Happy April Fool’s Day! While the upside-down jelly (Cassiopea sp.) may seem foolish—and appropriate for today—it lives upside-down in order to better survive by hosting photosynthetic zooxanthellae, just like corals! These dinoflagellate algae (i.e. zooxanthellae) require sunlight to live, and by being upside-down, these jellies provide a guarded place for their sunbathing friends to get more sunlight. In turn, the zooxanthellae provide this resourceful jelly with nutrients. This is an example of symbiosis: when two species form a mutually beneficial relationship.

upside-down sea jelly

Even though Cassiopea don’t have tentacles, they do have stinging cells (nematocysts), which they excrete along their bodies and oral arms. Unlike some other jellies, they also excrete their nematocysts in a mucus that fouls the water around them. So, if they are disturbed in the water, animals and people will begin to feel the sting even though they aren’t touching the jellies!

Their oral arms trap food sifting in the water column above the jellies and transport it toward the stomach at their center. So, they get metabolic energy both from digesting food (small plankton-sized animals) and from energy exchange with their zooxanthellae. Just like corals, if conditions are poor the jellies will bleach, or expel their zooxanthellae. This is why the Aquarium biologists routinely cycle the jellies on exhibit with a holding tank behind the scenes to boost their light exposure. Visitors are often surprised that this is necessary because the exhibit looks so bright, but it is not as bright as the sun!

jellies light get additional behind the scenes
These jellies are treated to additional light behind the scenes.

These jellies can (and do) swim. They use the same pulsing mechanisms as other jellies and will move themselves into more favorable conditions. Like other jellies, they have minor control over the direction they move, but can’t compete against strong currents or movement in the water. We don’t know much about how they choose to move, but we do know they can sense lights and nutrient loads (food density) around them.

sea jelly lifecycle
upside-down sea jellies

The upside-down jellies can be found in mangroves and shallow-water areas all along tropical seacoasts. At the New England Aquarium, you can find these jellies in the mangrove tank, across from the Trust Family Foundation Shark and Ray Touch Tank on Level 1. 

For this jelly, being upside down may be somewhat odd, but it is definitely not foolish, as it is benefits from allowing the most sunlight to its symbiotic algae. Happy April Fool’s Day from the New England Aquarium, our upside-down jellies, and all our marine animals.

Read more about jellies.