Blue-ringed octopus

A small octopus, mantle to about 5 cm and around 12 cm overall, carrying roughly sixty iridescent blue rings. The rings are only fully displayed when the animal is aroused; at rest it is a drab mottled brown and very easily walked past. Against its congeners: the rings of this species are noticeably larger, 7 to 8 mm across, where the southern blue-ringed octopus has many small ones and the blue-lined octopus has elongated lines and chains rather than discrete rings. The species name needs care. Four species are described, around six more are undescribed and under study, and one of the four is known from two specimens in total. In Indonesian waters the honest field call is “a blue-ringed octopus” rather than a binomial.

Solitary and den-dwelling in rubble and sand, coming out to feed and to mate. Mating is unusually well studied: males mount indiscriminately and cannot tell the sex of another octopus at a distance, so male-female pairings run about 160 minutes while male-male encounters are broken off in about thirty seconds. The female lays 50 to 100 eggs, carries them in her arms for around fifty days, and dies. On the hazard: the venom is tetrodotoxin and it is real, concentrated in the posterior salivary glands, and there is no antivenom. It is also routinely overstated. The highest whole-animal load measured in this species came to roughly two per cent of a human lethal dose, and nearly every fatality on record involves Australian species and a handled animal. Published counts of those fatalities disagree with each other and none of them traces to a case review. A bite is typically painless. The practical position for a diver is simple and undramatic: the risk is close to zero unless the animal is picked up, and there is no reason to pick it up.