Cassiopea andromeda, commonly known as the upside down jellyfish, lives in the waters of the Indo-Pacific and the Hawaiian Islands. These unusual jellies can grow up to 30cm in diameter and are usually brown or green with pale spots, although they may sometimes be brightly colored. The coloration of these jellyfish depends on the specific location where they are found. The upside down jellyfish feeds on drifting zooplankton but can also acquire energy through a symbiotic relationship with Symbiodinium, photosynthetic algae that live inside the tissue of the jellyfish. These jellyfish live in shallow tropical lagoons and mangrove areas in waters up to 30m deep.
Most jellyfish are bell-shaped, live in the open water column, swim and drift around in search of food. The upside down jellyfish is quite unusual in looks and in behavior. These jellyfish spend most of their time resting upside down on the seafloor. While most jellyfish are bell-shaped, the upside down jellyfish’s bell flattens out, allowing the animal to easily rest on the bottom. These jellyfish are typically observed sitting in this manner, gently pulsating their bells to create water flow over their tentacles. When resting on the ocean floor, upside down jellies are often mistaken for plants or sea anemones.
Even though upside down jellyfish spend most of their lives on the seafloor, they are not restricted to completely sedentary lifestyles. These jellies can swim quite well, however they only tend to swim from one resting spot to another, never continuously. When agitated the upside down jellyfish will pulse rapidly, release a slime-like substance into the surrounding water, and may even swim away.
Upside down jellyfish are not highly dangerous to humans. Like most jellyfish they have nematocysts, or stinging cells, that are used for defense and to capture food. Due to their diet consisting mainly of plankton and other small organisms, the sting of upside down jellyfish is proportionally weak. They have a mild sting, since they are primarily photosynthetic, but sensitive individuals may have a stronger reaction.