The name on the masthead is a plain one. A ring octopus, in these notes, is a blue-ringed octopus: one of the small animals in the genus Hapalochlaena. At rest they are the color of wet sand and dull gold. Disturbed, they switch on a set of blue rings, or in one species a set of blue lines, and the meaning of that color is not ambiguous. It is a warning.
They are smaller than the octopus of most posters. An adult can sit in a person’s palm. The mantle, the bag of body behind the eyes, is often no bigger than a golf ball, and the whole animal, arms included, is commonly somewhere between about twelve and twenty centimeters. Weight is measured in tens of grams, not in kilos. None of that smallness makes the animal minor. The rings are there because the bite behind them is serious.
Their home water is shallow and warm-temperate to tropical, from Japan south and west through the Philippines, Indonesia, New Guinea, and northern Australia, and on around the southern and eastern coasts of Australia, with one poorly known species described from the Bay of Bengal. Tide pools, reef flats, rubble, and seagrass are the usual address. Empty shells, and sometimes a discarded bottle, will do as a door.
This journal is a set of field notes about that animal: the four named species, the way the color is made, the short life, the crabs they hunt, and the venom. It is written for a reader who wants the creature rather than the campfire version. The practical rule is short enough to put above every tide pool. Look. Do not pick it up.