Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.08.23.263426v1?rss=1 Authors: Crook, R. J. Abstract: Tonic pain is an ongoing, negative affective state arising from tissue damage or inflammation. Because pain is aversive and its relief is innately rewarding, mammals learn to avoid a context in which pain is experienced, and prefer one where pain relief occurs. It is generally accepted that vertebrate animals experience pain, however, there is currently no conclusive evidence that pain occurs in any invertebrate. Here we show that octopuses exhibit tonic pain behavior after subcutaneous injection of dilute acetic acid into the skin of one arm. In conditioned place preference assays, octopuses avoid contexts in which pain was experienced, prefer a location in which they experienced tonic pain relief, and show no conditioned preference in the absence of pain. Octopuses are thus the first invertebrate shown to experience pain. Copy rights belong to original authors. Visit the link for more info