Not every member of the genus draws a circle. Hapalochlaena fasciata, the blue-lined octopus, carries iridescent blue lines along the body and arms. The warning is the same kind of signal, made of the same kind of structural color, arranged as a stripe instead of a ring. A person scanning a pool for “rings” can miss it, which is a poor reason to pick up a small striped octopus on an eastern Australian shore.
Hoyle named it in 1886. The accepted range is the coast from southern Queensland down to southern New South Wales, in shallow water. A total length around fifteen centimeters is a fair picture of the adult. Like the others, it is a soft-bodied hunter of small crustaceans, and like the others it would rather be in a crevice than in a hand.
The lines are a useful lesson in not letting a common name do all the identification. “Blue-ringed” is the phrase that traveled. The genus includes an animal that abandoned the ring and kept the blue. If the color flashes on, the instruction does not change because the geometry did. Leave it alone, and let the display finish its work, which is to end the encounter.
There is a further, more technical footnote. Work on animals close to this species has suggested that tetrodotoxin is not confined to the bite. Some related octopuses appear able to release toxin when they recognize a threat by sight or smell, without waiting to be touched. That is a research finding, not a reason to invent a cloud of invisible danger around every rock pool. It is a reason to stop treating the animal as safe until the moment of a bite.