How smart are my pet jellyfish?

If you know me, you probably know that I have pet jellyfish. Moon jellies, to be specific. It may sound like a strange pet to have—and I admit, they were a not-very-well-thought-out impulse purchase—but, even despite a few painful run-ins with Portuguese man-of-wars in my childhood, I have always been enchanted by the majestic beauty of jellyfish. They have long been my favorite exhibit in any aquarium—ghostly creatures, seemingly suspended in space and time, floating gracefully to an eery, dream-like soundtrack. A work colleague of mine eloquently described the impression he got while watching them at a recent trip to the aquarium: “They looked like raw pieces of life desperately trying to be more than mere matter.”

But it was not until I started observing my own Khal Drogo, Khaleesi, and Jorah daily on my kitchen counter that I realized just how stupid they are.

First off, they seem to exhibit no foraging behavior. They cannot “search” for food. The only way they can “eat” is to randomly swim around, while whatever “food” they happen to swim through gets caught in their tentacles and drawn up through their digestive systems. Astounding, really, how these species have been able to survive for the millions of years they have. And still going strong.

Furthermore, they seem to have only one swimming motion, which is to synchronously contract all their muscles, creating thrust by expelling water from their ventral side, thereby propelling themselves in the opposite direction. [More on swimming →] As a result, they can only swim in one direction, and must rely on the current of the water to change direction. So another behavior that I observe time and time again is that they swim down to the bottom of the tank, such that their bell is pushed up against the bottom or into a corner, and continue to “swim,” executing the same pumping motion over and over again, continually pushing themselves into the same corner. I wonder to myself, Do they not sense that they are swimming into an object? Or can they simply do nothing about it? I do know that they have some “touch” receptors, for they do seem to react by contracting in a funny way when I touch them….

My sister visited a few weeks ago, and tried to explain how stupid they are. “How can they be so stupid?” she asked. “They don’t have brains,” I answered. “What do you mean they don’t have brains?!?! How do they swim and eat if they don’t have brains?” “Well, they do have neurons. They at least have motor neurons to swim, and probably something like pacemaker neurons to control the rhythm of their swim. And they must have some sensory neurons too,” I speculated. This was followed by a funny discussion about whether or not jellyfish sleep (they don’t).

So this got me thinking: Just how many neurons do jellyfish have? They are obviously primitive. More primitive than any species that has a brain, like fish. But where do they fall on the scale with creatures like nematodes (302 neurons) and sea slugs (18,000 neurons)? This number has been surprisingly hard to find. Wikipedia reports 800, but I was unable to verify their reference.

During my research into this question, I found that I was right about the primitive nature of their nervous system: They are said to have the most basic nervous system of any multicellular organism. I think this is said because they have no “brain” (centralized network of neurons), but instead have a loose network of neurons referred to as a “nerve net,” located throughout the epidermis (skin). However, I learned they are not nearly as stupid as I thought.

Their nervous system is made up of three main subsystems: the rhopalium, the motor nerve net, and the diffuse nerve net. The rhopalium includes sensory receptors for light, gravity, touch, and chemicals (aka smell), as well as pacemaker neurons for controlling the basic swim rhythm. The motor nerve net is responsible for activating the muscles cells in response to signals from the pacemaker cells for swimming. The diffuse nerve net “induces marginal tentacle contraction” and “is believed to relay sensory information to the musculature indirectly be affecting the pacemaker activities, and directly by providing modulatory input to the musculature.” So this most primitive nervous system is actually capable of some fairly sophisticated behaviors, including “sun compass navigation, diel vertical migration, avoidance of low salinity, escape from contacts with predators, and formation of aggregates.” [reference]

Motor nerve net (A) and diffuse nerve net (B) from a single location in the subumbrella (area of swim musculature) in Aurelia aurita (moon jelly). Figure from [Satterlie]

Another type of jellyfish, cubozoans (box jellyfish), have much more sophisticated nervous systems, including “highly developed image-forming eyes with cornea, pupil, lens, and retina, much like our own.” These species can accomplish even more sophisticated behaviors, such as vision-based navigation to their preferred habitats as well as vision-based obstacle avoidance. Some even engage in courtship rituals, where males gravitate towards females who exhibit dark orange spots, called “velar spots,” which are a sign of sexual maturation. [reference]

I have definitely not seen the sophisticated behaviors that my jellies are supposedly capable of. But, they are still pretty entertaining roommates.

I leave you with my latest production, a music video called “Moon Jelly Disco.”

PS: If you want to see some really crazy jellies, and other bizarre creatures, check out the BBC documentary, The Blue Planet, Episode 2: “The Deep” (available streaming on Netflix.)