You’re swimming in the ocean when something brushes your leg. When the tingling sets in, you realize you’ve been stung by a jellyfish. How do these beautiful, gelatinous creatures pack such a painful punch? Jellyfish are soft because they are 95% water and are mostly made of a translucent gel-like substance called mesoglea. With such delicate bodies, they rely on thousands of venom-containing stinging cells called cnidocytes for protection and prey capture. Even baby jellyfish, the size of a pencil eraser, have the ability to sting. Larval jellyfish, ephyrae, look like tiny flowers pulsating in the sea. As they grow, they become umbrella-shaped with a bell at the top and descending tentacles around the margin.
The largest species of jellyfish, the lion’s mane, has tentacles that can extend more than 100 feet, longer than a blue whale. These tentacles contain most of the stinging cells, although some species have them on their bells, too. Venom is ejected via a nematocyst, a whip-like hollow tubule, which lies coiled under high osmotic pressure. When mechanical or chemical stimuli activate an external trigger, the lid of the cell pops open and sea water rushes in. This forces a microscopic barbed harpoon to shoot out, penetrate and inject venom into its victim. Nematocyst discharge can occur in less than a millionth of a second, making it one of nature’s fastest biomechanical processes.
Nematocysts can continue to fire even after a jellyfish has died, so it’s important to remove lingering tentacles stuck to the skin. Rinsing with vinegar will usually render undischarged nematocysts inactive. Seawater can also help remove residual nematocysts. But don’t use fresh water because any change in salt balance alters the osmotic pressure outside of the cnidocyte and will trigger the nematocyst to fire. That’s why urinating on the affected area, a common folk remedy, may do more harm that good, depending on the composition of the urine. Most jellyfish stings are a painful nuisance, but some can be deadly. An Indo-Pacific box jelly, also called a sea wasp, releases venom which can cause contraction of the heart muscles and rapid death in large doses.
There’s an anti-venom, but the venom is fast-acting, so you’d need immediate medical intervention. Despite the impressive power in their tentacles, jellies aren’t invincible. Their stinging cells are no match for the armor of thick-skin predators, like the leatherback turtle and ocean sunfish. These predators both have adaptations that prevents slippery jellyfish from escaping after they are engulfed: backwards pointing spines in the turtle’s mouth and esophagus and recurved teeth behind the sunfish’s cheeks.
Even tiny lobster slipper larvae can cling to the bell of a jellyfish and hitch a ride, snacking on the jelly while they preserve their own energy for growth. Small agile fish use the jellies as moving reefs for protection, darting between tentacles without ever touching them. Nudibranchs, which are sea slugs covered in protective slime, can actually steal the jelly’s defenses by eating the cnidocytes and transferring them to specialized sacks for later use, as weapons against their own predators. Even humans might benefit from the sting of a jellyfish one day. Scientists are working on manipulating cnidocytes to deliver medicine, with nematocysts rarely 3% of the size of a typical syringe needle. So, the next time you’re out in the ocean, be careful. But also, take a second to marvel at its