The Common Sea Urchin, Echinus esculentus, is constantly browsing any available surface for algae or invertebrate life. In the illustration you will see that the rocks are almost bare and that is largely due to the urchins activities. In this zone, which is around about 20m, there is little wave action so the urchins are not disturbed and huge numbers are present at times. On the underside of the urchin is a five-pointed beak-like arrangement which rasps food off the rocks. This is comparitivley powerful and few organisms can resist it. In shallower areas, and particularly on steep surfaces, wave action dislodges urchins that have managed to get up there in settled weather and the algae and invertebrate life is only affected minimally.
See separate picture showing a common sea urchin's tube feet which it uses to cling on to rocks when necessary.
St Abbs Marine Reserve. North Sea