, they recreated its environment in their lab, situating the box jellyfish in a controlled setting. In this space, the team adjusted the light-dark contrasts to study its impact on the jellyfish’s responses.

Their observations revealed that the learning curve for these jellyfish is often paved with unsuccessful evasive actions. Specifically, they adapt and learn after misjudging light contrasts, leading to collisions with roots. The integration of their visual perceptions with the physical jolt from such encounters teaches them to navigate better.

Garm added, “Our behavioral experiments demonstrate that three to five failed evasive maneuvers are enough to change the jellyfish’s behavior so that they no longer hit the roots. It is interesting that this is roughly the same repetition rate that a fruit fly or mouse needs to learn.”

The team further validated these learning patterns using advanced electrophysiology and classical conditioning techniques, pinpointing the exact locations within the jellyfish’s neural system where this learning transpires.

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