Sea urchins can scrape their way through solid rock to make themselves homes. This ability has long been suspected but never demonstrated, until now.
Michael Russell at Villanova University in Pennsylvania and his colleagues studied purple sea urchins (Strongylocentrotus purpuratus), which live along the west coast of North America. These animals look like purple balls with hundreds of spines. They also have structures called tubefeet, which they use to walk and move food to their mouths. They also use their tubefeet to attach themselves to rock, making it difficult to dislodge them. The safest homes are pits and holes, which offer a larger surface area for an urchin to lock onto.
In the lab, the researchers placed single sea urchins on flat pieces of soft mudstone, moderately hard sandstone and tough granite. After a year, they measured the weights of the rocks, how the surfaces looked, and how much the rocks were eroded.
The sea urchins had eaten holes in all the rocks, although they made slower progress on the harder ones. Field measurements showed that holes in mudstone were about 220 cubic centimetres, whereas holes in sandstone were 63 cubic centimetres and holes in granite were just 45 cubic centimetres.
“We were not surprised that they excavate rock,” says Russell. “What shocked us was… how fast they were able to form pits, particularly in the sandstone.”
Russell says the sea urchins’ drilling abilities are a by-product of how they eat. Each animal has five sharp teeth on its underside. Even when they are not chewing on food, the teeth are constantly scraping the rock they sit on, sculpting it in the process. The urchins eat the rock scrapings.
The team calculates that sea urchins can create an immense amount of sediment every year: perhaps 200 tonnes per hectare. That is comparable to the amount of sediment carried to the sea by many rivers.
The sea urchins’ drilling may significantly erode rocky reefs in the temperate seas where they live. However, unlike voracious tropical sea urchins, they may be too slow to be a cause for concern, says Timothy Davidson of California State University, Sacramento.
Journal reference: PLoS One, DOI: 10.1371/journal.pone.0191278
Article amended on 9 March 2018
We have corrected our description of sea urchin anatomy, where rock-eating rates were measured and where they live
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