The seas and oceans around Australia harbour numerous venomous jellyfish. Chironex fleckeri, the box jellyfish, is the most lethal causing rapid cardiorespiratory depression and although its venom has been characterised, its toxins remain to be identified. A moderately effective antivenom exists which is also partially effective against another chirodropid, Chiropsalmus sp. Numerous carybdeids, some unidentified, cause less severe illness, including Carybdea rastoni whose toxins CrTX-A and CrTX-B are large proteins. Carukia barnesi, another small carybdeid is one cause of the 'Irukandji' syndrome which includes delayed pain from severe muscle cramping, vomiting, anxiety, restlessness, sweating and prostration, and occasionally severe hypertension and acute cardiac failure. The syndrome is in part caused by release of catecholamines but the cause of heart failure is undefined. The venom contains a sodium channel modulator. Two species of Physalia are present and although one is potentially lethal, has not caused death in Australian waters. Other significant genera of jellyfish include Tamoya, Pelagia, Cyanea, Aurelia and Chyrosaora.