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 & 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