The sea urchins are related to the starfish and sand dollar.  They are grazers and will graze algae until it’s eaten for that particular area.  They normally feed on broken off parts of the plant, but have been known to overeat, which cause devistation to kelp beds.  This is why it’s very important to maintain the natural predators of the urchins.  Urchins are the responsible for urchin barrens which are areas cleared away of kelps mainly by the grazing of Strongylocentrotus purpuratus.