Jellyfish, those seemingly simple and ethereal creatures drifting through the ocean, have recently defied scientific expectations by demonstrating their ability to learn from past experiences. In a groundbreaking study published on September 22 in the journal Current Biology, researchers have unveiled that Caribbean box jellyfish (Tripedalia cystophora) can acquire and apply knowledge, challenging long-held notions that advanced learning necessitates a centralized brain. This discovery provides valuable insights into the evolutionary origins of learning and memory.
Despite lacking a central brain, these Caribbean box jellyfish, no larger than a fingernail, boast a surprisingly complex visual system comprising 24 eyes embedded within their bell-shaped bodies. Thriving in the mangrove swamps, these enigmatic creatures employ their visual prowess to navigate the murky waters and gracefully maneuver around underwater tree roots while hunting for prey. The study demonstrated that these jellyfish have the capacity for associative learning, a process in which organisms establish mental connections between sensory stimuli and behavioral responses.
Jan Bielecki, the first author of the study and affiliated with Kiel University, Germany, emphasized the significance of leveraging an animal’s natural behaviors when teaching it new skills. He stated, “Learning is the pinnacle performance for nervous systems,” emphasizing the importance of aligning the training process with an organism’s innate behaviors to unlock its full learning potential.
To replicate the natural environment of these jellyfish, researchers designed a circular tank adorned with gray and white stripes, with the gray stripes emulating distant mangrove roots. Over a span of 7.5 minutes, the jellyfish were observed in this tank. Initially, they swam close to the distant stripes, frequently colliding with them. However, as the experiment progressed, the jellyfish displayed a remarkable improvement in their ability to navigate, increasing their average distance from the tank wall by approximately 50%, quadrupling the number of successful pivots to avoid collisions, and halving their contact with the tank wall. These findings highlight the jellyfish’s capacity to learn from both visual and mechanical stimuli.
Anders Garm, the senior author of the study and associated with the University of Copenhagen, Denmark, emphasized the advantages of studying relatively simple nervous systems, such as those found in jellyfish, to gain a deeper understanding of complex neural structures and behaviors. He stated, “Looking at these relatively simple nervous systems in jellyfish, we have a much higher chance of understanding all the details and how it comes together to perform behaviors.”
The researchers delved further into understanding the associative learning process by isolating the visual sensory centers within the jellyfish, referred to as rhopalia. Each rhopalium houses six eyes and generates pacemaker signals that govern the pulsing motion of the jellyfish, which intensifies when avoiding obstacles. To simulate the jellyfish’s approach to objects, researchers presented the stationary rhopalium with gray bars. Initially, the rhopalium remained unresponsive to light gray bars, interpreting them as distant objects. However, after the researchers trained the rhopalium by delivering mild electrical stimulation upon the approach of bars, it began generating signals for evading obstacles in response to the light gray bars. This revealed that combining visual and mechanical stimuli is essential for associative learning in jellyfish and established the rhopalium as a key learning center.
In the next phase of their research, the team aims to delve deeper into the cellular interactions within jellyfish nervous systems to unravel the intricacies of memory formation. Additionally, they seek to gain a more comprehensive understanding of how the bell’s mechanical sensor operates to provide a comprehensive understanding of the jellyfish’s associative learning capabilities.
Anders Garm expressed his astonishment at the speed of learning displayed by these creatures, comparable to that of more advanced animals. This discovery hints at the existence of a profoundly fundamental cellular mechanism for advanced learning, possibly originating at the dawn of the evolution of the nervous system.
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