The Irukandji jellyfish, which is a highly venomous type of Australian box jellyfish, has nearly invisible tentacles and a tiny, transparent bell that makes it very hard to see. Though the Irukandji's bell is typically less than an inch wide, its translucent tentacles, which contain venomous darts, can be up to 3 feet long.
In humans, an Irukandji sting can bring about a condition known as Irukandji syndrome, which produces a range of different symptoms ranging from psychological effects to cardiovascular symptoms and painful cramps. Typically, the syndrome is not fatal, especially if treated properly. The symptoms tend to set in several minutes, and sometimes up to 2 hours, after a sting.