One of the first warnings to snorkelers exploring the Caribbean waters around the Virgin Islands is to avoid the fire coral. That is because they can deliver extremely painful stings that are often described as having the same intensity as the Portuguese man o’ war. This is not a stretch as fire coral is not a true coral, but a hydrozoan, putting them into the same bigger family that includes jellyfish.

In the USVI waters around St. John, fire coral can be found in three different forms: flat sheets that coat rocks and dead coral, blade-like plates, or as little branching tree-like structures. Often shaped like flames, their colors —brownish yellow to darker orange— can also appear as fires of varying temperatures. At their tips where active growth occurs, the color can be a pale white, resembling a hotter fire, for example.

Image: Fire Coral, Location: Virgin Islands, Credit: Jane Conrad Fahrner

Fire coral are very common, especially in places like Virgin Islands National Park, and causes many minor injuries every year. It doesn’t go after anyone as it isn’t mobile, like jellyfish, but does happen to grow in most of the places where tourists swim, wade, and likely to grab

How Dangerous Is Fire Coral

Fire coral’s stringers are made of nematocysts, which are microscopic stinging cells that activate immediately on contact. Just brushing against these creatures can cause an intense burning and stinging sensation, though sometimes reactions may be delayed five to thirty minutes. Afterwards, victims of fire coral stings can expect local redness, itching, and raised welts (also called wheals) or small blisters.

In more severe cases, there can be tissue irritation and in some rare situations, small areas of the skin can become necrotic. While these stings are painful and something everyone wants to avoid, they aren’t usually considered dangerous, like a shark bite might be. The risk to watch for is really more of a secondary infection from broken blisters, etc.

First aid includes rinsing with seawater or vinegar (don’t use fresh water, which triggers nematocysts to fire again), removing fragments with tape or tweezers, then using a hydrocortisone cream or a cold compress to treat itching and swelling. Naturally seeing a doctor becomes necessary if a wound opens up, looks infected, or with signs of a broader reaction.

Fire Coral Sting vs. Jellyfish Sting

Both the fire coral and jellyfish sting with nematocysts, meaning the underlying mechanism is similar, and that's part of why people might say a fire coral sting feels like being stung by a jellyfish. However, there are differences.

A fire coral sting happens when someone brushes the stationary colony; the reaction is very local to the exact spot it touched. A jellyfish sting comes from a mobile organism that often has trailing tentacles, which means the contact area is usually longer and more diffuse, like stripes all the way down the back and legs. The severity varies hugely by species too. For example, moon jellyfish sting might be barely noticeable, while a Portuguese man o' war or a box jellyfish sting can cause intense pain and be quite medically serious.

A Stinging Fire Worm That Eats Fire Coral

If you get a close enough look at fire coral you might see a bearded fireworm. This worm actually dines on fire coral. It can also sting people, but it does not get it’s venom from the fire coral. Its white, hair-like bristles (chaetae) have their own venom which is injected when touched and these bristles break off in the skin on contact. It can cause a painful, itchy, burning sensation on top of the sting the fire coral inflicts. A double sting awaits, if you’re not careful out there.

Why Fire Coral Isn’t Dying Like Other Coral

Fire coral is one of the few reef organisms gaining ground while others, especially true corals die off. Researchers say it’s because it is able to spread as flat sheets in calmer conditions or build up into taller branching structures where competition is tougher. This high degree of adaptability is its real secret. Plus, it stings and people actively avoid touching or even kicking fins near it.

Extensive heatwaves have however had significant impacts on the shallow-water fire coral populations in the USVI, so even this comparatively hardy organism is not immune to climate change. It is definitely taking some hits, just not as noticeably as the true corals like staghorn, elkhorn, and stone corals.

Ultimately, the safest approach to take around fire coral in USVI is the one park rangers recommend for coral in general: keep your distance, don't stand on the reef, and if you absolutely must touch down, shoot for a patch of bare sand.