Our brains are filled with lots of specialized structures that do things like process visual information, handle memories, or interpret language. One of the ways we try to understand what a brain is capable of is by comparing it with the brains of other species—what structures are present in the brain, and what behaviors those brains support.

But what if the animal doesn't have a brain? Presumably, most of the behaviors we've looked at require at least some sort of centralized nervous system. But there are a lot of species, including anemones, corals, and jellyfish, that have a fairly diffuse nerve net and lack anything that's clearly brain-like. But apparently, that's enough to perform associative learning, the sort most often (forgive me) associated with Ivan Pavlov.

Is our cnidarian learning?

Associative learning is pretty much what it sounds like: Through repetition, an animal learns to associate an event with something that's otherwise unrelated to that event. In Pavlov's case, he trained dogs to associate a specific sound with being fed. Once trained, the dogs would start salivating once they heard the noise—even if food wasn't present. A huge range of animals are capable of associative learning, and it's easy to see how it can provide a selective advantage.

But does it require a brain? Animals in the phylum Cnidaria, which includes everything from the tiny hydra to enormous jellyfish, lack a centralized structure that we can recognize as a brain. Instead, they have a diffuse web of neurons called a nerve net. Obviously, the nerve net is capable of coordinating activity across the entire body, as we can watch jellyfish swim through rhythmic contractions. Cnidarians can also respond to environmental stimuli; a number of them have structures that are analogous to eyes. So, while the nerve net lacks the sort of specialized structures seen in brains, it's clearly capable of performing some of the functions we normally associate with a brain.

(Cnidarians also have the distinction of being radially symmetric, as opposed to bilaterians like ourselves, which have distinct sides.)

But can it handle learning? A group of European researchers (Gaelle Botton-Amiota, Pedro Martinez, and Simon Sprecher) got curious about the question and found that it had largely gone unanswered. There were a couple of papers that hinted some cnidarians could form stable associative memories, there hadn't really been a rigorous look at the question, and nobody had followed up on the initial work.

Shock the anemone

The experiment they did was remarkably simple, making for a very compact and straightforward research paper. The researchers knew that anemones can sense light (they worked with a species called Nematostella vectensis), although they lack the eyes seen elsewhere among the cnidarians. So, they combined exposure to light with an unpleasant sensation—namely an electric shock.

On its own, the shock would cause the animals to contract their bodies and pull in their tentacles. Exposure to light would cause about 20 percent of the animals to respond in the same way. But, after an hour-long training period in which the animals had the light and shock paired repeatedly, things changed dramatically. The percentage of animals that responded to light alone dropped by about half, to roughly 10 percent. The shock alone caused a similar frequency of responses.

To get a consistent contraction, the researchers had to give the light and shock simultaneously, which caused about 70 percent of the animals to contract. (This is lower than the "strong, fast, and reversible retraction" seen at the start of the experiment, but the animals had been receiving shocks for over an hour by this point, and some level of acclimation is expected.) Separating the light from the shock by a minute did trigger contractions, but at a reduced frequency (about 30 percent of the time).

So, this isn't the sort of conditioning seen with Pavlov's dogs, where the animal learns an association in a way that allows it to respond to an unrelated cue. Instead, the animals here learned to interpret the two stimuli as a single trigger, and so started needing both to respond.

All of this indicates that there must be some form of central coordination within the nerve net that allows the animal to integrate environmental signals and trigger a response that involves most of the organism. Many cnidarians have sections of the nerve net with higher concentrations of neurons, but these don't seem to have been associated with any specific functions. So, it's not clear whether this memory and coordination is associated with any sort of physical structure.

It's possible to interpret these results in light of recent results showing that disorganized neurons in a culture dish could "learn" to play Pong. This was interpreted as an indication that neurons natively develop expectations for the inputs they will receive and adjust their behavior when those inputs don't match. That behavior could produce an associative memory without requiring any central organization. It also would suggest that the nerve net should be able to easily unlearn the association if the two triggers stop being linked—something that would be very easy to test.

PNAS, 2023. DOI: 10.1073/pnas.2220685120  (About DOIs).