Behind every jellyfish sting in the sea stands an ancient and formidable weapons system: the nematocyst. This tiny organelle fires a hollow thread at staggering speed, and it works based on a biological mechanism that has endured for hundreds of millions of years. Following years of research into the mechanism, we already know how to protect ourselves from stings in the water with a simple lotion, and—on the medical side—how to harness this effective mechanism to deliver drugs through the skin.

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The last jellyfish are still out there. If you have already felt a suspicious sting on your leg, know that it is not just a sting, but a microscopic shot powered by osmotic pressure, fired by none other than… a cell. More precisely, by a remarkable organelle called a nematocyst, undoubtedly the tiniest weapon you will ever feel on your skin. It has existed in nature for hundreds of millions of years and is considered one of the standout specializations of an entire animal phylum: the Cnidaria. This ancient group of marine creatures includes jellyfish, sea anemones, hydras, and corals—organisms that appeared in the oceans about 600 million years ago, long before fish, insects, or trees.

Although their bodies look soft and simple, cnidarians have survived entire ages and even mass extinctions, armed with a unique and highly efficient mechanism—the stinging cells. What is no less astonishing is that their structure has barely changed since then. When a mechanism works this well, evolution has little reason to alter it [1].

The nematocyst is an intracellular organelle located inside the stinging cells, called cnidocytes, and it functions like an ultra-fast single-use syringe. It consists of a capsule containing a hollow, spring-like thread. The thread is coated with barbs that carry venom or other compounds, depending on the required task: offense, defense, or anchoring. At the top of the capsule is a lid-like structure called the operculum, which seals the thread until the moment of discharge. Protruding next to the capsule is a sensory hair named the cnidocil, an exceptionally sophisticated sensor [2]. Inside the capsule reigns enormous pressure, osmotic pressure generated by a very high concentration of ions and soluble proteins within it: water flows in from the surroundings to balance the concentration of molecules, building pressure inside the capsule that can exceed 150 atmospheres, equivalent to the pressure at a depth of 1.5 kilometers under the sea.

If we compare the nematocyst to parts of a gun, the capsule is the barrel, the thread is the bullet, the osmotic pressure is the gunpowder, and the cnidocil hair serves as the trigger. The hollow thread is densely coiled inside the capsule, like a loaded spring waiting to be released. The tremendous pressure is maintained constantly until the sensor detects a combination of stimuli—such as a slight touch, a suspicious molecule, a disturbance in the water, or even a sudden change in light. Then, like a sprung trap, the capsule opens and the thread is fired at staggering acceleration. How staggering? Measurements have shown more than five million g, i.e., five million times Earth’s gravity and roughly fifty times the acceleration of an average rifle bullet! [3].

But… how does the nematocyst avoid firing at every grain of sand drifting by? The secret lies in the combination of stimuli. Only when both physical contact and the appropriate chemical signal—say, a protein coating the skin of potential prey—occur together does the sensor “decide” that the target justifies firing [4]. This way it saves the biological bullet for the right moment, which is vital because a single shot, and it is over. The nematocyst is a one-time weapon: once the thread is fired, it disintegrates and cannot be reused.

Structure of the nematocyst and its mechanism of action. Source: Wikimedia Commons

Yet we are not talking about just one kind of cell. Jellyfish and every other cnidarian possess an impressive arsenal of stinging cells, each containing a different type of nematocyst designed for a specific mission—injecting venom, capturing prey with sticky threads, defending itself with stinging spines, or adhering. The nematocyst’s structure, the thread’s length, and the active compounds vary according to the cell’s role, making the system extraordinarily precise.

And when something in nature works this well, biotechnology pays attention. Researchers are trying to exploit this mechanism exactly as it is, as a system for delivering substances into cells, for example to transport drugs directly to their target. One of the most intriguing technologies for using nematocysts as drug-delivery tools harnesses the firing mechanism to inject medications through the skin instead of via a needle. Nematocysts can act like natural microscopic syringes, shooting the drug directly into the dermis or bloodstream [5].

Another application, already on the shelf, is a unique sunscreen lotion developed in Israel—Safe Sea. The lotion shields you from the sun and simultaneously protects you from jellyfish stings. Its principle is simple yet brilliant: it creates a protective layer on the skin that confuses the nematocyst and prevents it from recognizing you as prey. In other words, it disrupts the דטןצוךן combination that would trigger the nematocysts, giving you a good chance to leave the sea sting-free [6].

So next time you are at the beach, first apply sunscreen, and remember that you also have a way to outsmart one of the oldest and most precise firing systems in nature.

Hebrew editing: Smadar Raban
English editing: Elee Shimshoni

References:

[1] A genomic overview of 500 million years of cnidarian evolution

[2] Structure of nematocysts and the biomechanics of firing

[3] Nematocysts as osmotic systems

[4] The molecular filtering that enables precise firing in cnidarian cells

[5] Biotechnological applications of nematocysts in medicine

[6] Official Safe Sea sunscreen site