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The lesser (Southern) blue-ringed octopus (5 cm long and arms to 10 cm) is a little bit larger than the similar Greater blue-ringed octopuss (less then 5 cm long with. arms to 7 cm). The characteristic bright blue rings when alarmed, are smaller (2 mm) when compared wiith Greater blue-ringed octopuss (up to 8 mm).

Probably the easiest way to distinguish the two is the location where you see it. The lesser blue-ringed octopuss lives in the more cooler waters of Australia. Let's say at the height of Sydney and more south. The greater blue-ringed octopuss lives in the tropical waters from Queensland (Australia) up to the Philippines.