What sort of jellyfish is poisonous?

Answer 1

All jellyfish are venomous, but most are not injurious to us because either their venom is not potent enough to harm large species such as humans, or their stingers cannot penetrate our epidermal layer.

Among the most harmful jellies to us is a hydrozoan species- Physalis physalia, or Portuguese Man of War. They have very painful stings, but are rarely lethal to humans.

The most harmful species to us are in the cubozoan class (box jellies), such as Chironex fleckeri and the Irukandji species. Their stings are excruciatingly painful and occasionally can require life support and hospitalization for several days. Deaths are rare, but happen often enough to cause great concern and effort to limit exposure to them. Australia closes some beaches entirely during box jellyfish season, and puts out 'stinger nets' to protect popular beaches. Not all cubozoans are lethal, but scientists haven't identified all box jelly species, and speculate that one or more of these unidentified species may also be responsible for life-threatening injuries to humans.

There is no need to panic, however. In all recorded history, and many millions of stings, there have been about a hundred reported deaths due to jellyfish, mostly in the Indo-Pacific region. While that figure sounds scary, and unreported deaths are probably numerous, that is less than one reported death per year. You are more likely to be struck by lightning on a Golf course than face death from a jellyfish in the sea. Just don't hold a putter up in a thunderstorm, or jump into the water during stinger season along Australia's north coast.