A team led by scientists at the Smithsonian's National Museum of Natural History, the University of Kansas and the U.S. Naval Research Laboratory report in the Feb. 13, 2020 issue of the journal Nature Communications Biology that they have discovered microscopic stinging structures inside the mucus secreted by upside-down jellyfish--gyrating balls of stinging cells that they call cassiosomes. These cassiosomes can sting swimmers and prey without coming into contact with the jellyfish themselves.
While its exact role in the ocean is not yet known, Ames said cassiosome-packed mucus may be an important part of upside-down jellyfishes' feeding strategy. 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.