Invertebrates
Upside-Down Jellyfish
Cassiopea spp. · also called mangrove jellyfish
Resting bell-down against the seafloor of a sunlit mangrove lagoon, an upside-down jellyfish pulses gently in place, its arms angled upward to catch as much light as possible for the photosynthetic algae living quietly inside its own tissue.
Overview
The upside-down jellyfish, genus Cassiopea, is a coastal jellyfish found across tropical and subtropical shallow waters worldwide. Its bell-down resting posture and symbiotic relationship with photosynthetic algae set it apart from the free-swimming, actively hunting behaviour typical of most jellyfish species.
A jellyfish that farms sunlight instead of hunting
Rather than pursuing prey through open water, the upside-down jellyfish has essentially outsourced much of its nutrition to algae living inside its own body, a lifestyle that trades active hunting for quiet, sunlit stillness.
Status
Least Concern — widespread and stable across suitable shallow tropical and subtropical coastal habitat worldwide.
Upside-Down Jellyfish: common questions
Why does the upside-down jellyfish rest with its bell facing down?
An upside-down jellyfish typically rests with its bell facing downward against the seafloor and its oral arms extended upward toward the sunlight, an orientation that exposes the photosynthetic algae living within its tissue to as much light as possible, supporting the nutritional relationship the jellyfish depends on.
How does the upside-down jellyfish get its food?
An upside-down jellyfish obtains much of its nutrition from photosynthetic algae living symbiotically within its tissue, a relationship similar to that found in reef-building corals, supplemented by capturing small zooplankton and other particles drifting through the water it gently pulses to circulate.
Can you get stung by an upside-down jellyfish without touching it?
Yes, an upside-down jellyfish can release small clusters of stinging cells suspended in mucus into the surrounding water, capable of causing a mild, itchy stinging sensation in nearby swimmers even without direct contact with the jellyfish's own body.
Where does the upside-down jellyfish live?
Upside-down jellyfish species are found across tropical and subtropical coastal waters worldwide, generally favouring shallow, calm, sunlit lagoon and mangrove habitat, conditions that support the photosynthetic activity of the symbiotic algae living within the jellyfish's tissue.