The phylum Coelenterata contains approximately 10,000 different species, of which several hundred are a danger to humans. This diverse group includes hydrozoans (including Portuguese man-of-war, stinging hydrozoans, and fire coral), scyphozoans (“true” jellyfish), and anthozoans (soft corals, stony corals, and anemones). They account for more marine envenomations than any other phylum. The important species involved in human injuries have stinging cells called nematocysts. Nematocysts are enclosed in venom sacs and are present in tentacles that hang from air-filled structures. After external contact, the nematocysts are discharged from their sacs, often penetrating the skin, and release their venom. Nematocyst venom is an extremely complex substance containing numerous proteins and enzymes. Clinical presentations following envenomation range from mild dermatitis to cardiovascular and pulmonary collapse. Mild envenomations usually result in a self-limited inflammatory eruption associated with burning and limited to areas of contact. Moderate to severe envenomations produce a spectrum of neurologic, cardiovascular, respiratory, and gastrointestinal symptoms. Anaphylactoid reactions—including hypotension, dysrhythmias, bronchospasm, and cardiovascular collapse—may occur, resulting in unexplained drownings.
Stinging Hydrozoan. Similar to fire coral, contact with stinging hydrozoan results in local burning pain followed by erythematous papules or urticarial eruptions and blisters. (Photo contributor: Shawn Miller.)
Hydrozoan Envenomation. This hydrozoan sting resulted from accidental direct contact with the diver’s finger while taking an underwater photograph. (Photo contributor: Ian D. Jones, MD.)
Fire Coral. (A) Fire coral. (Photo contributor: Shawn Miller.) (B) Fire coral envenomation. After contact, fire coral most commonly causes immediate local burning pain, followed by erythematous papules or urticarial eruptions. Pruritus may last for several days. (Photo contributor: Emily R. Stack.)
Portuguese Man-of-War. A beautiful Portuguese man-of-war with multiple tentacles dangling in the water. The tentacles, filled with venomous nematocysts, can extend several meters in length. (Photo contributor: Adam Laverty.)
Box Jellyfish (Under Blue Light). The box jellyfish delivers an unbearably painful sting. Envenomations can be life-threatening and require immediate treatment. (Photo contributor: Shawn Miller.)
Coelenterate Envenomation. The sharp angulations and undulations characteristic of jellyfish envenomation. (From Halstead BH. Venomous Marine Animals of the World. Washington, DC: US Government Printing Office; 1965.)
Coelenterate Envenomation. Jellyfish envenomation on the lower extremities. (Photo contributor: Department of Dermatology, Naval Medical Center, Portsmouth, VA.)