Upside Down Jellyfish Attacks by Launching Venomous Grenades at Prey

casseiopea upside down

Lying inverted on the sea bed, upside-down jellyfish are often mistaken for corals  (Photo: Panjaporn Nawathong/Shutterstock)

As if the underwater world wasn't already weird enough, research published last week in the online journal Communications Biology has found that a commonly encountered jellyfish stalks and kills its prey by lobbing venom-filled mucus grenades into the water.

Cassiopea xamachana, from the genus Cassiopea – known as 'upside-down jellyfish' because they present their arms and mouths upwards rather than dangled underneath their bell – is commonly encountered in warm and shallow water in the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico. The wobbly cnidarian rebels are particularly partial to mangroves, where they conduct stealthy grenade warfare against unsuspecting snorkellers who often report being stung without ever making contact with a jellyfish. The phenomenon has become known as 'stinging water'

It had been presumed that the stings were either the result of detached tentacles or stinging cells – a common occurrence among creatures of the phylum Cnidaria, which also contains corals and hydroids – or perhaps by tiny juvenile jellyfish that were not large enough to be easily visible in the water. Cassiopea differ from the archetypal jellyfish form of a floating bell from which arms and tentacles are suspended in that they have no tentacles and lie, inverted, on the sea bed with their arms protruding upwards.

cassiopea cassiosome

Sketch of the composition of the cassiosome 'grenades' (L) and (R), sections of the popcorn-shaped structures seen under a microscope (Images: Ames, Klompen et al/Communications Biology)

The upside-down jellies are known to secrete a large amount of mucus, but the researchers found that not only does Cassiopea xamachana produce mucus, but suspended within the mucus are 'popcorn-shaped' balls of nematocysts – or stinging cells. When the water surrounding a C. xamachana medusa (the sexually productive adult phase of a jellyfish) was agitated, as it would be if prey was passing by, the mucus and its venomous 'grenades' – known as cassiosomes – were released. While the venom is described as causing a 'burning and itching' sensation in humans, it was more than sufficient to kill prey animals such as the brine shrimp that the scientists used to conduct the study.

Motile cassiosome causing fatal envenomation of brine shrimp (Video: Ames, Klompen et al/Communications Biology)

The grenades – a term used in the study itself – are capable of independent movement once released into the water column, and remain motile for up to ten days before disintegrating. Once the prey animals have been immobilised, the pulsating bell of C. xamachana draws them into the arms of the upside-down jellyfish where they can be digested.

The study shows how little we know about common, everyday phenomenon within the aquatic realm. Following the publication of the research, scientists hope that further study will shed light upon the evolutionary biology of jellyfish, and how their ability to produce mucus and stinging cells may be of importance to other marine organisms, and the development of human biotechnology.


The study, by lead authors Cheryl L. Ames of the Graduate School of Agricultural Science, Tohoku University, Japan, and Anna M. L. Klompen of the Department of Ecology and Evolutionary Biology, University of Kansas, USA was published under a Creative Commons license in Communications Biology, 13 February 2020.




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