Diana Nyad was forced to abort her storied attempt to swim from Cuba to Florida as a result of repeated stings from Portuguese Man-of-War on her face, arms and side. As disappointed as Nyad was by her failed attempt to cross the straits of Florida, she was lucky in one respect: A sting can result in severe muscle pain, fever, nausea and vomiting.
Small and deadly, the 'Irukandji' jellyfish has a bell which measures between 0.20 to 0.39 inches wide. Its tentacles can measure as little as a few centimeters, up to more than 3 feet in length. But if you happen to encounter a pint-sized version of the animal, don't get carried away by its non-threatening appearance. The Irukandji is considered to be 100 times as venomous as the bite of a cobra. Also, pay attention to this other distinguishing feature: Unlike other jellyfish, 'Irukandji' jellyfish have stingers on their tentacles as well as on on their bells.
Like the Portuguese Man-of-War, the sting from a box jellyfish can be deadly to humans. Beautiful but deadly, the box jellyfish produces venom that attacks the heart, nervous system and skin. Despite official statistics, it's estimated that box jellyfish stings account for dozens of human deaths per year.
Lion's mane jellyfish
The sting from the lion's mane jellyfish has been described as more akin to a burning sensation than that of a sting. Its size varies though the largest known specimen was longer than a blue whale. While not thought to be deadly, the sting nonetheless can leave a nasty and painful calling card. In a bizarre episode in July 2010, around 150 people swimming around Rye, New Hampshire wound up stung by what was reported to be the remains of a dead Lion's mane jellyfish that had earlier broken into pieces.
Sea Nettle jellyfish
When summer bathers hit the beaches, so, it seems, do the Sea Nettle jellyfish. Periodically, massive swarms appear in the water, a phenomenon still not fully understood by marine biologists. Pain from the animal's stings usually disappears within an hour of initial contact.
This species of jellyfish is found through the world's oceans. The Moon jellyfish falls into the category of being slightly venomous. Contact can lead to prickly sensations to mild burning.