Jellyfish tentacles contain microscopic barbed stingers (nematocysts). Each nematocyst is made up of a tiny bulb that holds venom and a coiled, sharp-tipped tube. The jellyfish uses the venom to protect itself and kill prey.

When something comes in contact with the tentacle — a fish or a human — tiny triggers on the surface of the tentacle release the nematocysts. The sharp tube penetrates the skin and releases the venom, which affects the immediate area of contact and may enter the bloodstream.

Jellyfish that have washed up on a beach may still release venomous stingers if touched.

Types of jellyfish

While many types of jellyfish are relatively harmless to humans, some can cause severe pain and are more likely to cause systemic reactions. Types of jellyfish known to cause more-serious problems in people include the following:

  • Box jellyfish. Also called sea wasps, box jellyfish are generally the most harmful jellyfish to humans and can cause significant pain. Life-threatening reactions — although still relatively rare — are more common with these species. The more dangerous species of box jellyfish are found in the warm waters of the Pacific and Indian oceans.
  • Portuguese man-of-wars. Also called bluebottle jellyfish, these species live mostly in warmer seas. A Portuguese man-of-war has a blue or purplish gas-filled bubble that keeps it afloat on the surface of the water and acts as a sail.
  • Sea nettles. Common in both warm and moderately cool seawaters, sea nettles are the most common species of jellyfish on the northeast coast of the United States and abundant in the Chesapeake Bay.
  • Lion's mane jellyfish. These are the world's largest jellyfish. The body of a lion's mane can reach a diameter of 10 feet (3 meters). They are most common in cooler, northern regions of the Pacific and Atlantic oceans.
Sep. 01, 2011

You Are ... The Campaign for Mayo Clinic

Mayo Clinic is a not-for-profit organization. Make a difference today.