Almost every aquarist will feel the sting from a cnidarian (anemones, jellyfish, ect.) at some point or another. For some, it can just be a mild annoyance, but for others it can be downright painful or even dangerous. The correct treatment depends on what you were stung by and how sensitive you are to it.
Cnidaria is a large phylum (one of the broadest scientific classifications) and includes jellyfish, corals, anemones, and hydroids that aquarist might encounter in their tanks. Cnidarians have specialized stinging cells known as “nematocysts” or “cnidocysts”. These cells can be used as defense mechanisms or to catch prey. Some are harmless to people, but effective on the cnidarian’s targeted prey. Some can be lethal to anything they touch – the Sea Wasp, a type of box jellyfish, is touted as the most venomous marine animal ever and is usually fatal. Different types of nematocysts have different functions and one animal can have more than one type of nematocyst at a time, but all function essentially the same way.
A nematocyst is like a spring-loaded harpoon. When the trigger is activated, either mechanically by something touching it or chemically by a change in its structure, the barb-like tip is discharged to deliver its venom. The most common “attacks” in home aquariums are a result of an aquarist (or a tankmate) rubbing against a stinging organism like an anemone, hydroid, or a few types of corals.
How the sting may affect you depends on your sensitivity. The venom in most stinging cnidarians is similar to that of a bee or wasp. If you know you are sensitive to or react poorly to bee or wasp stings or other stinging organisms like poison ivy or nettles, use caution around cnidarians! Gloves or other protections can help you avoid being stung. If you are stung, it is important to treat the sting to help avoid further injury.
If you experience shortness of breath, dizziness, nausea, muscle soreness or any other such symptoms, seek medical attention! This basic treatment and advice is no substitute for professional medical care. Though the cnidarians you normally see in aquaria are not usually as threatening as the Sea Wasp, everyone reacts differently and these symptoms may be a sign of a severe reaction.
Once you are aware of having been stung, you first want to get rid of any nemaocysts that may be stuck to your skin. Avoid rubbing the area with your hand since that may result in your hand being stung as well. Rub the area with sand, a paper towel, or the edge of a credit card or similar object and rinse with saltwater (freshwater may cause any untriggered nematocysts to fire). Once the area is free of any more nematocysts, rinse and clean it well with warm freshwater. Cleaning the area with a paste of plain meat tenderizer or a white vinegar rinse helps some people since the enzymes in the tenderizer and the acidity of the vinegar can help to break down the venom. You can apply a topical hydrocortizone cream to the sting area or rash and ice can help relieve any pain from the sting. Benadryl or another diphenhydramine can also help cut down on a mild allergic reaction from the sting. This treatment is good for most mild reactions similar to that from an insect sting or poison ivy. Again, if you are experiencing more severe symptoms, get professional medical attention! The treatment also varies and stings are more severe from venomous fish like lionfish and require more medical attention.
Awareness can prevent most cnidarian stings and knowing your own limitations are important. Most anemones don’t bother me, personally, but I can’t touch a carpet anemone without paying for it later. While avoiding that first sting is important, most aquarists will think twice before not wearing gloves or bumping against an anemone again!
You can see a video of nematocysts firing here.