The blue ringed octopus is a small brown octopus which develops brilliant blue ring-shaped markings when disturbed. The genus is found throughout Australia's coastal waters and contains H. maculosa (the southern blue ringed octopus) in southern regions, H. lunulata (the greater blue ringed octopus) in more tropical areas, and H. fasciata (the blue lined octopus) which is found along the east coast of Australia. This genus has been associated with severe envenomations resulting in respiratory failure and human fatalities. The blue ringed octopus is found in tidal rock pools and is very attractive, especially to children and tourists, who are at risk of envenomation when they pick up the octopus. Bites can also occur when the creature is trodden on by waders.
The venom contains tetrodotoxin, (also found in puffer fish) and is secreted in the saliva of the octopus. It causes blockade of sodium channels and thus neurological problems such as weakness, numbness or paraesthesia, breathing difficulties and paralysis. The patient may be completely paralysed and unable to respond, sometimes with fixed dilated pupils, but the sensorium is often intact, and care should be taken to avoid negative remarks which the alert patient may hear. Envenomations are uncommon (11 cases had been reported up to 1983, including 2 fatalities) but may require supportive treatment including mechanical ventilation until the effects of the venom wear off. There is no antivenom available in Australia for blue ringed octopus envenomation.