Snorkelers in mangrove forest waters inhabited by the upside-down jellyfish Cassiopea xamachana report discomfort due to a sensation known as ‘stinging water.’ When Tohoku University’s Dr. Cheryl Ames and colleagues looked at toxin-filled mucus the jellyfish release into the water, they were surprised to discover gyrating balls of stinging cells — dubbed cassiosomes — inside the mucus.
“This discovery was both a surprise and a long-awaited resolution to the mystery of stinging water,” Dr. Ames said.
“We can now let swimmers know that stinging water is caused by upside-down jellyfish, despite their general reputation as a mild stinger.”
When Dr. Ames and co-authors first placed a sample of the jellyfish mucus under a microscope, they found bumpy little balls spinning and circulating in the slimy substance.
They then turned to several more sophisticated imaging methods to examine the mysterious masses closely, and eventually a clearer picture emerged.
The bumpy blobs were actually hollow spheres of cells, probably filled with the same jelly-like substance that gives jellyfish their structure.
Most of the outer cells were stinging cells known as nematocytes.
Other cells were present, too, including some with cilia — waving, hairlike filaments that propel the cassiosomes’ movements.
Puzzlingly, inside the jelly-filled center of each sphere was a bit of ochre-colored symbiotic algae — the same sort that lives inside the jellyfish itself.
Taking another look at the jellyfish themselves, the team was able to detect cassiosomes clustered into small spoon-like structures on the creatures’ arms.
When the scientists gently provoked a jellyfish, they could see cassiosomes slowly break away, steadily leaving the appendages until thousands of them mingled with the animal’s mucus.
They also found that the cassiosomes were efficient killers of lab-fed brine shrimp, and videos that they produced show tiny crustaceans succumbing quickly to the venomous spheres in the lab.
Further molecular analyses identified three different toxins within the cassiosomes.
“While its exact role in the ocean is not yet known, cassiosome-packed mucus may be an important part of upside-down jellyfishes’ feeding strategy,” Dr. Ames said.
While the photosynthetic algae that live inside upside-down jellyfish provide most of the animals’ nutritional resources, the jellyfish likely need to supplement their diet when photosynthesis slows — and toxic mucus appears to keep incapacitated critters close at hand.
“Venoms in jellyfish are poorly understood in general, and this research takes our knowledge one step closer to exploring how jellyfish use their venom in interesting and novel ways,” said Anna Klompen, a graduate student at the University of Kansas.
The study authors also identified cassiosomes in four additional closely related jellyfish species.
They are eager to learn whether they might be even more widespread.
“This study shows the power of harnessing multi-institution collaboration to solve a problem that has baffled scientists and swimmers around the world,” said Dr. Gary Vora, deputy laboratory head at the U.S. Naval Research Laboratory.
“What stood out most was the team’s ability to experimentally pursue where the data was taking us, given the breadth of the tools that were required to come to these conclusions.”
The study was published in the journal Communications Biology.
C.L. Ames et al. 2020. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana. Commun Biol 3, 67; doi: 10.1038/s42003-020-0777-8