Smallest jellyfish
irukandji box jellies
5 centimetre(s)
Australia ()

The world's smallest jellyfishes are the irukandji box jellies, of which there are approximately 16 species, predominantly native to Australia's marine waters, especially those off Queensland, but also turning up elsewhere from time to time, including Japan, Florida and occasionally even off the English coast. They sport a bell that can measure as little as 5 cm across and boast a volume as small as 1 cubic cm, and they possess just four tentacles that can measure as little as just a few centimetres long. Despite their tiny size, however, they are extremely venomous, occasionally having proven fatal to humans, and unlike all other jellyfishes they bear stinging cells (nematocysts) not only on their tentacles but also on their bell.

Owing to the fact that they are not only tiny but also virtually transparent, irukandji box jellies remained undiscovered by science until as recently as December 1961, when a specimen of the first documented species, Carukia barnesi, was captured by Australian marine biologist Dr John Barnes using a special device.

Ironically, the severe effects of their stinging cells had long been known, including vomiting, intense headaches, chest pains and anxiety, as reported by people swimming in waters where these tiny creatures existed unseen. However, these effects' cause had remained a medical mystery until the first documented irukandji specimen was captured and Barnes bravely applied its tentacles to his own skin to observe their effects, which proved to be the very same ones that had been mystifying everyone for generations.

In recognition of Barnes's discovery and bravery, when it was formally described in 1967 Carukia barnesi was named in honour of him.