[2] They can be identified by their yellowish skin and characteristic blue and black rings that can change color dramatically when the animal is threatened.

[3] Despite their small size—12 to 20 cm (5 to 8 in)—and relatively docile nature, they are very dangerous if provoked when handled because their venom contains a powerful neurotoxin called tetrodotoxin.

In the greater blue-ringed octopus (Hapalochlaena lunulata), the rings contain multi-layer light reflectors called iridophores.

[7] Similar to other Octopoda, the blue-ringed octopus swims by expelling water from a funnel in a form of jet propulsion.

In the southern blue-ringed octopus, body mass is observed to be the strongest factor that influences copulatory rates.

[10] The blue-ringed octopus, despite its small size, carries enough venom to kill 26 adult humans within minutes.

[12] The octopus produces venom containing tetrodotoxin, histamine, tryptamine, octopamine, taurine, acetylcholine, and dopamine.

The venom can result in nausea, respiratory arrest, heart failure, severe and sometimes total paralysis, blindness, and can lead to death within minutes if not treated.

[citation needed] The major neurotoxin component of the blue-ringed octopus is a compound originally known as "maculotoxin"; in 1978, this maculotoxin was found to be tetrodotoxin,[17] a neurotoxin also found in pufferfish, rough-skinned newts, and some poison dart frogs; the blue-ringed octopus is the first reported instance in which tetrodotoxin is used as a venom.

[18] Tetrodotoxin blocks sodium channels, causing motor paralysis and respiratory arrest within minutes of exposure.

[20] Estimates of the number of recorded human fatalities caused by blue-ringed octopuses vary, ranging from seven to sixteen deaths; most scholars agree that there have been at least eleven.

Even sensitive areas such as the Needham's sac, branchial heart, nephridia, and gills have been found to contain tetrodotoxin, and it has no effect on the octopus's normal functions.

First aid treatment is pressure on the wound and artificial respiration once the paralysis has disabled the victim's respiratory muscles, which often occurs within minutes of being bitten.

Because the venom primarily kills through paralysis, victims are frequently saved if artificial respiration is started and maintained before marked cyanosis and hypotension develop.

[25] Currently, the blue-ringed octopus population information is listed as Least Concern according to the International Union for the Conservation of Nature (IUCN).

This genera of octopus provides stability of habitat biodiversity as well as expanding the balance of marine food webs.