Australia has declared a state of emurchincy. Police and naval forces were called in last week after "unexploded mines" showed up near a popular snorkelling destination – but upon closer inspection, the danger balloons proved to be nothing more than a pair of sea urchins. Oh, humans.
The call came in after a swimmer spotted the urchins near Clifton Gardens Reserve in New South Wales. "He obviously got out of the water quite quickly," the NSW police said. "The water police weren't immediately available, so we went down for a look."
When authorities arrived, the concerned citizen got a chance to study photographs of both actual underwater mines and local urchin species, but he adamantly stood by his claim.
"We had to take him at his word – though of course we were a bit suspicious. It would be unusual for any [naval mines] to show up in the harbour. The water police eventually turned up, and set a guard up overnight because the navy had to be called down the next day."
Because naval specialist divers weren’t immediately available, a maritime exclusion zone was put in place, banning both beachgoers and vessels from nearby waters. It was only the following morning that the culprits were revealed: the "mines" turned out to be urchins in the genus Goniocidaris, a group known for its distinctive club-shaped spines.
"The divers went down, I imagine had a quiet giggle underwater, and then came back up and said, 'It's some sea urchins, you idiots'," the police said.
You may notice that the urchins' blunt appendages look slightly encrusted. These invertebrates belong to the family Cidaridae, a group whose spines lack skin (yes, most urchin spines are covered in a thin layer of skin). Because of this, algae – and in some cases even other animals like sponges and worms – are able to stick to them.
"Most of these encrusting animals are filter feeders (they pick food out of the water), and living on sea urchin spines lifts them into the water water column and off the gritty, dirty bottom," explains the Echinoblog's Dr Christopher Mah. "It's all about prime real estate baby!"
The urchins themselves are omnivorous, and can be found feeding on everything from plant matter to dead fish.
UPDATE (8:04 PST APRIL 29, 2016): It's also possible that this urchin belongs to the genus Phyllacanthus. Also known as "pencil urchins", these animals also possess blunt, club-like spines.
Top header image: Philip/Flickr