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Blue-ringed octopuses are undeniably stunning. When they are alarmed, these animals will show off the eponymous iridescent blue rings that cover their body and arms.
But their adorably small size and Instagrammable appearance is deceptive: blue-ringed octopuses are some of the ocean's most toxic animals.
Blue-ringed octopuses are a group of highly venomous cephalopods.
These little octopuses have been found in the Pacific and Indian oceans. They typically live on coral reefs and rocky areas of the seafloor, though some may be found in tide pools, seagrass and algal beds. They are usually nocturnal, emerging at night to hunt crustaceans and sometimes small fish.
As their name suggests, these octopuses feature numerous blue rings on their bodies and arms. These rings are an example of aposematism, where animals use bright colours as a warning to would-be predators.
But unlike other aposematic animals such as poison dart frogs or velvet ants, which permanently display their bright colours, blue-ringed octopuses only show theirs off when they feel threatened.
Octopuses have thousands chromatophores under their skin. These are specialised cells that allow them to change colour instantaneously. Blue-ringed octopuses put on a threat display using these cells and iridophores, a specialised type of light-reflecting chromatophore that gives the blue rings their iridescence.
When alarmed or attacked, these animals quickly change colour. They use muscles to make their blue patterns appear, displaying them on top of a yellow or cream-coloured base with dark pigmented cells underneath the rings, which helps to intensify their colour.
Like lots of other octopuses, blue-ringed octopuses can also use their chromatophores to help camouflage themselves. They spend a lot of their time tucked away in crevices, hiding from animals that might try to eat them.
There may be around 10 different species of blue-ringed octopus, though only four have been given scientific names.
The greater blue-ringed octopus (Hapalochlaena lunulata) is about 12 centimetres long including its arms. This species prefers the shallows, up to 20 metres deep. It has been found in the waters around Indonesia, the Philippines, Papua New Guinea, Vanuatu and the Solomon Islands.
The Southern blue-ringed octopus (Hapalochlaena maculosa) is found up to 50 metres deep along Australia's southern coastline, including around Tasmania. It reaches about 22 centimetres long and its base can range from grey-green to cream, with 50-60 blue rings.
Despite its name, the blue-lined octopus (Hapalochlaena fasciata) is also a member of the group. It features iridescent blue lines on its mantle (the muscular structure behind the octopus's head, which holds all its organs), but it also has the signature blue rings on its arms. This species reaches 15 centimetres long and is seen in the waters off eastern Australia, from southern Queensland to southern New South Wales at depths of up to 20 metres.
The fourth species is Hapalochlaena nierstraszi, which has only been officially recorded twice: in the waters around the Andaman Islands in 1938, and in 2013 when one was caught in a trawling net near Chennai in southeast India. Little is known about this species, though the 1938 specimen had a 16-centimetre-long mantle.
The vibrant threat displays of blue-ringed octopuses aren't just for show.
Symbiotic bacteria in blue-ringed octopus salivary glands produce tetrodotoxin (TTX). This substance is potently neurotoxic, blocking the transmission of nerve impulses. This stops muscles from being able to contract and has potentially deadly consequences. Some reports state that TTX is over 1,000 times more toxic than cyanide.
TTX is found in a variety of animals including fish, amphibians and shellfish. Pufferfish are one well-known example. These fish can be eaten, but they sequester TTX in their internal organs, so if they aren't properly prepared, they can cause fatal poisoning.
Blue-ringed octopuses disperse TTX throughout their body. If they are eaten by another animal, the TTX acts as a poison. There has been at least one case of human poisoning caused by someone mistakenly eating a blue-ringed octopus, though a study found that swallowing TTX can make it about 50 times less toxic than when it is delivered through other means.
Blue-ringed octopuses can inject TTX too, delivering it as a venom via a near-painless bite.
Bites from blue-ringed octopuses are reasonably rare, but these animals do carry a dose of TTX that is lethal for humans. There have been a handful of fatal encounters with these animals.
TTX can take effect quickly, rapidly weakening and paralysing muscles alongside a host of other potential side effects, such as vomiting and dizziness. While increasingly unable to move, TTX victims generally remain conscious and aware until a lack of oxygen renders them unconscious. Death is usually by respiratory failure, the diaphragm having become paralysed. This can occur within a matter of minutes.
There is no antidote for TTX, and most care is supportive, such as ventilation to keep a patient breathing until the effects of the toxin wear off.
Experts aren't sure how many blue-ringed octopuses there are, and it's unclear whether their populations are in decline. While none of the species are formally considered to be at risk, human activities may still be affecting them.
One potential impact is to their key habitats. Blue-ringed octopuses are known to inhabit coral reefs, which are facing threats such as from warming waters, ocean acidification and pollution, to name but a few. Seagrasses are similarly in decline. The destruction of these habitats threatens the survival of the animals that rely on them.
Blue-ringed octopuses are also internationally traded to be kept in aquaria. While some of their range falls within Marine Protected Areas, these animals have been known to be harvested from the wild in areas where there can be very few regulations on collecting them. They also have short lifespans, living for 2-3 years, and reportedly don't fare well in transit.
Arguably, blue-ringed octopuses wouldn't make good pets. Like other octopuses, they are escape artists, able to get themselves out of even the most secure tanks. This would make them a risk to curious children and well-meaning people who might unwisely scoop them up to return them to their tank. This naivety is perhaps proved by multiple examples of people picking up wild blue-ringed octopuses and showing them off on social media, unaware that they are holding one of the ocean's most venomous creatures.
Blue-ringed octopuses are incredibly beautiful animals, especially when they show off their bright patterns. But despite their small size and pretty colours, it's important to remember that their flashy rings are not an idle threat. If you are lucky enough to see one in the wild, it's safe to watch them from a respectful distance, but you should never pick one up.