A recent study in the journal of Current Biology titled "Associative learning in the box jellyfish Tripedalia cystophora" presents evidence that some species of gelatinous medusa are capable of associative learning — which is to say, they can remember things, and apply that knowledge to their future decision-making — despite not having brains. This is particularly remarkable because the Tripedalia cystophora is so damn small — about the size of a fingernail — and that this kind of operant conditioning has rarely been seen in other, similar animals, even those with more obviously complex nervous systems. From the summary:
The obstacle avoidance behavior (OAB) is a species-specific defense reaction (SSDR) for T. cystophora, so identifying such SSDR is essential for testing the learning capacity of a given animal. Using the OAB, we show that box jellyfish performed associative learning (operant conditioning). We found that the rhopalial nervous system is the learning center and that T. cystophora combines visual and mechanical stimuli during operant conditioning.
Here's how the researchers studied the jellies, according to Nautilus:
Garm and Bielecki set up an experiment. They outfitted a fish tank with a bunch of gray and white plastic strips, which were meant to mimic the tree roots and light streaming through. The trick was that to the jellyfish, the light gray color looked like a root that was simply far away. "In reality it wasn't far away. It just seemed far away because we painted it gray," Garm explains. Initially, the jellies perceived the gray "roots" as being remote—and bumped into them. But after a few bumps, they would learn to avoid the gray strips. "They get the mechanical sensory input telling them, OK, this root was much closer than it originally appeared," says Garm—and they start changing their behavior.
Less than 10 minutes into the experiment, the jellies quadrupled the number of successful pivots to avoid collision, scientists found. "They learned that in this condition, low contrast still means that the 'root' is close by and then within three to five errors of bumping into the root they learn to turn earlier and not bump into it," Garm says. "We were surprised at how quickly they learned." This form of learning is called associative learning: The jellies learn to associate sensory stimuli—such as images of the gray strips—with bumps, to remember this association and then adapt future behavior.
Jellyfish are, of course, giant nerve clusters without any sort of central nerve processing system like a brain. And that's the fascinating takeaway from this study: the idea that having a central nerve processing system might not actually be a necessity for forming memories. The question remains, however: where do they store this memory-like information? The hypothesis right now is that the memory information is basically distributed amongst the cells of the jellyfish's distributed nervous system: "These neurons form a memory of sensory stimulation, which then becomes associated with a particular behavior: The image of the root, plus the bump, leads to avoidance."
I, for one, welcome our jellyfish overlords.
Associative learning in the box jellyfish Tripedalia cystophora [Jan Bielecki, Sofie Katrine, Dam Nielsen, Gösta Nachman, Anders Garm / Current Biology]
How to learn without a brain [Lina Zeldovich / Nautilus]