The phylum Cnidaria (coelenterates), numbering over 10,000 species, includes fire coral, jellyfish (including Portuguese man-o-war, box jellyfish, sea nettle), and anemones. Despite considerable morphologic variation, all these organisms deliver venom through specialized microscopic organelles called nematocysts. Of the 10,000 different species of cnidaria, 100 are known to injure humans with nematocysts capable of penetrating the human dermis
Each nematocyst contains a small, ejectable thread soaking in venom. The thread has a barb on the tip and is fired from the nematocyst with enough velocity to pierce human skin. The nematocysts are contained in outer sacs (cnidoblasts) arranged along the tentacles of jellyfish or along the surface of fire coral and the finger-like projections of sea anemones. When the cnidoblasts are opened by hydrostatic pressure, physical contact, changes in osmolarity, or chemical stimulants that have not been identified, they release their nematocysts, which eject the thread and spread venom into the skin of the victim. The venom contains numerous chemical components, including neuromuscular toxins, cardiotoxins, hemolysins, dermonecrotoxins, and histamine-like compounds.
Each time a nematocyst is opened, all the contained venom is released. The extent of toxicity is dependent on the number of nematocysts that successfully discharge venom, the envenomation site, the contact time, the particular species involved, and individual patient sensitivity. Hundreds of thousands of nematocysts may be discharged with a single exposure.
Deaths from jellyfish stings in the Northern Hemisphere are rare and almost always are due to the Portuguese man-o-war (Physalia physalis), although Chiropsalmus quadrumanus (a type of box jellyfish) was implicated in the death of a child off the coast of Texas.
The Australian box jellyfish (Chironex fleckeri, "Assassin's Hand") is the most venomous marine animal and responsible for numerous fatalities. It should not be confused with the Hawaiian box jellyfish (Carybdea alata), a related but significantly less toxic species.
Stinging produces immediate burning pain, pruritus, papular lesions, and local tissue inflammation, which may progress to pustules and desquamation.
Nausea, vertigo, dizziness, muscle cramping, myalgia, arthralgia, anaphylactic and anaphylactoid reactions, and transient elevation in liver transaminases may follow.
Severe envenomation may result in respiratory distress, severe muscle cramping with hypotension, arrhythmias, shock, and pulmonary edema. Lethal outcomes are associated with rapid onset of cardiovascular collapse. Fulminant hepatic failure and renal failure have been reported after sea anemone stings.
"Irukandji syndrome" is associated with stings from Carukia barnesi, found mostly in the oceans off Australia's Northern Territory and less commonly near Hawaii and Florida. These stings can induce a severe catecholamine rush that often leads to severe hypertension, cardiac dysrhythmias, pulmonary edema, cardiac myopathy, and death. Skin findings are often absent. Muscle spasms, frequently involving the back preferentially and coming in waves, are described as unbearable and parenteral analgesia is often necessary....