Swimmers have long been wary of the calm, placid waters where Cassiopea – the upside down jellyfish – are found. Many have experienced the itch and inflammation that follows a swim through the stinging water, where the jellyfish can deliver a nasty sting to humans or to small prey like shrimp without ever having direct contact. Now, a collaborative team of marine scientists affiliated with the National Museum of Natural History, the U.S. Naval Research Laboratory, the National Oceanic and Atmospheric Administration (NOAA), Tohoku University, and the University of Kansas have shown for the first time how the sting is carried through the water.
They found that the water around these jellyfish is filled with tiny mucus grenades containing structures they have named “Cassiosomes.” These are hollow balls of cells called nematocytes, responsible for delivering the jellyfish sting. The balls also contained cells with cilia, structures that help the mucus move rapidly.
Mucus itself is an important structural material in the aquatic environment. It is formed from a large protein that forms a strong watery gel, and has many different biological functions across many types of animals. A famous example from the marine environment is the hagfish, which produces huge amounts of mucus when threatened, using the slippery material to evade capture.
The hagfish produces mucus all over its body when poked. The team investigating the jellyfish grenades found that similar stimuli could provoke Cassiopea to release Cassiosome grenades from some tiny spoon-shaped structures on their arms. This is a fascinating finding, but it won't protect you from jellyfish stings – so next time you go swimming in warm, coastal waters, keep your eyes out for upside-down jellyfish (and their mucus)!