This tooled-up bruiser leaves its mark on the underwater environment, says Paul Naylor
SWISS ARMY KNIFE An urchin's globe carries an impressive array of tools. The spines, attached by ball-and-socket joints, can be moved to help walk the animal along, or be held rigid for defence. Among them are flexible tube feet to help it move, and long-stemmed structures with lockable plier-like jaws for removing debris and any creatures seeking a free ride.
FAMOUS GREEK Around a grazing sea urchin's mouth is an elaborate structure which houses five chisel-like teeth used for scraping food off rock surfaces. The teeth grow continuously as they are worn down in use. The whole mechanism, which can be protruded and withdrawn, is called 'Aristotle's lantern', after the man who first described it.
UNDERGROUND MOVEMENT The obvious urchins live on rock faces, but there are also those that spend all their lives burrowing beneath sandy sea floors. They have a much more streamlined shape, shovel-shaped spines for digging, and highly elongated tube feet for reaching, like a periscope, to the surface of the sediment. One common species rejoices in the apt name of 'sea potato'.
MY STAR COUSIN Urchins look unique, but in fact belong to the same animal group as starfish and share many features (imagine a starfish curled up into a ball). A starfish has small bony plates embedded in its body wall. An urchin has similar but much larger plates, which interlock to form the globe-like skeleton or 'test'.
CUTTING A DASH The common sea urchin is a powerful, unfussy grazer, eating animals such as barnacles as well as seaweed, and leaving virtually bare swathes across a rock face in its wake. If sea urchins are thriving in a particular area, they have a major impact on the type of community that can develop, so are very important to the ecosystem.
NO SEX PLEASE Much of the internal space of an urchin is taken up with its gut and the reproductive organs. The sexes are separate but, perhaps wisely, urchins don't get intimate. They simply shed their sperm or eggs into the sea for fertilisation to take place. A large female might contain 20 million eggs.