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by Elaine K Howley

July 15, 2021

Move over sharks—jellyfish are the real threat for open water swimmers

If your open water swimming adventures ever take you into salt water, you should be prepared to meet some stinging, gelatinous creatures while you’re submerged.

There are more than 1,600 known species of jellyfish, a group of marine invertebrates that drift and pulse through Earth’s oceans. They inhabit primarily salt water, but there are a few freshwater species. Despite their Jell-O-like texture, jellyfish are actually made mostly of water—95% water to be exact, according to the National Oceanographic and Atmospheric Administration.

These umbrella-shaped critters in the phylum Cnidaria are heartless—really, they don’t have a heart. They also lack brains and blood. Instead, the bell top of these simple links in the oceanic food chain are composed of three layers:

  • An outer skin called the epidermis
  • A middle layer called mesoglea, a thick, jelly-like substance.
  • An inner layer called the gastrodermis, which, as the name suggests, is where digestion happens.

Jellyfish also have a nerve net, which is a sort of rudimentary nervous system that allows the creature to detect light and smell and respond to other stimuli.

Dangling down below the dome on most jellyfish are an array of long, sometimes stringy or even filament-like tentacles that are covered with venomous stinging cells called cnidocytes. Jellyfish use these cells for both protection and to stun or kill prey.

Cnidocytes contain a microscopic structure called a nematocyst, which responds to the faintest stimuli by releasing a venom-filled barb into the unfortunate passerby or meal prospect.

These nematocysts can cling to your skin and continue to fire their tiny, venomous harpoons long after the jellyfish has died. Many species also shed these cells into the water leaving behind a trail of misery for any unlucky human who might cross their path.

Jellyfish exist to eat and be eaten. Although humans are not on the typical jellyfish menu, the more we venture out into the big blue, the more frequently we’re likely to come into fiery, prickly, stinging, burning contact with these mindless creatures. And depending on the species, those interactions can range from merely irritating to downright deadly. 

Common Species of Jellyfish

Off-shore waters in North America are home to many species of jellyfish. Here are just a few of the more common ones you might encounter on your next ocean swim.

Moon jellies (Aurelia aurita). Moon jellies are super common in the coastal waters off North America. These mostly transparent jellyfish cause virtually no sting and have very short tentacles that look like a fringe of hair rimming the edge of the bell. They have four looping structures in the center of the underside of the bell that looks a bit like a flower, often in a pale shade of purple. They can grow to be about 8 or 9 inches across. When they wash up on land and harden a bit in the sun, they make great frisbees.

Lion’s mane (Cyanea capillata). Also known as the giant jellyfish, the hair jelly, or the arctic red jellyfish, these giant creatures are the largest jellyfish you’ll come across and their sting can really pack a punch. Some of these jellyfish have tentacles that stretch more than 100 feet in length, making them longer than a blue whale. They tend to prefer colder water, so are more often spotted in Maine and off the coasts of Northern California, Oregon, Washington, and Alaska. They’re not deadly, but they hurt an awful lot and have such long tentacles, it’s easy to get tangled up if you’re unfortunate enough to swim into one.

Portuguese man of war (Physalia physalis). Also known as a bluebottle or a floating terror, these creatures are not technically jellyfish, but rather a closely related animal called a siphonophore. These strange beasts are actually a colony of creatures that all work together as one animal. Despite this taxonomic creepiness, they sting just like jellyfish and can be truly terrible to tangle with. They’re shaped like a floating, purple balloon and have long, dark, dangling tentacles. They’re rarely deadly to people, but they can leave a big welt cause a lot of pain.

Sea nettles (Chrysaora quinquecirrha and Chrysaora fuscescens). Atlantic sea nettles, Chrysaora quinquecirrha, hang out in the Chesapeake Bay and other temperate waters along the Atlantic seaboard. They’ve got a distinctive golden-brown bell and fluffy, flowy white-and-brown tentacles. They can get big—up to about 3 feet across in the wild. They aren’t deadly, but as the name implies, they sure can needle your skin. Their pacific counterparts, Chrysaora fuscescens, are also a pretty golden color and an aquarium mainstay for their attractive swimming displays.

Pink meanies (Drymonema larsoni). These giant jellyfish were first observed in the Gulf of Mexico in 2000 and thought to be a species native to the Mediterranean. But in 2010, the pink meanie was found to be an entirely new species. This animal can grow quite large, with a bell several feet across and tentacles more than 12 feet long. They eat moon jellies and have been observed gobbling 30 of them at once.

Remedies for Jellyfish Stings

In some people, jellyfish venom can trigger an allergic reaction. Seek immediate medical attention if you’ve gotten a bad sting, are having an allergic reaction, or have been swimming in a location known to be home to dangerous species of jellyfish.

In less high-stakes situations, you can ameliorate the pain of the sting in a few ways:

  • Rinse with salt water. Rinsing the area gently in salt water will help remove any lingering bits of tentacle to help prevent additional stings. And make sure it’s salt water, not fresh, because any change in the salt balance around the nematocyst will encourage it to fire and can actually make the stinging feel worse. This is also the same reason why urinating on the sting—a commonly believed folk remedy—doesn’t work and can actually make the sting feel worse.
  • Take an allergy medication. If you’re having a mild allergic reaction, taking a Benadryl or other over-the-counter allergy medication can help ease symptoms.
  • Ice the area. Your skin will likely show signs of the sting in the form of a raised red welt or swelling. Appling an ice pack to the area can help reduce this inflammation.

Conventional wisdom says that you should rinse the area with vinegar, but vinegar has been found to be unhelpful in most cases and could actually increase the release of venom in some cases. Similarly, meat tenderizer, a long held favorite home remedy along the mid-Atlantic coast, is not likely to be of much real use and will just make you smell like BBQ.

For some species of jellyfish, submersion of the area in very hot water can denature the protein in the venom and stop the stinging. But be careful not to burn yourself.

Preventing Stings

The best way to cope with jellyfish is to do your best to avoid them. That’s a tall order for most open water swimmers, but jellyfish have seasons, and if you know they’re in bloom, maybe head to the pool or a freshwater swimming hole until they thin out.

Over the last decade or so, some jellyfish repellant products have come on the market, sold as a lotion or spray that you apply to the skin before swimming. You can also opt to invest in a stinger suit, a type of thin, full-body swimsuit that creates a barrier between your skin and whatever the ocean contains.

Lastly, sea turtles and sunfish are both predators of jellies, so if you want to take the long road to reducing your risk of a negative interaction with a jellyfish, support conversation efforts aimed at protecting these two gentle giants in their quest to gobble up all the squishy little devils.


  • Open Water


  • Safety