Dyani Lewis in Nature:
A tiny jellyfish has, for the first time, demonstrated a mighty cognitive capacity — the ability to learn by association. Although it has no central brain, the finger-tip-sized Caribbean box jellyfish (Tripedalia cystophora) can be trained to associate the sensation of bumping into something with a visual cue, and to use the information to avoid future collisions. The experiment shows a type of learning called associative learning — made famous by neurologist Ivan Pavlov’s experiments with dogs in the late-nineteenth century — in which an animal learns to associate one stimulus with another through training. “Associative learning is now considered solid evidence of cognitive capacity,” says Ken Cheng, an animal behaviour researcher at Macquarie University in Sydney, Australia. Many other animals — from humans to birds, octopuses and even insects — have the ability to learn by association.
“The box jellyfish finding is very important because it shows that a centralized nervous system, or brain, is not necessary for associative learning,” says Pamela Lyon, a cognitive biologist at the University of Adelaide, Australia. “It’s super,” says Gaëlle Botton-Amiot, a neurobiologist at the University of Fribourg in Switzerland, who published a study in March1 showing that the sea anemone Nematostella vectensis is also capable of associative learning. Sea anemones and jellyfish both belong to a group of organisms known as cnidarians, and Botton-Amiot thinks that “this ability to do associative learning is present across probably the entire cnidarian tree”.
More here.