Marine Wildlife Encyclopedia
Purple Sea Urchin Strongylocentrotus purpuratus
This small sea urchin has been responsible for the demise of large areas of giant kelp forest off the North American coastline. Like most sea urchins, it feeds by scraping away at seaweeds and fixed animals and its favorite food is the giant kelp Macrocystis pyrifera. Its numbers reach densities of up to several hundred animals per square yard, and it can chew through kelp holdfasts, setting the plants adrift. Populations are normally kept in check by sea otters and by large fish such as sheepheads. In the past, when sea otters were hunted, urchin numbers increased explosively in some areas.