Jellyfish and Other Cnidaria
The large 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 have venom contained in microscopic structures like water balloons, called nematocysts.
Mechanism of toxicity. Each nematocyst contains a small, ejectable thread soaking in viscous 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.
Toxic dose. Each time a nematocyst is opened, all the contained venom is released. The degree of effect depends on the particular species, the number of nematocysts that successfully discharge venom, the envenomation site, the contact time, and individual patient sensitivity (eg, children may be more susceptible owing to their smaller body size). 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 species), 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,
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