Immortal jellyfish

Turritopsis dohrnii, the immortal jellyfish (Oceaniidae), is a small hydrozoan with a bell up to about 4.5 mm wide, 80–90 marginal tentacles and a bright red, cross-shaped stomach. It is widely distributed in temperate to tropical seas, living in coastal shallows, on docks and buoys, at roughly 14–25 °C and salinities of 18–40. It feeds on small zooplankton such as copepods, cladocerans and fish eggs. Under injury, starvation or abrupt change in temperature or salinity, the medusa can transdifferentiate back into a polyp, repeatable in the laboratory. As of 2026 it has not been assessed by IUCN.