Irukandji jellyfish and box jellyfish are two different types of jellyfish. Box jellyfish are much larger than Irukandji jellyfish. Also, box jellyfish produce a complex venom that can cause cardiac arrest and attack the nervous system, skin cells, and other organs. Irukandji jellyfish have a smaller amount of poison, but it contains a potent neurotoxin that can cause severe pain, nausea, and breathing difficulties.