Pacific sea nettles, Chrysaora fuscescens. These beautiful jellyfish occasionally grow to more than a metre in diameter in the wild. They feed as they drift, paralysing prey with stings from their 24 undulating maroon tentacles. For humans, their stings are often irritating, but rarely dangerous.
This image was taken by Jessica Chatburn for the Royal Photographic Society's International Images for Science 2016 competition.