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Box jellyfish belong to the most highly developed jellyfish and have a very well developed nervous system as well as eyes that enable them to see and follow movements. This is one of the reasons why these actively swimming hunters can come too close to divers and swimmers.
The fast-acting toxin produced by some species of box jellies is one of the most potent marine toxins. The fast and strong effect is needed so that these frail creatures can paralyze their prey immediately in order not to be torn apart by a panicking fish. One of the side effects is that the toxin can also cause severe harm to humans or even be fatal.