Nematocysts are concentrated on the tentacles or oral arms. A single tentacle can have hundreds or thousands of nematocysts embedded in the epidermis. Triggers of nematocysts are activated when contact is made with another object. Pressure within the nematocyst forces the stinging thread to rapidly uncoil. The thousands of nematocysts act as small harpoons, firing into prey, injecting paralyzing toxins.
Stings usually paralyze or kill only small creatures but some jellyfish are harmful to humans.
Stings occur when swimmers or beachcombers come in contact with nematocysts. Severity of stings depends on the species of jellyfish, the penetrating power of the nematocyst, the thickness of exposed skin of the victim and the sensitivity of the victim to the venom. The majority of stings from jellyfish occur in tropical and warm temperate waters. Most species off the southeastern coast are capable of inflicting only mild stings resulting in minor discomfort. Although most jellyfish that inhabit South Carolina waters are harmless to humans, there are a few, like the sea wasp, that require caution.