Sean Carroll in The New York Times:
I have long suspected that fish are smarter than we give them credit for.
As a child, I had an aquarium with several pet goldfish. They certainly knew it was feeding time when my hand appeared over their tank, and they excitedly awaited their delicious fish flakes. They also exhibited a darker, disturbing behavior. Evidently, a safe life with abundant food was not fulfilling. From time to time, either sheer ennui or the long gray Toledo winter got to one of the fish and it ended its torment with a leap to my bedroom floor. Maybe my anthropomorphizing is a bit over the top. But, really, just how smart are fish? Can they learn?
A 10-gallon tank with a plastic sunken pirate ship is certainly not the most stimulating habitat. But in the colorful, diverse and dangerous world of coral reefs, fish must be able to recognize not only food, but also to discriminate friends from foes, and mates from rivals, and to take the best action. In such a complex and dynamic environment, it would pay to be flexible and able to learn. A series of studies has recently revealed that reef fish are surprisingly adaptable. Freshly caught wild fish quickly learn new tasks and can learn to discriminate among colors, patterns and shapes, including those they have never encountered. These studies suggest that learning and interpreting new stimuli play important roles in the lives of reef fish.