Greater Blue-Ringed Octopus - Hapalochlaena lunulata
This Blue-Ringed Octopus - Hapalochlaena lunulata are small in size, up to 10 cm including the tentacles.
They are known to be one of the most venomous marine animals, with enough toxins to kill up to 20+ adult humans. However, that does not means they are dangerous. Being venomous and dangerous is two different things.
Like most cephalopods, they feeds mostly on crustaceans. And all crustaceans have hard shells and sharp claws. Cephalopods have soft body and when they hunt crustaceans, they also run a risk of being injured if they do not possess strong venom to immobilise the crustaceans. As such, they inject their strong venoms into the crustaceans to paralyse them within seconds, as a way to protect themselves, and once the prey is helpless and not moving, they can then slowly starts to feed on them.
In this regards, they are not likely to bite humans (scuba divers, snorkelers and beach goers) intentionally unless they are badly harassed and provoked. I have seen video footages of local kids who are not aware of their venoms, handling the Blue-ringed Octopus, holding it in their hands and throwing it at each other without getting bitten. Note, I am not condoning this actions, but just illustrating the false fears among those who are ignorant of this.
Check-out this video of local kids in Malaysia handling Blue-ringed Octopus :
The greater blue-ringed octopus is one of four species of venomous blue-ringed octopuses belonging to the octopodidae family.