Researchers have finally solved the long pending mystery between jellyfish and stinging water

A jellyfish of the genus Cassiopea (different from that of the study). This is a Cassiopea andromeda. | Pete Oxford

Researchers at the US Naval Research Laboratory have discovered that a particular species of mangrove jellyfish hunts for prey by launching venom grenades, creating areas of “stinging water”. The discovery solves a long-standing mystery about how they gather food without using their tentacles.

The Cassiopea xamachana jellyfish , commonly called “upside-down jellyfish” by English speakers, was first described by marine biologist Henry Bryant Bigelow in 1892. In French, the common name indicates its habitat, or “mangrove jellyfish”. These two names apply to all jellyfish of the genus Cassiopea.

It is found in particular in the shallow waters of Florida, the Caribbean and Micronesia. The animal is a frequent nuisance for divers and surfers, who seem to be bitten without even touching it.

Cassiopea: a kind of atypical jellyfish

Cassiopea are not common jellyfish already by the simple fact that they are photosynthetic, seeming to draw a large part of their energy from solar radiation thanks to their dorsal symbiotic algae.

So far, some have believed that the bites came from loose tentacles or from younger specimens. But recently, a team from the US Naval Research Laboratory realized that Cassiopea had actually developed a new way of hunting, without using its tentacles. The results of the study were published in the journal Communications Biology.

To hunt, C. xamachana generally lands on the seabed, on the back, and sends globs of mucus filled with venom above it. These structures, called cassiosomes, can kill small prey and are probably the cause of the aforementioned ghost bites, a phenomenon particularly experienced by snorkelers and fishermen in tropical waters.

The life cycle stages of C. xamachana and its cassiosome-laden mucus. Credits: Ames et al., Communications Biology, 2020

The team analyzed the cassiosomes expelled and discovered that the outer layer was covered with thousands of stinging cells. Although the venom is not powerful enough to pose a significant risk to humans, it is known to destroy skin cells and is fatal for small organisms.

a, b : C. xamachana (5-12 cm in diameter) resting on their apex (white arrow) with the oral arms (cyan arrows) facing upwards, observed by researchers in their natural habitat (mangroves) in Key Largo, Florida (United States). c - f: Cassiosome nests (pink arrows) observed in the form of curved white spots at the end of the vesicular appendages (green arrows), on the oral arms of the jellyfish (cyan arrows). Scale bars: a = 2 cm; b = 5 cm; c, d = 1 mm; e, f = 0.5 mm. Credits: Ames et al., Communications Biology, 2020

According to Cheryl Ames, of the Tohoku University Graduate School of Agricultural Science in Japan, the animal's hunting method "causes itching and burning sensations and, depending on the person, can cause enough discomfort to make him want to to get out of the water". According to her, the results of the study could help tourists, divers and even aquarium staff to avoid this type of discomfort in the future.

Ames said the scientific community still has a lot to learn about jellyfish. "They have a complex and coordinated behavior with their eyes, and some specimens are even capable of killing humans in a few minutes," she explained to AFP. "There is still a lot to learn about them and their applications to biotechnology."


Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana

Cheryl L. Ames, Anna M. L. Klompen, Krishna Badhiwala, Kade Muffett, Abigail J. Reft, Mehr Kumar, Jennie D. Janssen, Janna N. Schultzhaus, Lauren D. Field, Megan E. Muroski, Nick Bezio, Jacob T. Robinson, Dagmar H. Leary, Paulyn Cartwright, Allen G. Collins & Gary J. Vora

Communications Biology volume 3, Article number: 67

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