Looking at cold-water corals, like Lophelia, initially reveals a structure that is dominated by a branching limestone skeleton. Moving closer in and hundreds of tiny polyps become visible, with their flexible tentacles extended to catch passing food. The tentacles are the most obvious feature of the polyp, with species like Lophelia having up to 16 tentacles surrounding the polyp’s single opening - which serves both as a mouth and anus.
Like sea-anemones and other cnidarians, the tentacles of cold-water corals contain specialised stinging cells called nematocysts. These cells can immobilise prey by injecting a potent poison. Some corals only produce a mild sting, causing an itchy rash, whilst others are very dangerous, especially to people who could suffer an allergic reaction to the sting