Cassiopea frondosa is a species of jellyfish, also known as the upside-down jellyfish, belonging to the family Cassiopeidae.

Description Cassiopea frondosa was one of the first species described of Cassiopeia. It was found in 1774 by Pallas, around the Caribbean. C. frondosa has a bell measuring around less than 15 cm in diameter, and has 12 sensory organs called rhopalia. This jellyfish has a unique position, unlike others, with its bell down and its short oral arms facing up. It is the only species in its group that can be clearly distinguished from others because of its size and unique features. C. frondosa come in a variety of colors; brown, blue, green, or mixes of different colors. Their color is contributed from their environment and from their symbiotic relationship with zooxanthellae. C. frondosa have stinging cells called nematocysts found all along their tentacles. They can also release toxins through their mucus into the water to capture prey or for protection.