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Marine Animal Encyclopedia

Upside-down Jellyfish Cassiopeia xamachana

Divers who find this jellyfish upside-down on the seabed often think they have found a dying specimen. However, the upside-down jellyfish lives like this, floating with its bell pointing downward and its eight large, branching mouth arms held upward. The mouth arms have elaborate fringes consisting of tiny bladders filled with minute single-celled algae called zooxanthellae. The algae need light to photosynthesize, and the upside-down jellyfish behaves as it does in order to ensure its passengers can thrive. Excess food manufactured by the algae is used by the jellyfish, but it can also catch planktonic animals with stinging cells on the mouth arms. Its bell pulsates to create water currents that bring food and oxygen. When it wants to move, the upside-down jellyfish turns the right way up with the bell uppermost. A very similar jellyfish, Cassiopeia andromeda, is found in the tropical Indian and Pacific Oceans and may actually be the same species.

Upside-down Jellyfishzoom image
  • Class Scyphozoa
  • Diameter Up to 12 in (30 cm)
  • Depth 0–33 ft (0–10 m)
  • Habitat Coastal mangroves
  • Distribution Tropical waters of Gulf of Mexico and Caribbean
Upside-down Jellyfish habitat mapzoom image