Cassiopea xamachana

Cassiopea xamachana, commonly referred to as the upside-down jellyfish, is a distinctive species found in warm waters of the western Atlantic Ocean, Caribbean Sea, and Gulf of Mexico. This jellyfish can reach a diameter of approximately 25 cm (10 in) and is characterized by its unique saucer-shaped bell that rests on the ocean floor, with its tentacles oriented upwards. The jellyfish often appears bluish-grey or bluish-green due to the presence of symbiotic zooxanthellae, which provide both color and nutritional benefits through photosynthesis. Instead of a central mouth, it features numerous oral openings along its tentacles that connect to its stomach. Typically, upside-down jellyfish are found in large groups on muddy substrates in shallow lagoons and inshore bays, usually at depths between 0 and 10 meters. When disturbed, they may pulsate their bells and rise briefly in the water before settling back down. Their diet includes small prey captured with the stinging cells in their tentacles, as well as nutrients absorbed from the surrounding water. Their life cycle alternates between a medusa phase and a polyp phase, with larvae that settle on surfaces like decomposing red mangrove. Breeding occurs when gametes are released into the water, leading to the formation of planula larvae. Diving encounters with Cassiopea xamachana can involve observing their resting behavior on the seabed or their reactions to disturbances. While they can cause a stinging sensation, they are generally not aggressive. The conservation status of this species has not been assessed by the IUCN, leaving its status uncertain.