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.
Cassiopea xamachana medusae (5-12 cm in diameter) resting on umbrella apex with oral arms facing up, observed by authors in the natural mangrove habitat in Key Largo, Florida, the United States. Scale bar – 5 cm. Image credit: A. Morandini.
“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.
Observations of mucus and cassiosome release in Cassiopea xamachana medusae: (a) Cassiopea xamachana releasing mucus (yellow arrows) following collection in the field; cassiosome nests (pink arrows) appear as light bulging spots at the termini of vesicular appendages (cyan arrow); (b) mucus (yellow arrow) released into the water by Cassiopea xamachana in the lab — small white flecks correspond to live cassiosomes (green arrows); (c) live cassiosomes (green arrows) suspended in mucus (yellow arrow) harvested after release from Cassiopea xamachana; (d) multiple motile cassiosomes isolated from Cassiopea xamachana mucus; (e) live cassiosome close-up (green arrows), showing irregular shape and centralized Symbiodinium dinoflagellates as amber spheres (red arrows); (f) confocal image of highly motile cassiosome immobilized on MatTek glass bottom dishes coated with Cell Tak adhesive; image collected with x60 objective (oil) reveals organization of the peripheral cell layer: NucBlue-Hoescht 33342 (1,100) stains nuclei (blue) of nematocytes with peripheral nematocytes bearing O-isorhiza nematocysts (blue arrows, DIC) and non-nematocyte ectodermal cells (purple); DIC shows centrally Symbiodinium (dark spheres, red arrows) occupy presumptive Cassiopea xamachana amoebocytes in an otherwise acellular core. Scale bars: 3 cm in (a), 3 mm in (b), 1 mm (c), 5 mm in (d), 300 μm in (e), 50 μm in (f). Image credit: Ames et al, doi: 10.1038/s42003-020-0777-8.
“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.
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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
Biologists Solve Mystery of ‘Stinging Water’ | Biology – Sci-News.com
