Flower sea urchins might look like underwater bouquets covered in delicate petals but they are highly toxic.
Their scientific name – Toxopneustes – gives you a clue to stay away from these organisms. According to the Natural History Museum, ‘Toxopneustes’ means ‘poison breath’.
Other sea urchins usually sting via spines but flower urchins deliver their venom through the same petal-like structures (known as pedicellariae) covering their bodies that make them look so pretty.
Each ‘petal’ is like a tiny claw. When the urchin detects a threat, its pincers clamp shut and inject venom. These little jaws can even detach completely from the urchin, continuing to grip onto the perceived attacker.
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“Venom delivered by the pedicellaria (small pincer-like organs) that cover the urchin's hard shell (test) can cause contractions in smooth muscle which lines many major organs, including cardiac (heart) tissue,” says Guinness World Records on its website, which rates these animals as the most dangerous sea urchin in the world. “This can lead to severe pain, respiratory problems and even paralysis.”
There have been reports of people drowning after being stung, according to Guinness World Records, but these have not been scientifically verified.
There are surprisingly few reports of what it’s like to get on the bad side of these urchins from people who have actually been stung. Most examples point to a report of a sting that happened nearly 100 years ago.
In 1930, Dr Tsutomu Fujiwara from the Hiroshima Zoological Laboratory in Japan was stung by a flower urchin that he’d accidentally touched with his bare hands. His 1935 account (quoted by Guinness World Records) says that, when he touched the urchin, seven or eight pedicellariae “stubbornly attached themselves” to his finger, causing immense pain.
“After a while, I experienced a faint giddiness, difficulty of respiration, paralysis of the lips, tongue and eyelids, relaxation of muscles in the limbs, was hardly able to speak or control my facial expression, and felt almost as if I were going to die,” he says. “About 15 minutes afterwards, I felt that pains gradually diminish and after about an hour they disappeared completely.”
His facial paralysis finally subsided about six hours after the sting.
An interesting behaviour that’s sometimes missed in all the talk of venomous stings is that flower urchins sometimes cover themselves with rocks and other debris.
Some have suggested this might be to protect them from the sun’s rays, as if they were creating their own parasol. But a study in the journal Marine Biology suggested the urchin might be trying to protect itself from strong wave surge. “The ratio of covering material [to] body weight and the percent cover of material held were highest at the site with the most surge,” the authors write in the paper.
Although all this talk of toxic stings might make it sound like we’d be better without flower sea urchins in our seas, they do have benefits. “These sea urchins play an important role in marine ecosystem by consuming algae, effectively keeping them from covering coral reefs,” write the authors of a 2025 scientific study published in Mitochondrial DNA Part B
