Cassiopea (upside-down jellyfish) is a genus of true jellyfish and members of the family Cassiopeidae.[3] They are found in warmer coastal regions around the world, including shallow mangrove swamps, mudflats, canals, and turtle grass flats in Florida, the Caribbean and Micronesia. The medusa usually lives upside-down on the sea floor in shallow areas, which has earned them their common name. These jellyfish partake in a symbiotic relationship with photosynthetic dinoflagellates and therefore, must lie upside-down in areas with sufficient light penetration to fuel their energy source.[4] Where found, there may be numerous individuals with varying shades of white, blue, green and brown.
Cassiopea species have a mild sting since they are primarily photosynthetic, but sensitive individuals may have a stronger reaction. The photosynthesis occurs because, like most corals, they host zooxanthellae in their tissues.[6] The stinging cells are also found in cellular masses, dubbed "cassiosomes", excreted in a mucus; people swimming near the jellyfish may come in contact with these cassiosomes and be stung.[7][8] The stings, appearing in the form of a red rash-like skin irritation, are known for being extraordinarily itchy. Sometimes this jellyfish is picked up by the crab Dorippe frascone and carried on its back[citation needed]. The crab uses the jellyfish to defend itself against possible predators.
Certain species of Cassiopea have been observed to enter a sleep state - exhibiting decreased pulsation rate, reduced responsiveness to stimuli, and compensatory rebound sleep after deprivation.[9] They are the first animals with no central nervous system to have been observed sleeping.[10]