Revealed: the mystery of stinging water


It’s the mucus of upside-down jellyfish.


The upside-down jellyfish Cassiopea xamachana.

National Aquarium

By Natalie Parletta

People who have experienced stingy, itchy skin from warm coastal waters might be familiar with some of the theories about what causes it – including sea lice, severed jellyfish tentacles and anemones – but the truth has been an enduring puzzle.

Now, a multidisciplinary team of scientists has revealed the source: deceptively placid-looking upside-down jellyfish, Cassiopea xamachana, which pulse rhythmically in groups of hundreds to thousands of individuals at the bottom of shallow bays, lagoons and mangroves.

The stinging is caused by contact with venomous cells inside the jellyfish’s mucus which they send out to catch prey, according to a paper published in Communications Biology, a journal from Nature Research.

A young adult Cassiopea.

Allen Collins

“We found that the mucus contains tiny moving clusters of cells,” says lead author Cheryl Ames from Tohoku University, Japan, and the Smithsonian’s National Museum of Natural History in Washington, DC.

“We call these self-propelled cell masses cassiosomes,” she explains. “Using high-tech microscopy methods, our team discovered that the cassiosome outer layer is lined with thousands of jellyfish stinging capsules called nematocysts.”

These venom-filled capsules are normally found in the tentacles of jellyfish.

“The tentacle contacts the prey, injects venom, paralyses or kills prey which is then ingested through a central feeding tube and digested internally,” says Ames. “Cassiopea has no tentacles or central feeding tube, and novel stinging strategy.”

But the Cassiopea’s sting is not known to be dangerous, just uncomfortable enough to propel affected swimmers, snorkelers or fishers out of the water.

Uniquely, single-cell algae, Symbiodinium, live symbiotically in its cells, enabling the Cassiopea to get nutrients via the algae’s photosynthesis – like a plant making its own food. But like other jellyfish, it is also a carnivore.

“Cassiopea uses the edges of its umbrella to push water over its frilly feeding structures,” says Ames, “sucking in mucus filled with prey – tiny shrimp and other plankton – it captures in the net-like mucus it casts.”

The upside-down jellyfish is pervasive – it is found in the warm waters around Australia, Bermuda, Fiji, Florida Keys, and the Caribbean and Hawaiian islands, and has now invaded the Mediterranean Sea near Turkey.

Ames and other marine biologists were motivated to investigate the source of stinging water through firsthand experience of the irritating sensation while studying upside-down jellyfish in mangrove forest waters and working with aquarists at public aquariums.

Now that the stinging water has a scientific explanation, people can be encouraged to wear protective clothing in areas where Cassiopea thrives.

The team also found cassiosomes in several other related jellyfish species that cause stinging water symptoms in humans, spearheading new lines of investigation into jellyfish mucus, says Ames.

“We are excited that this discovery of cassiosomes – an evolutionary novelty in jellyfish that have a history of over 600 million years – has opened up a totally new field of research and presents many opportunities for further investigation.”

CREDIT: André C. Morandini University of São Paulo, Brazil

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Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
  1. https://doi.org/10.1038/s42003-020-0777-8
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