Which cnidarians can deliver painful or dangerous stings to humans?

While all cnidarians possess nematocysts—specialized stinging cells used to capture prey and defend themselves—most species are not harmful to humans. However, certain cnidarians can deliver painful or dangerous stings due to the toxicity of their venom.12

The most dangerous group is the class Cubozoa, or box jellyfish. The Australian box jellyfish (Chironex fleckeri) is considered the deadliest, as its potent venom can cause extreme pain, paralysis, cardiac arrest, and death within minutes. Other box jellyfish, such as the Caribbean species (Tripedalia cystophora), may cause intense pain without being fatal, while some can induce Irukandji syndrome, characterized by severe muscle cramps, vomiting, and heart complications.34

Other cnidarians known for painful or dangerous stings include:
* Portuguese man-of-war (Physalia): Can cause extremely painful stings, leading to paralysis, shock, or even death in sensitive individuals.12
* Gonionemus: Specifically G. vertens, which can cause burning sensations, blistering, joint pain, and respiratory distress.5
* Sea anemones: Certain species, such as those in the genus Actinodendron, are known to impart painful stings.2