The upside-down jellyfish rests with its bell facing downward and its tentacles pointing upward, gently pulsating while “standing” inverted in shallow waters. This distinctive upside-down posture sets it apart from other jellyfish and leaves a lasting impression on those who encounter it. Because the tentacles of the upside-down jellyfish contain symbiotic algae, it can obtain energy through photosynthesis. During the daytime, it adopts this inverted position to expose its tentacles to sunlight. Sometimes, in order to receive more sunlight, groups gather in very shallow areas—only to be stranded and dried into “jellyfish jerky” when the tide recedes before they can return to deeper waters.

The upside-down jellyfish found in the waters of Dongsha Island, Cassiopea andromeda, belongs to the phylum Cnidaria. Its scientific name is quite interesting: the genus name Cassiopea refers to the constellation Cassiopeia, while the species name andromeda means Andromeda—perhaps alluding to the noble and graceful posture of its swimming form.

According to the Marine National Park Headquarters, upside-down jellyfish are distributed throughout tropical seas worldwide. They primarily inhabit sandy environments from the intertidal zone to depths of about 10 meters. In addition to deriving energy from photosynthesis, they also feed on zooplankton using the stinging cells (nematocysts) on their tentacles. As these nematocysts contain mild venom, it is advisable to avoid close contact or touching them.

In Dongsha, upside-down jellyfish are mainly concentrated in the inner lagoon, with occasional sightings in seagrass beds. Because they are not strong swimmers, the calm waters of the inner lagoon reduce the likelihood of being swept away by currents. Moreover, water exchange in the inner lagoon is relatively slow, and nutrient levels are higher than in the outer lagoon, making it an ideal and tranquil habitat for their growth.