Sea urchins are well-known residents of ocean reefs worldwide. These living pin cushions can be found on coral reefs in the tropics and rocky reefs in both north and south polar regions, and everywhere in between. The two most common species of sea urchins found off the coast of southern California are the purple sea urchin (Strongylocentrotus purpuratus) and the red sea urchin (Mesocentrotus franciscanus, formerly Strongylocentrotus franciscanus). The purple urchin lives in a delicate balance with the giant kelp (Macrocystis pyrifera) in kelp forest ecosystems along the Pacific coast of North America. Feeding preferentially on giant kelp, purple sea urchins rely on a constant supply of kelp raining down to the bottom of the surrounding forest.
At the same time, sea urchins are favored prey of spiny lobsters (Panulirus interruptus), sheephead fish (Semicossyphus pulcher), and southern sea otters (Enhydra lutris). For that reason, they prefer to hang out in hard to reach places to provide protection from these voracious predators. It is a well-known phenomenon that purple urchins are often found in cavities in the rocky reef, the openings of which are too small for the urchin to have passed through. For that reason, it has been assumed that these creatures were capable of somehow boring into solid rock, but until very recently there has been no verifiable evidence of this.
Published just this past February, researchers from Villanova University in Pennsylvania have shown that purple sea urchins are able to bore into rock, but to do it at surprising speeds. In laboratory experiments, urchins were able to excavate depressions in sandstone in a matter of months. Based on these results it was calculated that over large areas with high urchin densities the amount of material created by urchin erosion of rock can be comparable to the amount of sediment carried by many rivers.
Another example of how living organisms can alter the environment in a big way, in spite of opinions to the contrary.
This photo was taken at Santa Rosa Island off Saint Augustin Canyon with a Canon EF100 mm f/2.8 Macro lens on a Canon EOS 5D Mk III in an Ikelite underwater housing. Lighting was provided by an Ikelite DS161 strobe in eTTL exposure mode. The exposure was set to 1/200 sec at f/6.7 and ISO 400.
Sources:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0191278
https://www.montereybayaquarium.org/animal-guide/invertebrates/purple-sea-urchin