Do Jellyfish have Brains? How can Jellies Hunt and Swim without Brains?

Jellyfish nervous system

As a biologist, I’m fascinated with one of the most successful creatures on our planet – the jellyfish.  Let’s take a closer look at how they manage to survive… even when it seems they do not have a lot going for them!!

Jellyfish do NOT have brains (or bones, teeth, blood…or even hearts).  Instead, they have nets of neurons and specialized sensory structures along the edge of their bells called “rhopalia.”  Using their senses, jellyfish can discern light from dark, up from down.  Some species, such as box jellyfish, actively hunt and kill their prey.

Jellyfish have lived in the oceans for at least 500 million years and possibly more than 700 MY – long before the emergence of fish, trees, or flowering plants.  They are remarkably well-adapted for the conditions they face and have survived as the oldest group of multi-organ animals on the planet. 

I think you may be surprised to learn how complex these seemingly simple creatures really are.  Even though they consist of about 95 percent water, they manage very well without a brain!!

How can jellyfish survive without a brain?

When you think of all the organisms on our planet, we see incredible diversity all around us.  The creatures that we see are the ones that are best adapted to conditions on earth as we know it today.  It is sobering to think that 99% of all the creatures that have ever lived are now extinct.  Millions and millions of species have gone extinct during the period that jellyfish have survived.

Jellyfish have managed to thrive in our oceans for hundreds of millions of years because they have what it takes to pass on their genes from one generation to the next.  And they manage to do very well without having a brain.

In general, jellyfish can discern light from dark, and up from down using “rhopalia” – the specialized sensory structures along the edge of their bells.  Rhopalia combine some of the functions of our eyes, middle ears, and cerebellum all in one place…and perhaps even our sense of smell.

Brains require a lot of resources, biologically speaking, so an animal that can thrive without a brain is perfectly fit from an evolutionary standpoint.  How can that be?  The word “fit” can easily throw a lot of people.  That’s because we’ve all heard the phrase “survival of the fittest” and we automatically think of some big tough guy winning over some puny weakling.  But that’s not what fittest means in evolutionary terms.

Fitness – in evolutionary terms – means that an organism is well-fitted to survive in its current environment, with all the challenges and opportunities entailed.  If an organism can continue to do well as the environment around it changes, it will continue to survive and pass on its genes.  If not, it will go extinct. 

Jellyfish have demonstrated remarkable resilience in adapting to changes in the environment over many, many millions of years.

Jellyfish have a decentralized nervous system 

Instead of a single, centralized brain, jellies have neurons throughout their body that act like a net of nerves.  Rather than connecting to one central brain, neurons in the net interconnect with each other without any apparent sort of hierarchy or some being more important than others.

The rhopalia are spread evenly along the lower edge of the bell and together they “act like a pacemaker,” writes Rebecca Helm, jelly biologist with the University of North Carolina, “helping coordinate jelly movement, similar to the way our cerebellum coordinates ours.”

In addition to the neurons in the rhopalia, the rest of the nervous system is broken into two parts or neural nets; a “Large Nerve Net” and a “Small Nerve Net.”

The Large Nerve Net

Why is this set of nerves called large?  Turns out that the nerve cells that compose this net are large – literally huge!  The reason is that, unlike many animals, including mammals like us, jellyfish don’t have a specialized kind of fatty cell called oligodendrocytes. These specialized fatty cells wrap around the neurons, like wire insulation, making the nerves better conductors.  Consequently, even thin human neurons can transmit massive amounts of information.  Since jellyfish don’t have these fatty cells their neurons have to be huge to conduct the required amount of information quickly.

The Large Nerve Net ties directly into the muscle layer found along the underside of jellyfish, coordinating the rhythmic pulsing of these muscles.  When we see a jellyfish pulsing through the water, we’re watching the Large Nerve Net turn the entire jellyfish into a big beating heart.

Rhythmic pulsing of the Large Nerve Net as Jellyfish swims. Click to view in motion.

The Small Nerve Net

The second nerve net is the “Small Nerve Net.” This nerve net coordinates all the non-swimming behaviors, which can be more demanding than the rhythmic pulsing actions.  One example is feeding behavior, where the jellyfish sucks the food from the tentacle that has managed to capture prey. 

How can the jelly “know” which tentacle has food without a brain?!

Somehow the small nerve net helps the jelly differentiate between the various parts of its body and to coordinate movements and tasks between them.

Jellies actively swim without a brain; they’re not just aimless drifters!

Even though jellyfish don’t have brains per se, we can see that they do have surprisingly complex nervous systems.  Using these three components of their nervous systems – rhopalia, the large net and the small net – jellyfish have mastered their environment and developed the means to thrive and survive in the oceans. 

Jellyfish don’t just swim aimlessly—some species can navigate, using the small net to coordinate refined activities, such as hunting and feeding.

For example, box jellyfish have advanced eyes similar to those of humans. Their complicated eyes allow them to actively hunt and seek favorable habitats.  According to a recent Current Biology study. “These behaviors require not only accurate vision but also precise control of speed and direction of swimming,” write the researchers.

Irukandji by Lisa-ann Gershwin
Irukandji box jelly with stinging cell nodes. Photo by Lisa-ann Gershwin CC by 4.0

“The box jellyfish solution may thus be linked to the absence of a central brain, but it defeats the idea that a central brain is a prerequisite for advanced behavior,” write the researchers.

Check out my article on Irukandji jellyfish to learn more about tiny – but deadly – Irukandji box jellies and their amazing technique for attracting fish with lures and then killing them with some of the most powerful venoms on earth!

In Summary

Jellyfish may not have a centralized brain, but they do have a complex and effective nervous system:

  • One part of the nervous system, that includes the rhopalia, forms a ring where the neurons are concentrated to process sensory and motor activity.  
  • “Large Net” neurons send chemical signals to the muscles to rhythmically contract, allowing the animals to swim.
  • “Small Net” neurons help to coordinate more specialized behaviors and activities, such as hunting and feeding.

When it comes to jellyfish, there are lots more questions than answers! 

I aim to delve further into fascinating jellyfish behavior and biology in future posts!

What do jellies feed on if they don’t have teeth?

Do they have a circulatory system?

How do they get rid of waste?

References

Katsuki, T, Greenspan, RJ. Jellyfish Nervous Systems: Quick Guide GUIDE| Current Biology. Vol 23, Issue 14, PR592-R594, JULY 22, 2013. https://doi.org/10.1016/j.cub.2013.03.057

Satterlie RA, Eichinger JM. Organization of the ectodermal nervous structures in jellyfish: scyphomedusae. Biol Bull. 2014 Feb;226(1):29-40. doi: 10.1086/BBLv226n1p29. PMID: 24648205.

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George Sranko

George Sranko, B.Sc., MA (Hons), is a retired professional biologist, photographer, author and speaker. He has explored fascinating nature topics and epic destinations for over 40 years, beginning with his first job as a National Park naturalist. George is a popular destination and science lecturer on cruise ships throughout the world, with hundreds of presentations under his belt. He has visited over 90 countries and happily shares his personal experiences and insights in a dynamic and entertaining style.

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