The upside-down jellyfish rests with its bell facing downward and its tentacles pointing upward, gently pulsing its bell while remaining “inverted” in shallow waters. This unusual upside-down posture and swimming behavior, unlike that of most jellyfish, leaves a lasting impression on those who encounter it. Because its tentacles contain symbiotic algae that provide energy through photosynthesis, the jellyfish adopts this inverted position during the day to expose its tentacles to sunlight. At times, in pursuit of more sunlight, they gather in very shallow areas, only to be stranded and dried into “jellyfish jerky” when the tide recedes before they can return to deeper water.

The upside-down jellyfish found in the waters around Dongsha Island, Cassiopea andromeda, belongs to the phylum Cnidaria. Its scientific name is particularly interesting: the genus name Cassiopea refers to the constellation Cassiopeia, and the species name andromeda refers to Andromeda—perhaps evoking the graceful and elegant swimming posture reminiscent of a celestial maiden.

According to the Marine National Park Headquarters, upside-down jellyfish are distributed throughout tropical seas worldwide. They mainly inhabit sandy environments from the intertidal zone to depths of about 10 meters. In addition to deriving energy through photosynthesis, they also capture plankton using the stinging cells (nematocysts) on their tentacles, similar to other jellyfish. As these stinging cells possess mild venom, it is advisable to avoid direct contact.

At Dongsha, upside-down jellyfish are primarily concentrated within the island’s inner lagoon, with occasional sightings in seagrass beds. Since they are not strong swimmers, the calm waters of the inner lagoon prevent them from being swept away by currents. Moreover, the inner lagoon experiences slower water exchange and contains higher nutrient levels than the outer lagoon, making it an ideal and tranquil habitat for their growth.