The waters of Southeast Asia are fringed with some of the most beautiful beaches in the world. Countless commercials, eager to underline the paradise factor of whatever it is they’re selling, are filmed on Thai shores. But things are not quite so white and fluffy when it comes to jellyfish — whether they’re venomous or completely harmless.

Let’s start with the basics. Thailand’s waters are home to an enormous variety of jellyfish. And while the country has dedicated institutions such as the Department of Marine and Coastal Resources, backed by years of research, the ocean remains, for all intents and purposes, only superficially understood. Fortunately, for our purposes, superficial is exactly what we need: we’re interested in the creatures you might actually encounter at the beach.

And by now, the “Jellyfish of Thailand” folder is already packed with useful and occasionally rather alarming facts about these slippery creatures.

So, what exactly are jellyfish?

They are marine invertebrates belonging to the phylum Cnidaria — a group that also includes the long-suffering corals and anemones. The most familiar jellyfish belong to the class Scyphozoa, but the seas are also home to box jellyfish, or cubozoans, as well as various members of the hydrozoan families.

Jellyfish have no brain and no heart. They are about 95 percent water. Some, however, possess venomous cells that allow them to hunt and defend themselves.

And once you descend the taxonomic ladder to the level of species, you find a colourful army of around 9,000 varieties. Only about 100 of them are considered dangerous to humans.

So, roughly speaking, that’s one symbolic percent.

Thai Jellyfish: Meet the Locals

According to Thailand’s Department of Marine and Coastal Resources, eight species of venomous jellyfish have been recorded in Thai waters, belonging to five major families.

1. Chirodropidae.
This is the family of box jellyfish — and the most dangerous bunch of the lot. Their venom affects the central nervous system and can lead to the least desirable of outcomes. For a little reassurance, though, these particular box jellyfish are most dangerous during the monsoon season and are concentrated mainly around Samui, Phangan and Thailand’s southern coast.

2. Physalia utriculus.
Also known as the bluebottle or Portuguese man o’ war. This creature has been heavily promoted as Phuket’s resident marine monster, although technically speaking, it isn’t a jellyfish at all.
Yes, it looks remarkably jellyfish-like. Yes, it has tentacles. Yes, its venom can leave you with a nasty burn. But scientifically speaking, Physalia is a siphonophore — essentially a colony of tiny marine organisms living together as one rather impressive-looking creature. In other words: plankton with ambitions.
That technical distinction, however, does absolutely nothing to make its venom kinder to human skin. So stay alert: the season for bluebottles in the Andaman Sea runs from January to May.

3. Chiropsalmidae.
Another family of Thai box jellyfish. So far, only one species from this family has been recorded in Andaman waters, but that is quite enough. An accidental encounter can leave you with a painful burn — unpleasant, certainly, but not life-threatening.

4. Carukiidae.
Another box jellyfish, although this one belongs to a different clan. Two colonies of this family have been discovered not only in the Andaman Sea but also around Samui. They are not considered life-threatening either, but their stings are highly toxic and can easily trigger severe skin reactions.

5. Pelagia.
These may belong to the Scyphozoa rather than the box-jellyfish crowd, but when it comes to stinging, they punch well above their weight. They are beautiful to look at — which is precisely why you should resist the temptation to touch them.

There’s a reason they’re popularly known as fire jellyfish. And, as we all know, playing with fire is rarely a sound strategy.
Phuket’s Siphonophores

On Phuket, things are considerably less dramatic. Most of the species mentioned above have not been recorded here. The Portuguese man o’ war, however, does occasionally drift into local waters.

So, once again: from January to May, take extra care when swimming.

The venom of this siphonophore remains active even after the creature has been washed ashore. Which means that dry land does not magically turn it into a harmless beach decoration. Avoid physical contact with it there, too.
No handshakes.

Vinegar Is Your Friend

If you happen to have an encounter with Physalia utriculus, the likely outcome is a fairly serious burn — but not the end of the world. The important thing is to take the right measures.
And this is where vinegar comes in.

Treat the affected area with vinegar for around 30 seconds. Forget the idea that fresh water is a universal cleansing agent: in this particular situation, it can actually make things worse by triggering further venom discharge.

Drink the water.
Treat the sting with vinegar.
Do not mix the two up.

Follow those simple rules, and by evening you may have completely forgotten that you were ever stung.
Given that jellyfish are active year-round, the general advice is simple: don’t swim where warning signs have been posted. Don’t swim at night.

And always take a bottle of good vinegar to the beach.
Even if you don’t encounter a jellyfish, you’ll look absolutely fantastic with vinegar in your beach bag.

Original text translated by Claude