The upside-down jellyfish (Cassiopeia xamachana) is another member of the Rhizostomae order. The name of the species, xamachana, means Jamaican although their natural habitat is by no means exclusive to Jamaican waters. Populations exist throughout the Gulf of Mexico and the Caribbean as well as along the Florida strata. They are also found halfway around the world in the Pacific Ocean. Although not native to these waters, upside-down jellyfish were introduced to the Hawaiian Islands during WWII, most likely from jellyfish polyps attaching to the undersides of returning warships from the Philippines.
This species is widespread in warm, shallow tropical waters such as mangrove swamps. They are often called mangrove jellyfish because they are frequently found in large aggregations in these marshy regions. Unlike many species of jellyfish, upside-down jellies are entirely marine. None were found in brackish or fresh water.
These jellies spend their lives completely differently from most jellyfish. Jellyfish generally spend much of their time drifting freely on ocean currents. Upside down jellyfish swim freely until they reach about 2 cm. Then their bell reverses and they sink to the bottom of the ocean floor. From there, they will spend the majority of their adult life upside down on the muddy substrate with their tentacles pointed upwards to capture the zooplankton always present in the water columns.
Like blue jellies, upside down jellyfish have a symbiotic relationship with zooxanthellae. It is the same symbiosis that occurs with many jellyfish and coral species. In addition to providing essential nutrients, these golden algae also produce oxygen to help support the metabolic respiratory functions that jellyfish need to survive in oxygen-poor environments. This is especially important for upside down jellyfish as they spend the vast majority of their lives nestled in a muddy substrate and must rely on their food to come to them. Due to their specialized eating habits, upside-down jellyfish are generally found in nutrient-rich waters with high concentrations of decomposing matter to support zooplankton in these marshy saltwater environments.
Upside down jellies have flat bells in the shape of a saucer. Their umbrellas are usually greenish gray or blue. They have a central depression or an exumbrella in their bell. The exumbrella acts as a suction device to help them stay anchored to the bottom of the ocean. Rather than a single mouth opening, they have 4 carefully branched mouth arms. These arms have a frilly look similar to lace, similar to many leafy green vegetables. They are often called cabbage-headed jellyfish because of these appendages. It is believed that this species is a filter feeder and also depends on some form of absorption of dissolved nutrients directly from the water to supplement its nutritional needs.
Upside down jellyfish do not directly inject their prey like most jellyfish. Their nematocysts (stinging cells) are controlled by the cnidocile. It is the equivalent of a mechanically or chemically triggered grenade launcher. The stinging cells launched from the cnidocile produce a cnidoblast that will stun or paralyze the prey in the immediate vicinity. The jellyfish then begin to ingest their prey with their primary mouth openings. Once the prey is reduced to food fragments, these nutritious particles are transmitted to the secondary mouths for better digestion.
The jellyfish cnidoblasts also function as a self-defense mechanism. If suddenly disturbed, large groups of these frosts launch upwards from the bottom of the ocean and release their nematocysts. This massive release of venom into the water is generally sufficient to ward off potential predators. The toxic compound is generally harmless to humans. This can result in an itchy or tingling sensation in the skin or a rash in people who are more sensitive to venom.
Upside down jellyfish can grow up to 14 inches in diameter in the wild. In captivity, a maximum growth potential of 8 inches is more realistic. Depending on their size in captivity, upside-down jellyfish can be fed on zooplankton, or small invertebrates and fish. In order to allow their symbiotic algae to photosynthesize well, a lighting system conducive to a marine reef aquarium is highly recommended. These jellies have a higher temperature tolerance than most scyphozoan jellyfish. Jellyfish or adult jellyfish can be found all year round. However, the optimal temperature for these adult frosts is between 75 and 78 F. This simulates the height of the adult season. Upside down jellyfish usually mobilize in summer or early fall. While most scyphozoa mobilize during the winter months.
by Stephen J Broy