Alexander Lowry/Science Source

How a sea urchin is like a rushing river

Colorful, spiky sea urchins sit snugly inside depressions pocking rocky reefs around the world, but naturalists have argued for centuries over whether they create these pits themselves or simply find existing ones and grow into them. Nobody has provided proof one way or another—until now. In a study published today in PLOS ONE, scientists spent a year observing purple sea urchins (Strongylocentrotus purpuratus, pictured above) growing on sections of sandstone, mudstone, and granite reefs that had been transported into a laboratory setting. They found the urchins indeed excavated their own cozy cavities within a few months, with shallower pits and flatter urchins occurring on granite reefs, and deeper pits with taller urchins on sandstone and mudstone reefs. After a year, they dissected the urchins and explored their digestive tracts, discovering the dug-out sediment in the animals’ guts. Sea urchins, it turns out, eat their substrate to shape their homey habitats in order to provide a stable place to grow and reproduce, as well as shelter themselves from predators—and they have quite the appetite." Across the species' range from Baja California, Mexico, to Alaska, bioerosion on urchin-covered sandstone reefs, the researchers report, produces sediment approximately equivalent to that delivered to the coast by a river—some 200 tons of sediment per hectare—suggesting that when you stroll along the beach, a not insignificant chunk of the sand is, in fact, sea urchin waste.