Hapalochlaena lunulata is the animal most people picture when they hear the common name. It is called the greater blue-ringed octopus, which is a relative compliment. Greater, here, means the rings are larger and more circular than those of its southern cousin, not that the octopus is large. Adults are still a handful. The mantle is often under about five centimeters, the arms may extend the animal toward something like twelve centimeters, and published weights sit roughly between ten and a hundred grams, with an average near the middle of that range. Females run a little larger than males.
The range is the tropical Indo-Pacific. Northern Australia, Indonesia, Papua New Guinea, the Philippines, the Solomon Islands, Vanuatu, and west as far as Sri Lanka are the places repeatedly named, with the genus as a whole also reaching toward Japan. The depth is shallow. Reef flats, tide pools, sandy and silty pockets among coral, and clumps of algae, mostly from the surface down to about twenty meters, are the usual map.
At rest the skin is tan, yellow-brown, or a dull gold, and the rings are faint enough to miss. A short blue line often passes through the eye. Papillae can roughen the outline until the animal looks like a scrap of reef. When it is provoked the background shifts toward a brighter yellow and the rings become a hard iridescent blue. On this species the count is often given as fifty to sixty rings.
It is a daytime hunter of small crabs, hermit crabs, and shrimp, with the occasional small fish. It also has the ordinary octopus talent for a borrowed home: a crevice, an empty shell, a bottle, a can. None of that habitat notes is an invitation to turn the rubble over with bare hands. The warning display exists because the bite does.