Pathways to complex cognition without a centralized brain: Insights from the moon jellyfish

Cnidarians, such as jellyfish, are often described as “brainless” and incapable of complex behavior. This assumption stems from the belief that cognitive complexity requires centralized brain structures; moreover, the lack of behavioral evidence has failed to challenge this view, reinforcing it in a circular way. Here, we question this notion by testing juvenile moon jellyfish (Aurelia spp.) for behavioral markers of cognition: specifically, short-term memory and novelty preference. We observed how 16 young jellyfish responded to familiar versus novel stimuli. During an initial exposure phase, individuals explored a single object. In a subsequent test phase, they were presented with both the familiar and a novel object. Jellyfish consistently explored the novel object, indicating a clear preference for novelty. In cognitive ethology, such exploratory behavior is considered a form of curiosity, an intrinsic motivation to seek new information. We demonstrate that jellyfish can discriminate between familiar and unfamiliar elements in their environment and show evidence of curiosity-like behavior. To date, the box jellyfish Tripedalia cystophora (Cubozoa), which features the most complex decentralized nervous system and rhopalia in Cnidaria, has been shown capable of associative learning by integrating visual and mechanical stimuli. Here, we report behaviors typically associated with higher-order processing in Aurelia spp., which possess a less developed nervous and sensory system. Rather than treating such cases as anomalies, we argue that these findings support a broader, more inclusive framework for studying animal cognition, one that recognizes how diverse neural architectures can support adaptive cognitive functions shaped by ecological context.