The blue ringed octopus though incredibly small are the most deadly of the cephalopods due to their highly toxic venom. There are multiple species of the blue ringed octopus. One of the most common is the lesser blue ringed octopus (Hapalochlaena maculosa). The lesser blue ringed octopus has a body up to 5cm long and arms up to 10cm long. The lesser blue ringed octopus’ name is derived from the size of the blue rings it creates when it is threatened. The size of the rings will generally be less than 2mm in diameter. The dorsal surface of the lesser blue ringed octopus has a rough appearance due to the irregularly arranged wrinkles. When the octopus is not under threat its colouration is beige with large light brown patches known as maculae. When the octopus is agitated these brown patches will darken dramatically and bright blue rings will appear within them and may even pulsate. There will typically be 50-60 blue rings covering the lateral and dorsal surfaces of the mantle.

Hapalochlaena maculosa can be found only in the temperate waters of southern Australia. They live in waters up to 50m deep. Another species of blue ringed octopus, the greater blue ringed octopus (Hapalochlaena lunulata) can be found in shallow reefs or tide pools from the northern regions of Australia to Japan and has even been found as far west as Sri Lanka. It can be found at depths between 0-20m.

The mating ritual for the lesser blue ringed octopus will begin when a male octopus approaches a female and then proceeds to caress her with his hectocotylus. The hectocotylus is a specially adapted arm. Once the male has finished caressing the female he will climb onto the back of her mantle. The hectocotylus is then inserted under the female’s mantle where the male will then release his spermatophores into the oviduct of the female. The male will die shortly after he has finished mating. The female will lay 50-100 eggs and will guard them by carrying them beneath her tentacles. This prevents the female from eating and so she will die shortly after the eggs have hatched. The eggs will usually take 50 days to hatch. When the eggs hatch the hatchlings will be about the size of a pea and will mature into an adult which is about the size of a golf ball. The hatchlings will mature very rapidly being able to mate the next year. Octopuses like squid and cuttlefish have a short life span of about 2 years. A study showed that male greater blue ringed octopus (Hapalochlaena lunulata) were not able to distinguish between males and females of the same species without the insertion of the hectocotylus under the mantle. The study recorded physical contact between pairs of octopus and copulation and also whether spermatophores were released. In the case of the male-male interactions 80% of the pairs made physical contact then proceeded to copulate. None of the male-male interactions had any spermatophores release recorded and the mounting male always removed his hectocotylus from the mantle of the other octopus and then dismounted. None of the male-male pairs were observed to show any aggressive behaviour to the other male before or after copulation.

Both the lesser and greater blue ringed octopus feed on small crabs and shrimp. The blue ringed octopus hunts during the day and uses the tetrodotoxin in its body to kill its prey. The method which the octopus uses to deliver the toxin is not known the octopus either bites its prey with its beak delivering the toxin in the saliva on the beak or the toxin is released into the water surrounding the prey in the octopus’s saliva. The tetrodotoxin in the blue ringed octopus is created by bacterium. Tetrodotoxin has been found to be present throughout the octopus’s body with the highest concentration in the arms.