Fire coral is a marine organism in the family Milliporidae which is notable for its stinging cells, which can cause persistent pain to organisms — such as unwitting divers — which come into contact with it. These corals appear in tropical and subtropical waters all over the world, and they can be especially perilous for divers since they appear in a variety of guises. Incidentally, people who are stung with fire coral should use an acid such as vinegar or lemon juice to treat the sting, and if acid is not available, the sting should be flushed with seawater and lightly covered until some vinegar can be obtained.
In strict point of fact, fire coral isn't a coral at all. It is in the are in the same phylum, Cnidaria, as coral, so it is a distant relatives, but they are actually radically different organisms. Fire coral is part of a class of creatures known as hydrozoans, more closely related to jellyfish than true corals.
However, fire corals do share some traits with true corals. They live in colonies which cover themselves in a calcareous skeleton for protection, and they have a symbiotic relationship with algae which is encouraged to grow inside the organism. The algae converts sunlight into energy, and the tiny hydrozoans inside the “coral” use this energy to survive.
The life cycle of fire coral is markedly different from that of true coral. In colonies, the organism takes the form of a polyp, as is the case with coral, and the polyps are capable of reproducing asexually, cloning themselves to expand the size of the colony. The polyps can also bud to create medusas, free-swimming versions of polyps which are capable of sexual reproduction. When medusas reproduce, their offspring can in turn create new colonies.
Fire coral can range in color from yellow to brown, and it takes a variety of forms. Sometimes it takes over, creating a crust on top of rocks and true corals, and it can also grow in plate-like forms and finely branched forms. At a casual glance, this organism often looks like seaweed, so divers may accidentally touch it or swim through it, not realizing the danger. Fire coral is also very hard, and capable of cutting skin and damaging diving suits. People who are cut should seek medical attention, as the wound may need to be cleaned to prevent infection and remove all of the painful stinging cells injected by defensive polyps.