Interesting Animals: Upside-Down Jellyfish

Cassiopeia in its usual upside-down position

Cassiopeia, better known as the "upside-down jellyfish" is a fascinating species of jellyfish commonly found in waters off the coast of Florida, the Gulf of Mexico, and the Caribbean. It is actually a genus that includes several different species, but their behavior is always the same. The position they are found in is laying on the bottom with bell to the sand and tentacles facing upward. This is because they are providing the tiny algae (zooxanthellae) in their tentacles with sunlight so that they can photosynthesize. This is not the only food source for the jellyfish, however it does provide a significant portion of the jelly's nutritional needs. They also capture plankton using their stinging cells (nematocysts) as is usually expected of a jellyfish. 

This jelly has a healthy crop of algae

 

These jellyfish are also one of the few jellyfish to have more than one mouth. They have a primary mouth, as well as several lesser ones. Crabs are known for enlisting other species to aide them  and will sometimes carry upside-down jellys around for protection.

 

If disturbed, they will swim away in the same position. As far as I know, they never voluntarily turn over to the more usually seen orientation of the Medusa stage of Cnidarians (bell up, tentacles down). They are only found in shallow water so that they can provide enough sunlight to their algae and are a common sight in Mangrove swamps. I observed these creatures in the wild during a trip to Florida.

Images are from Wikimedia Commons under a creative commons license: one, two