Upside-Down Jellyfish

About

The upside-down jellyfish (Cassiopea andromeda) does exactly what its name promises. It parks itself bell-side down on sand or rubble and points its oral arms upward toward the light. That inverted resting posture is not laziness; it is a feeding strategy. The tissues of this jellyfish host symbiotic zooxanthellae, the same photosynthetic algae found in corals, and by lying with arms exposed to the water column, the animal maximizes light exposure for its algal partners while also capturing passing plankton and organic particles. In the wild it inhabits warm, shallow lagoons, seagrass beds, and mangrove margins across the Indo-Pacific, places where water is calm and sunlight reaches the bottom. In the aquarium it behaves much the same way, pulsing rhythmically and anchoring itself to a sandy substrate. One important caveat: the mucus this jellyfish releases contains stinging nematocysts that drift freely through the water column, which means any tank mates, fish especially, are at real risk of irritation or injury. Cassiopea is best kept in a dedicated species setup or alongside other jellies. Provided it has adequate light and calm flow, it is a surprisingly approachable invertebrate for a keeper ready to meet those specific needs.