British zoologist Guy Coburn Robson gave the genus its name in 1929. The animals themselves had already been noticed. The greater blue-ringed octopus, Hapalochlaena lunulata, was described by Quoy and Gaimard in 1832. The southern species, H. maculosa, and the blue-lined octopus, H. fasciata, were both named by William Evans Hoyle in the 1880s. H. nierstraszi waited until 1938, when William Adam described it from a single specimen.
Four confirmed species is the honest current list. It is not a claim that the ocean has been fully counted. Researchers have pointed to further blue-ringed octopuses that do not sit cleanly under those four names, still undescribed, still being sorted. A field guide that stops at four is telling the truth about the published names and staying quiet about work that is not finished.
The names are worth using when a photograph or a place makes them usable. Lunulata is the tropical animal with the larger, rounder rings. Maculosa is the southern Australian animal, often with more and smaller rings. Fasciata draws lines where the others draw circles. Nierstraszi is a name attached to very few animals and a great deal of uncertainty. If all you can see is the warning color and a shallow Indo-Pacific shore, “a blue-ringed octopus” is the accurate phrase, and it is not a lesser one.
The rest of this journal takes the four names one at a time, then goes back to the things they share: the rings, the dens, the short life, and the toxin.