Taking the sting out of swimming with jellyfish

Jellyfish like warm water as much as swimmers do - here's how to understand the risks and swim on

Aaron Ross on Unsplash

As the sea warms, swimmers and jellyfish alike appear on our shorelines. The warming of the oceans has led to jellyfish expanding their habitat, resulting in significant blooms in some areas. Though it is not yet clear whether the overall jellyfish population is increasing, it only takes one or two for many a swimmer to tense up and consider a return to the shore. But Susanne Masters advises understanding the risks and swimming on.

Jellyfish inhabit an awkward place in our thoughts at the crossroads of wondrously beautiful and excruciatingly painful.  It is true that in some tropical waters there are jellyfish powerful enough to kill a person who doesn’t reach medical treatment, such as irukandji and box jellyfish. But in temperate waters our jellyfish tend more towards a negligible or easily treated sting, perhaps occasionally painful, and powerful enough to disrupt a swim. But that’s not a reason to fear sea swimming or jellyfish season. So, lets explore the diversity of jellyfish that inhabit our earth’s salty waters by taking a closer look. 

Knowing your local jellyfish can make you feel a lot happier in the water when jellyfish are about. Key is being able to tell the difference between one that will give you a nasty sting or has long trailing tentacles that are hard to avoid, and jellyfish with negligible stings or easy to avoid tentacles.  Help is at hand with the Marine Conservation Society jellyfish identification guide.



Compass jellyfish. Zuoqi Liu on Unsplash

Jellyfish that sting:

  • Compass jellyfish (left)
  • Lion’s mane jellyfish
  • Mauve stinger
  • Portuguese man-of-war

Covering up with a long-sleeved swimsuit is a good idea if you plan to swim near viciously stinging jellyfish like lion’s mane. Similarly, swim leggings will cover your legs but not interfere with swimming. However, if you do get stung, in order to treat a jellyfish sting it helps to understand the mechanism by which their stings work. Jellyfish are characterised by having nematocysts – stinging cells.  Contact with a jellyfish tentacle can leave nematocysts that have not been triggered on the surface of your skin. So the first step in treating jellyfish stings is to remove nematocysts from the surface of your skin without triggering them to release their stinging chemicals.

Nematocysts respond to changes in salinity, pH, and pressure. Rinsing with seawater might wash some off, but rinsing with freshwater or other liquids will cause the sting to be released. So, rinse the stung area with sea water. Running a hard edge e.g. a credit card over the surface of the stung area is suggested as a means of scraping off stinging cells. Some jellyfish stings, most notably those of warm water residents the box jellyfish, are remedied with vinegar. While the tendency is to reach for an ice pack to soothe pain, recent research is suggesting that at least with some species of jellyfish the venom is broken down by heat. Therefore, treating stung areas with a heat pack will provide relief whereas treatment with an ice pack might prolong the pain of being stung. 

Leatherback turtle. Tanguy Sauvin on Unsplash.

“Some people feel more positively inclined towards jellyfish when they hear that they are a key part of the diet of leatherback turtles. No jellyfish = no leatherback turtles.”

Some people feel more positively inclined towards jellyfish when they hear that they are a key part of the diet of leatherback turtles. No jellyfish = no leatherback turtles. As charming as turtles are, I’m more excited about the human life-saving potential of jellyfish. Chemicals in their sting that can cause us pain might also be the source of new pharmaceutical drugs for treating life-threatening conditions. For example Nemopilema nomurai venom has anticancer activity via selective cytotoxicity. If scientists can work out how immortal jellyfish (Turritopsis dohrnii) change from adult to juvenile it might not give our more complicated bodies immortality but could show us ways to reverse some ageing processes.  With jellyfish blooms creating large amounts of biomass they are also being considered as candidates for making protein-based plastic that is biodegradable. 

In these respects, for all the simplicity of creatures that are dome plus tentacles, see through, and have a few chemicals in them, jellyfish have roles within an ecosystem to which we also belong. And there is a simple beauty to swimming past a jellyfish that has caught the sunlight. 

  • Help the Marine Conservation Society monitor marine wildlife by registering your jellyfish sightings here. The form records information including location, type of jellyfish and number, so it may be useful to familiarise yourself with it before going for your next swim.
Susanne Masters