And if comes under sustained attack it responds by sending its compatriots into a super-breeding frenzy in which millions of replacements are created.

The really bad news is that the box jellyfish and another equally poisonous species, Irukandji, are on the move. Scientists are warning that their populations are exploding and will pose a monumental problem unless they are stopped.
[...]
The only deterrent so far found is a colour - red. The jellyfish simply ploughed through white-coloured poles in the water and swam round black poles but they stayed as far away as they could from red poles.