Visitors to a popular beach in Sydney's north shore have been warned to watch out for Blue-ringed octopuses after several bites were reported in the last week.

Finally, why do octopuses have blue blood? Are you still wondering why octopus blood is blue and what the three hearts do? Well, the blue blood is because the protein, haemocyanin, which carries ...

That's the reality for those unlucky enough to be bitten by a blue-ringed octopus. Initially its bite is painless. But quickly the fast-acting toxin goes to work, paralysing its target.