The irukandji jellyfish are tiny, but they could equally fit into the dangerous category.
These tiny transparent species of box jellyfish with 16 known species grow from 5 millimeters / 0.2 inches to 25 millimeters / 0.98 inches wide. They have four long tentacles, ranging from only a few centimeters to 1 meter / 3.3 feet in length.
Irukandji jellyfish are found in the coastal waters of northern Australia and are considered to be one of the most venomous jellyfish in existence. They have stingers on both their tentacles and, unusually, their bell and can fire them into swimmers causing painful stings known as Irukandji syndrome, which can be deadly.
It is said that between 50 and 100 people are hospitalized each year after being stung by an irukandji jellyfish. Fortunately, when properly treated by medical professionals, stings are not usually fatal.