Even without brains, creatures like jellyfish and sea anemones can learn from experience.
Sea stars, jellyfish, sea urchins, and sea anemones don’t have brains — yet they can sense danger, catch prey, and react to what’s happening around them. So, does that mean creatures without brains can actually think?
According to Simon Sprecher, a neurobiology professor at the University of Fribourg in Switzerland, brainless doesn’t mean neuron-less. Except for a few very simple organisms like marine sponges and placozoans, almost all animals have neurons — the cells responsible for processing information.
Many “brainless” marine species, such as jellyfish, hydras, and sea anemones, have what’s called a nerve net — a web of neurons spread throughout their bodies and tentacles. “This network can process sensory input and create coordinated movements like swimming, feeding, or stinging — all without a central brain,” explained Tamar Lotan, a biologist at the University of Haifa in Israel.
This decentralized nervous system is far from primitive. Sprecher’s research showed that the starlet sea anemone (Nematostella vectensis) can actually form memories. In one experiment, anemones learned to associate a harmless flash of light with a mild shock. Eventually, just the light made them retract — clear evidence of associative learning.
Another study revealed that sea anemones can even recognize genetically identical neighbors and show less aggression toward them after repeated contact — suggesting a form of self versus non-self awareness.
Meanwhile, research by Jan Bielecki from Kiel University found that box jellyfish can learn to connect what they see with what they feel — avoiding obstacles by linking visual cues to the sensation of bumping into things. “I believe learning can occur even within a single neuron,” Bielecki said.
So, do these abilities mean brainless animals are capable of thought?
That depends on how you define “thinking.” As Ken Cheng, an animal behavior expert at Macquarie University in Australia, pointed out, scientists often prefer the term “cognition.” It refers to how organisms process information and use it to make decisions — not necessarily the kind of conscious thought humans have.
“If we define cognition as using information from the world to guide behavior, then all living organisms do it,” Cheng said. Even simple creatures like sponges process environmental cues to survive. However, advanced cognition, which might involve consciousness or self-awareness, is still uncertain in animals without brains.
According to Sprecher, basic cognition can be seen as any behavioral change that goes beyond simple reflexes — and by that definition, these marine animals clearly show it.
Lotan added that cnidarians — the ancient family that includes jellyfish and anemones — have survived for over 700 million years, outlasting many species with more complex brains. “Their success suggests they’ve evolved a unique way of adapting and thriving without a brain,” she said. Their neurons may allow them to perceive and respond to the world in a primitive but effective way — perhaps the earliest form of thinking on Earth.