Can Animals Without Brains Still Think? What Jellyfish and Sea Anemones Reveal About Cognition

Can animals without brains still think?

Many familiar sea creatures — including jellyfish, sea anemones, sea urchins and starfish — lack a centralized brain yet still capture prey, detect threats and respond to their environment. That raises an intriguing question: does having no brain mean an animal cannot think?

Neurons without a brain

"Brainless" does not automatically mean neuron-free. Aside from a few primitive animals such as marine sponges and placozoans, most animals possess neurons. Instead of a brain, some species have a diffuse nerve net: a web of interconnected neurons spread across the body and tentacles. These networks can collect sensory information and coordinate organized motor responses like swimming, contracting, feeding and stinging.

Surprising behaviors and learning

Despite their simple organization, nerve-net animals show complex behaviors. Research on the starlet sea anemone (Nematostella vectensis) demonstrated associative learning: an innocuous flash of light paired repeatedly with a mild shock eventually caused the anemone to retract when presented with the light alone. Other studies found that anemones can learn to recognize genetically identical neighbors and reduce their territorial aggression after repeated encounters, suggesting they can discriminate between "self" and "non-self."

Work on box jellyfish shows they can link visual cues to the tactile experience of bumping into obstacles, improving navigation around objects. Some researchers even argue that individual neurons can support forms of learning and memory.

Thinking, cognition and definitions

Whether these abilities count as "thinking" depends on definitions. Scientists more commonly use the term cognition, broadly understood as information processing: using sensory input and internal states to guide behavior. Under this wide definition, nearly all life-forms could be said to perform cognitive processing.

When researchers talk about advanced cognition — abilities beyond basic associative learning, such as abstract reasoning, self-awareness or conscious thought — they are less certain that animals without centralized brains qualify. Terms like "thinking" and "consciousness" remain imprecise and are defined differently by psychologists, biologists and neuroscientists.

Why this matters

Cnidarians (the group that includes jellyfish and sea anemones) evolved more than 700 million years ago and have endured while many brain-bearing lineages went extinct. Their resilience suggests their neural architectures provide robust adaptive strategies that allow sensing, interpretation and behavioral flexibility — arguably a rudimentary form of information-based problem-solving.

Bottom line: Animals without centralized brains can sense, learn and change behavior in response to experience. Whether this amounts to "thinking" depends on how strictly the term is defined; most scientists prefer the more precise concept of cognition and distinguish simple information processing from higher forms of conscious thought.