Upside-down Jellyfish

• biologíaUnlike most jellyfish it spends most of its adult life lying upside down on the sea floor, holding the photosynthetic algae in its tissues up to the sunlight.
• It releases into the water small packets of stinging cells called cassiosomes, which drift and cause the itching swimmers feel without ever touching a jellyfish, a mechanism described in 2020.
• It is one of the few brainless animals in which a sleep-like state has been demonstrated, with periods of reduced activity and reduced response to stimuli.
• It lives upside down. It settles on the bottom with its bell beneath and its oral arms pointing up, to expose to the sun the symbiotic zooxanthellae held in its tissues.
• Those algae supply much of its food by photosynthesis, so in practice it behaves more like a plant than like a hunting jellyfish.
• You do not have to touch it to be stung: it releases cartridges of mucus loaded with stinging cells into the water, producing the so-called stinging water that swimmers feel without having brushed against anything.
• In 2017 it was shown to enter a resting state comparable to sleep, with reduced activity and reduced responsiveness to stimuli, the first evidence of sleep in an animal without a brain.
• It has spread through the eastern Mediterranean via the Suez Canal and is one of the most conspicuous Lessepsian species.

Habitat and environment

Shallow bottoms of tropical mangroves, lagoons and sheltered bays

Sources consulted: Forsskål (1775); Nath et al. (2017), Current Biology; Ames et al. (2020), Communications Biology