Our brain is filled with many specialized structures that process visual information, process memories, interpret language, and more. One way to try to understand the capabilities of the brain is to compare it with the brains of other species. That is, to compare what structures exist in the brain and what behaviors those brains support.
But what if animals don’t have brains? Perhaps most of the behaviors we’ve seen require at least some kind of central nervous system. However, there are many species such as sea anemones, corals, and jellyfish that have fairly diffuse neural networks and apparently lack something like a brain. This is most often related to Ivan Pavlov (sorry).
Are our cnidarians learning?
Associative learning is exactly what it sounds like. Through repetition, animals learn to associate an event with something otherwise unrelated to the event. In Pavlov’s case, dogs were trained to associate certain sounds with feeding. After training, the dogs began to salivate when they heard the sound, even without food. It is easy to see that a vast range of animals are capable of associative learning and how it can provide selective advantage.
But does it require a brain?phylum animal cnidarianIncluding everything from tiny hydras to giant jellyfish. Instead, we have a diffuse network of nerve cells called the neuropil. Apparently, the neural network can coordinate whole-body activity, just as jellyfish can be seen swimming in rhythmic contractions.Cnidarians can also respond to environmental stimuli. Many of them have structures similar to the eye. So, while the neural network lacks the specialized structures found in the brain, it is clearly capable of performing some of the functions we normally associate with the brain.
(Cnidarians also have the distinction of being radially symmetrical, which contrasts with bilaterians like us, who have different sides on each side.)
But can it handle learning? A group of European researchers (Gaelle Botton-Amiota, Pedro Martinez, Simon Sprecher) were intrigued by this question and found it largely unanswered. There have been several papers alluding to the fact that some cnidarians can form stable associative memories, but no rigorous research has been done on this issue and no one has followed up on the initial studies. .
shock anemone
Their experiments are very simple, and the research paper is very compact and easy to understand. Researchers knew that sea anemones can sense light (they Nematostella vectensis), although lacking the eyes found elsewhere in cnidarians.
By itself, the shock causes the animal to contract its body and attract its tentacles. Approximately 20% of animals respond similarly when exposed to light. But after her hour-long training period, in which the animals were repeatedly exposed to light and shock, the situation changed dramatically. The percentage of animals that responded to light alone decreased by about half to about 10%. Shock alone produced a similar frequency of reactions.
To get consistent contractions, the researchers had to apply the light and the shock simultaneously, which caused approximately 70% of the animals to contract. (Although this is lower than the “strong, fast, reversible contractions” seen at the start of the experiment, the animals have been in shock for over an hour at this point and some adaptation is to be expected.) Labor was induced within 1 minute of shock, but less frequently (about 30% of the time).
So this is not the kind of conditioning we see in Pavlov’s dogs. Pavlov’s dogs learn associations so that they can respond to irrelevant cues. Instead, the animal here learned to interpret her two stimuli as one trigger he did and began to need both in order to respond.
All of this indicates that there must be some form of central coordination within the neural network that allows the animal to integrate environmental cues and provoke responses that include most of the organism. There are sections of the neural network in which neurons are concentrated, but these do not appear to be associated with any particular function. It’s not clear.
These results can be interpreted in light of recent results showing that disordered neurons in culture dishes can ‘learn’ play. ponThis was interpreted to indicate that neurons natively develop expectations for the inputs they receive and adjust their behavior when those inputs do not match. That action could generate associative memory without the need for central organization. It also suggests that the neural network should be able to easily undo the association if the two triggers are no longer linked. This is very easy to test.
PNAS, 2023. DOI: 10.1073/pnas.2215324120 (About DOI).
