Corals are often mistaken for plants, but these complex organisms are marine animals belonging to the phylum Cnidaria, a group that also includes jellyfish and sea anemones. The defining characteristic of all cnidarians is the presence of specialized stinging cells, which means some corals can and do sting. This stinging ability is a defense mechanism and a means of capturing prey. Understanding this biological detail is important for safely navigating tropical marine environments.
The Cellular Mechanism of the Sting
The sting originates from microscopic weapons called cnidocytes, which are specialized stinging cells found throughout the tissues of all cnidarians. Within each cnidocyte is a capsule-like organelle known as a nematocyst, containing a tightly coiled, hollow, harpoon-like thread. This structure is one of the fastest mechanical processes in the natural world, operating as a microscopic injection system.
When a trigger on the cell’s surface, called a cnidocil, is activated by touch or chemical stimulus, the nematocyst fires explosively. The firing mechanism involves a rapid influx of water, creating immense internal pressure. This pressure forcefully ejects the coiled thread, which turns inside out (everts) as it shoots out.
The harpoon penetrates the target organism, injecting a complex venom cocktail that can paralyze or kill small prey. Since each nematocyst can only fire once, the cnidarian must continuously replace these single-use cells. The purpose of this stinging apparatus is to subdue planktonic organisms for feeding and to deter predators.
Differentiating Stinging and Non-Stinging Corals
The term “coral” is broad, and a crucial distinction exists between true reef-building corals and the species responsible for human stings. The vast majority of hard, stony corals, which form the skeletal structure of reefs, are in the class Anthozoa and do not typically possess nematocysts powerful enough to penetrate human skin. Their stings are calibrated for microscopic prey like plankton.
Painful stings commonly experienced by humans come from “Fire Corals,” belonging to the genus Millepora. Despite the name, fire corals are not true corals; they are hydrozoans, classifying them more closely with jellyfish and hydroids. They are sometimes referred to as hydrocorals.
Fire corals can be difficult to identify because they grow in various forms, including branching, blade, and encrusting shapes, often resembling true corals. Visually, fire corals often have a yellowish-brown or mustard-color, frequently with white tips. A distinguishing feature is their fuzzy or mossy appearance due to the transparent, hair-like stinging tentacles protruding from minute pores across their smooth surface. True stony corals, by contrast, have visible cups, called corallites, where their polyps reside.
Immediate Symptoms and First Aid
Contact with fire coral results in a nearly immediate and intense reaction due to the injected venom. The typical symptoms begin within minutes, characterized by a sharp, burning sensation often described as feeling like fire, which is the source of the common name. This initial pain is rapidly followed by the appearance of a red, raised rash or welts, sometimes accompanied by small blisters or vesicles.
The immediate first aid response should focus on stopping further venom injection and managing the pain. Immediately rinse the affected area with seawater to remove any remaining nematocysts; avoid using freshwater, as this can trigger unfired stinging cells to discharge. Applying a mild acid, such as vinegar or isopropyl alcohol, can help stabilize and inactivate any undischarged nematocysts remaining on the skin.
After decontamination, carefully remove any visible coral remnants with tweezers. Immersing the affected area in hot water, as hot as can be tolerated without scalding, is an effective way to break down the heat-sensitive venom and relieve pain. Avoid rubbing the area, as friction can also cause more nematocysts to fire. If symptoms such as swelling of the face or throat, shortness of breath, or signs of infection develop, medical attention is required promptly.