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Upside-down jellies fire stinging grenades

“Upside-down Jelly” by nashworld is licensed under CC BY-NC-SA 2.0

Ames, C.L., Klompen, A.M.L., Badhiwala, K., Muffett, K., Reft, A.J., Kumar, M., Janssen, J.D. 2020. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea Xamachana. Communications Biology 3 (1): 1–15. https://doi.org/10.1038/s42003-020-0777-8.


Typical jellyfish life cycle, depicting the medusa stage as full grown jellyfish. This Photo by Unknown Author is licensed under CC BY.


The name Jellyfish represents a large group of commonly known marine species. Jellies are classified in Phylum Cnidaria (which includes anemones and corals) and the clade Medusozoa. Medusozoa includes 3 classes of Jellies, all of which share the characteristic of having a ‘medusa stage’ in their life cycle. Medusa – like the gorgon from Greek mythology – have long, slender appendages that are often able to stun or kill (tentacles). Tentacles line the bell-shaped, jelly-filled body. They are usually oriented with the smooth outer bell surface up, and the mouth and tentacles facing down.





Jellyfish tentacles are lined with cnidocytes – explosive cells that, when triggered, shoot little harpoons which are often filled with toxins. However, there are many forms and functions of these specialized cells and not all are toxic. Cnidocytes can be grouped into three major categories:

  1. Penetrant – skin-piercing; venomous
  2. Glutinate – sticky
  3. Volvent – lasso-like

A single jellyfish can have many different types of cnidocytes. The collection of different types varies from species to species, and may even change throughout their lifetime.


One unique type of jellyfish is Cassiopea xamachana, referred to as the upside-down jellyfish. As their name suggests, they are upside-down (relative to typical jellies). In other words, the bell is face-down, typically resting on the ground, and the tentacles face upwards. The bell of C. xamachana can grow up to 25 cm wide and is able to suction to the seabed! As a warm-water species, and inhabitant of mainly mangrove habitats, the upside-down jellyfish is of ecological importance. Mangroves are very unique and highly threatened environments and C. xamachana is an indicator species – their health tells us about the health of the environment. Several mangrove regions are under protection which also protects the upside-down jelly.

Upside-down jellyfish resting on seafloor. Image from Microsoft Word creative commons.


Other cool facts about Cassiopea xamachana

1. Their color is variable and dependent on symbiotic plankton

(ocean plants living in their cells)

2. They eat sunshine!

(Indirectly, from their cellular plant friends)

3. Instead of a central mouth, they have numerous mouths!

On their arms!



In this paper, the authors set out to investigate a phenomenon known as ‘stinging water’. Many swimmers in these habitats claimed to have been stung, but there was no animal evidence to support their claims (no visible jellyfish present). Many people thought that the stinging sensation was from microscopic jellyfish, dislodged tentacles, or some other unknown organism.


The authors of this study hypothesized that C. xamachana were involved and they set out to investigate. This was a very thorough study that paired both field and laboratory work. They also employed a large suite of methods which goes beyond the scope of this article – but they make a compelling case! Some of the methods used include microscopy, videography, and DNA analysis.



First of all, the authors found that the upside-down jelly releases a lot of mucus – both in their natural habitat, and in the lab. When they looked closer at this mucus, they found freely suspended (tiny) grenades!

These grenades were named cassiosomes by the authors and are hollow irregularly shaped ball-like structures. The outer layer of cassiosomes are covered in stinging cells, and the inner layer is filled with symbiotic plankton! Furthermore, these cassiosomes use hair-like structures called cilia for locomotion and can continue moving outside of the jellyfish for up to 10 days!


Medusae of the upside-down mangrove jellyfish Cassiopea xamachana. a, b C. xamachana medusae (5–12 cm diameter) resting on umbrella (white arrow) with oral arms (cyan arrows) facing up. c–f Cassiosome nests (pink arrows) observed as white bulging spots at the termini appendages (green arrows) on medusa oral arms (cyan arrows).

Characterization of mature cassiosomes in Cassiopea xamachana.









Cassiosomes were found to subdue and kill small prey (artemia, mm-sized shrimp). Between the abilities to swim, stin, and kill, it is not surprising that these structures were previously thought to be a unique organism of their own!


Check out some videos of cassiosomes moving and killing prey here:



If you ever go swimming in mangrove habitats wear a wetsuit!

**C. xamachana is a mild stinger, stings may result in rash, vomiting, swilling and irritation.**





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