How upside-down jellyfish can make the water sting

In some tropical waters like the Florida Keys mangrove forests, for snorkellers to stay safe, it’s not enough to not touch anything. It may not be a good idea to enter the water in the first place.

That’s because sometimes it seems that the water itself can sting, causing rashes on a snorkeller's skin.

The real culprit is not some mutant strain of water molecules. Rather, it is a species of upside-down jellyfish that lives on the ocean floor. Called Cassiopea xamachana, this jellyfish can also be found in places like the Caribbean Sea and the Gulf of Mexico.

This jellyfish's unique abilities is the subject of a paper published in the Communications Biology journal.

When it senses a disturbance in the water, this little critter releases a large amount of mucus containing tiny defenders that pack a punch.

Called cassiosomes, these popcorn-shaped, jelly-filled packages of microscopic stinging cells are powerful enough to irritate possible predators (and anyone nosy enough to swim in the vicinity). Hence, the researchers in the study would always cover up completely before diving to study the jellyfish. Nonetheless. they would still come up covered in “itchy and irritating” stings.

The cassiosomes can propel themselves through the water (using wavy hairs called cilia) and survive in the water for up to ten days. Likening them to mobile grenades, the research team suggested that the cassiosomes' functions were to protect the jellyfish from predators and to incapacitate prey, which would then fall downward, to where the jellyfish would be waiting.