She told of her fantail goldfish, badly injured by an animal attack three years ago, and asked me if it feels pain like we do. The short answer is no. In other words, only humans feel pain. Human awareness of pain itself may add a whole new dimension to the experience, but we can also safely presume the Dictionary's editorial board was staffed by only one species. This question is not so easily answered.
Bacteria have receptors to many environmental stimuli.
Single-celled pond creatures such as bacteria, algae, and protozoa are like responsive but unfeeling aquatic automatons. There are multiple-celled animals with specialized pain sensors and nerves, yet no detectable central nervous system. About fifteen years ago, researchers presented cotton-wrapped electrodes, soaked in the juices of marine mussels, to hungry starfish. After several hundred repeats, about a third of the starfish lost their taste for mussels. Despite the lack of a brain to accumulate and process memories, shocks penetrated beyond the moment of experience and triggered learning.