Last weekend, a lone, gray heart-shaped shell drifted ashore, nestling itself upon a beach in Cornwall, England. It was quickly joined by another—and then another. Before long, hundreds of these bizarre, hollow orbs speckled the sands, baffling and alarming innocent locals and tourists hoping for a relaxing mid-morning stroll along the coast.
“I took one home with me, then panicked and put it in the bin in case it attacked,” one dog walker told Damien Gayle of The Guardian. The strange spheres similarly spooked the resident’s spaniel, Gayle reports, and the pup refused to go near them.
In truth, it was a graveyard, stretching as far the eye could see.
These odd little orbs are actually the ghosts of a species of sea urchin, Echinocardium cordatum, often fondly referred to as “sea potatoes.” Their Latin name translates literally to “spiny heart,” a nod to their endearing physique. (In fact, both the suffix “-cardium” and “cordatum” translate to “heart” or “heart-shaped”: these urchins looked so nice, they were nicknamed twice.)
To make their homes, these sea urchins use spoon-shaped spines on their undersides to plow through sediment on the ocean floor. But because the long, soft bristles on their backs can poke tiny holes through the surrounding sands, they are still able to breathe in these burrows, which are typically a few inches deep. While the urchins are alive, these furrowed yellow spines cover most of their bodies, resembling a fine shag rug. Just like humans, these little critters hate coming home with sand stuck in unsightly places, so they stew nonchalantly in a layer of mucus they produce to keep the mud at bay.
But even with these safeguards, sea potatoes are fragile. Though they can live around ten years in the wild, they rarely survive collection. When they die, their delicate spines fall away, leaving a hollow husk, or “test,” that can wash ashore completely bare. These dried-up remnants—brittle and white, gray or brown in color—give the sea potato its common name; though each urchin shell, with its signature criss-crossing stitches, perhaps more resembles the flattened corpse of a crusty, forgotten baseball.
The appearance of hundreds of sea potatoes at once has fueled some concern amongst Cornish residents. But “mass strandings” are apparently not that uncommon, and not necessarily the result of some malicious massacre. BBC reports that a similar display of urchin carnage occurred on Long Rock Beach, also in Cornwall, in 2016.
So who—or what—is responsible?
Martin Attrill, a marine ecologist at the University of Plymouth in the United Kingdom, told BBC in 2016 that these events can happen when sea urchins come together to breed. Rather than copulating through physical contact, males and females will congregate and liberate their sperm and eggs into the surrounding waters. Thus, it’s to their benefit to huddle close together during mating season, upping the odds that their gametes will be ready to mingle. Most sea urchins mate in spring, but summer reproduction isn’t unheard of.
In any case, whether for an orgy or other cordial gathering, a sea potato party can quickly go south if a storm blows through, Attrill tells BBC. In keeping with this, the appearance of 2016’s Cornish sea potato graveyard immediately followed a couple “pretty strong” storms.
“I think such things happen from time to time and are entirely natural—bit like bushfires,” Attrill explains to Gayle at the Guardian. “Many of these marine seabed species have real boom-and-bust cycles where some years they do really well and others not so well. It is all part of the circle of life.”
Luckily, these sea urchins are in no danger of going extinct anytime soon. They’re found all over the world, except in the iciest of oceans. We may never find out exactly what brought about the portentous passing of so many sea potatoes last weekend, but these hardy little baseballs will likely endure whatever curve ball is thrown at them next.