Just don’t touch: that’s the motto when you encounter a poisonous jellyfish in the water. But this strategy is of little use for Cassiopea xamachana. Because this cnidarian known as mangrove jellyfish can even cause skin irritation, apparently without contact, as researchers report. Her trick: The jellyfish secretes mucus into the water with content that is truly impressive. They are spherical structures that contain, among other things, toxic nettle cells. These free-floating weapons of the jellyfish now seem to provide the explanation for an enigmatic phenomenon: “stinging water”.
Jellyfish are beautiful to look at with their sometimes colorful tentacles and bizarre shapes. But you shouldn’t get too close to some of them: Many of these cnidarians are poisonous. Depending on the species, contact with their tentacles can give people slight complaints such as uncomfortable wheals or even symptoms of paralysis. The Cassiopea xamachana known as the mangrove jellyfish seems to be a rather harmless specimen in this context. Because this jellyfish does not float through the water, but sticks upside down on the sea floor – its relatively short arms are turned upwards towards the sunlight. Thanks to this stationary lifestyle, divers and other water visitors can easily avoid the animals. Nevertheless, people keep reporting itchy, stinging skin if they are only near Cassiopea jellyfish. How can this phenomenon called “stinging water” be explained?
Toxic nettle cells
It is known that the magrove jellyfish release mucus into the water from time to time. It could be the cause of the symptoms. So far, however, what is wrong with this assumption is completely unclear. According to experts, separated parts of jellyfish tentacles, fish lice, anemones or other poisonous sea creatures could just as well be behind the stinging water. Cheryl Ames of the US Naval Research Laboratory in Washington and her colleagues have now set out to finally solve this riddle. In search of an explanation, the researchers first searched the scientific literature for clues to the mysterious phenomenon. In fact, they repeatedly came across mentions of the mucus secreted by Cassiopea xamachana. Accordingly, there seem to be conspicuous cell collections.
Following this trail, the scientists took a closer look at the slime of a specimen of Cassiopea from an aquarium. Microscopic examinations and other imaging methods revealed: Small ball-like structures circulate in the jellyfish’s mucus. These are collections of cells that Ames and her colleagues call cassiosomes. The important thing: These balls, which are hollow on the inside, are covered on the outside with nettle cells, the nematocytes. These contain poison-filled capsules and are usually found predominantly in the tentacles of jellyfish. As the researchers report, the cassiosomes in the Cassiopea jellyfish are filled with three different toxins. In the test, they were able to successfully kill brine shrimp, among other things.
Useful for catching prey
For the scientists, it is clear that it is these special structures in the mucus of the mangrove jellyfish that are responsible for the stinging water. They apparently act as “mobile grenades”. “This discovery was a surprise and provided the long-awaited explanation for the mysterious phenomenon. Now we can say to all swimmers: the stinging water is actually caused by the Cassiopea jellyfish, ”states Ames. According to the team, it is still unclear what exactly the nettle animals use their armed slime for. However, it seems obvious that he will help them catch prey. Because the mangrove jellyfish are mostly supplied with nutrients by photosynthetic algae symbionts. But if photosynthesis slows down due to a lack of light, they have to take in additional food – with the help of the slime, they can then kill small organisms in their vicinity and fish them out of the water, the guess is.
Interestingly, the researchers found that Cassiopea xamachana is apparently not the only jellyfish with cassiosomes. They also identified such structures in four closely related species. Like Cassiopea, these all belong to the order of the root mouth jellyfish (Rhizostomeae). A species from the sister line of the flag jellyfish, however, did not have these cassiosomes, as the studies showed. Ames and her colleagues therefore suspect that the development of this particular poisoning strategy can be traced back to a single evolutionary event and has not arisen independently several times. “The poisons of the jellyfish are poorly researched overall. Our work expands our knowledge of the interesting and sometimes unique ways in which cnidarians use their toxins, ”concludes co-author Anna Klompen of the Smithsonian in Washington.
Source: Cheryl Ames (US Naval Research Laboratory, Washington) et al., Communications Biology, doi: 10.1038 / s42003-020-0777-8