In mangrove forests, deep below the Caribbean Sea, floats the tiny box jellyfish. These jellyfish have no brain; they rely on their simple nervous system to receive and interpret environmental information. While it may seem very simple to observers, recent experiments indicate that the jellyfish Tripedalia cystophora is the third cnidarian, an aquatic invertebrate animal of the phylum Cnidaria, capable of learning. In addition, the discovery of jellyfish learning may be one of the keys for scientists to understand better the history of organismal brains and nervous systems, which are important factors in our own evolutionary and learning history.
These fascinating discoveries can be credited to a group of researchers who, less than a week ago, published their quantified findings on the Tripedalia cystophora jellyfish’s behavioral responses to varying stimuli. The behavior the scientists focused on was the jellyfish’s avoidance of obstacles, in this case, prop roots, which they often find in their natural environment. Lacking complex visual abilities, the jellyfish see the roots as mere black obstacles in contrast to the pale appearing water. However, the water’s contrast is constantly shifting; weather, waves, other life forms, and many other factors can influence the general “murkiness” of the water. Jellyfish must learn to avoid collisions even in inconsistent waters, thereby increasing their general survivability. How did the researchers prove this learning actually occurs?
Prior to any testing, a hypothesis was formed. The researchers had to identify exactly what the jellyfish experiences after a failed maneuver to avoid an obstacle. The team hypothesized that the jellyfish interpret both images of contrast and distance, along with a physical stimulus prompted by hitting roots on the seafloor. Through this experience, specifically the exposure to visual and physical stimuli, these jellyfish learn to avoid hitting obstacles.
To test their hypothesis, the researchers set up an experiment with a combination of visual and physical stimulation, along with two control group experiments that had only one naturally functioning stimulus. They believed that both visual and physical stimuli were necessary for Tripedalia cystophora learning and that those tests without one of the two should not exhibit any. The test with both visual and physical stimulation placed jellyfish in a container with roughly 6-inch black and white stripes lining the walls. To jellyfish, this appears similar to their natural root-filled habitat in the Caribbean. The jellyfish swam freely in the tank, bumping into the low-contrast walls. However, as time passed throughout the 7.5-minute trial, the jellyfish collided with the wall-less frequently and performed their obstacle avoidance behavior more often. Collisions decreased as time passed, making it clear that the jellyfish learned to interpret the distance and contrast images through the physical stimulus and experience of hitting the wall. The researchers’ predictions that the jellyfish would learn under these circumstances were correct!
However, to prove their hypothesis, the researchers had to demonstrate that this learning depends on both physical and visual stimuli. The way to do this was by isolating each stimulus to see if the jellyfish could learn from a single working stimulus. To isolate the visual stimulation, the jellyfish was placed in the same container and shocked by low-electrical pulses to mimic contact. The jellyfish responded to this by performing a U-turn to avoid obstacles, even though they had not hit anything. Although they had access to their visual abilities, malfunctioning physical stimuli caused the jellyfish to perform a disadvantageous behavior, showing that they do not have the same learning capabilities only with vision.
To isolate the physical stimuli, scientists placed jellyfish in a tank surrounded by a high white contrast background. Having no dark objects to guide them, the jellyfish spent the 7.5-minute trial swimming into the tank’s wall with little to no collision dodges. This experiment shows that the jellyfish are also incapable of learning with only physical stimuli. The researchers have demonstrated that their original hypothesis, that jellyfish rely on the use of both physical and visual stimuli to learn to dodge obstacles, was correct.
This research will come as a surprise to many people, as scientists have long believed that learning is exclusively possible for animals with much more complex nervous systems. Nonetheless, it is now indisputable that our old perception of learning was too limited. This new information about animal nervous systems will likely have many researchers reconsidering previous animal learning and evolutionary history beliefs.
References
Bielecki J, Dam Nielsen S, Nachman G, Garm A. Associative learning in the box jellyfish Tripedalia cystophora. Current Biology. 2023; 1-16.
https://www.cell.com/current-biology/fulltext/S0960-9822(23)01136-3?utm_campaign=Press%20Package&utm_medium=email&_hsmi=274356635&_hsenc=p2ANqtz-9_AW0rUk-j0VizHbaZvuqlPwEkm_treDatWBhyCsXtcjsxltDeth-AdtS2eeEBzi_Hmf6fplNQU5ewxL-JJExqSSZCfhwl88XhczSTWdUMxVVe27w&utm_content=274356635&utm_source=hs_email
