Beachgoers in Cornwall got quite the surprise this past weekend when hundreds of the tennis-ball-sized marine invertebrates known as sea potatoes suddenly appeared on the shore.
Penzance local Rosie Hendricks was strolling along the water at Wherrytown when she and her family first spotted the mass stranding. She told the BBC that she had never seen anything like it before and “wasn’t sure what they were”.
Yet despite being a rare sight for the general public, these unusual creatures are in fact incredibly common in temperate coastal waters around the world and off Britain and Ireland in particular – they’re simply hidden in the sand most of the time. Also known as heart urchins due to their recognizable shape, the animals Ms Hendricks encountered belong to the species Echinocardium cordatum, from the urchin family Loveniidae. E. cordatum live inside tunnels that they dig 10 to 15 centimeters (4 to 6 inches) deep into the sand on the sea floor at water depths down to about 200 meters (656 feet). Just like their relatives, sand dollars and sea stars, sea potatoes have tube feet that they use to explore their environment, collect food, and pass it to a central mouth cavity.
During their estimated 15-year lifespan, the invertebrates are covered with a mat of fine yellow to brown spines that gives them a furry-like appearance. After death, however, their bodies quickly shed these spines as they are battered by waves and against rocks and sand, revealing the five-point star that gives away their relationship to other echinoderms.
According to Plymouth University marine ecologist Professor Martin Attrill, mass strandings such as the one in Wherrytown are not unusual. Speaking to the BBC in August 2016, following a similar event at Long Rock beach in Cornwall, he noted that large groups of E. cordatum gather in the late summer to spawn, and thus a storm or especially turbulent waves during this period can carry a large number onto the shore.
If you happen to encounter one (or an entire beach full) of these urchins, you can appreciate them without worrying about toxic stings or dangerous spines. Unlike some species, E. cordatum’s spines are unlikely to pierce the skin and don’t contain any poison.
“They are quite cute," Attrill said.