3. Irukandji jellyfish

A fully grown Irukandji jellyfish.

A type of small box jellyfish, the Irukandji jellyfish can be the size of a finger nail in the bell, and its tentacles can be up to one metre (3.2 feet) in length.

Scientists are not certain of the full range of locations where they occur, Robinson said, but there have been remarkably similar stings along the Australian coastline. While the box jellyfish is usually found in shallow water, the Irukandji jellyfish is most often in deep water.

Danger rating: 9/10

Scientific name: Carukia barnesi

How to know if it's got you: Getting stung by these little buggers can bring on Irukandji syndrome. Taking about 30 minutes to set in, the syndrome can be marked by severe lower back pain, cramps, sweating, anxiety, nausea and other, more fatal, symptoms.