Mertens' Carpet Anemone

The biggest anemone in the world by disc area: a metre (39 in) and more of short-tentacled carpet, draped over reef rock and following its contours rather than lying flat as its shallow-water relative does. The column carries orange or purple warts, visible where the edge lifts clear of the substrate.

It lives on reef slopes down to about 30 m (100 ft), and its size makes it a habitat rather than an animal: clownfish, damselfishes, cleaner shrimp and porcelain crabs all use one, sometimes simultaneously. A large individual can be a hundred years old, which is worth knowing before touching one - anemones recover slowly from damage, and the residents have nowhere else to go. The sting is milder than the gigantic carpet's and harmless through a wetsuit. Mertens' name comes from the naturalist on a nineteenth-century Russian circumnavigation.