Cassiopea pulses its bell against the sediment, which anchors it, and holds its branched oral arms towards the light. Those arms contain dinoflagellate symbionts of the genus Symbiodinium, the same broad group that powers reef-building corals. Photosynthate transferred from the algae supplies a substantial fraction of the jellyfish's carbon budget, supplemented by captured plankton.
The orientation follows from that. An animal deriving energy from light needs its symbiont-bearing tissue exposed to the sun rather than shaded under a bell, so the usual jellyfish posture is inverted. Continued bell pulsation drives water over the arms, delivering nutrients and removing waste.
Cassiopea also releases mucus containing clusters of stinging cells into the water column, sometimes called cassiosomes. Swimmers passing over a bed can develop itching and rash without touching an animal, which explains the long-reported phenomenon of stinging water in mangrove lagoons. The common assumption that a group of Cassiopea has washed ashore or died is wrong; upside down is the working position.