The Deadly Beauty – Blue-lined Octopus

By Gomen See, Marine Science Team of Hong Kong Maritime Museum (HKMM)

Among the ocean’s most mesmerising creatures, the blue-lined octopus together with their cousins, blue ringed octopuses, stand out with their dazzling appearance and deadly secret. Found in tide pools and coral reefs across the Pacific and Indian Oceans, including Hong Kong, these tiny cephalopods—often no larger than a golf ball, around 10-20 cm long? —commands attention with its vivid, iridescent blue.

These patterns are not just for show. They serve as a warning signal to potential predators, including humans. Usually, the octopus will first mimic the surrounding environment to hide from other animals. However, when being noticed and feeling threatened, the octopus intensifies its patterns to scare away the predators. This visual cue is nature’s way of advertising the octopus’s most dangerous trait: its venom.

Despite its small size, the blue-lined octopus carries enough tetrodotoxin (TTX)—a neurotoxin also found in pufferfish—to kill an adult human. This venom is incredibly potent, blocking nerve signals and causing paralysis. What makes it even more alarming is that there is no known antidote. Victims may experience numbness, slurring of speech, and in severe cases, respiratory and/or heart failure which usually followed by death. When completely paralysed, the victim may still be conscious until shortly before death. 

Although the blue-lined octopus has such powerful weapon, it does not use it all the time, as each octopus only has a limited amount of toxin. TTX is a valuable resource to the octopus, and it is unaffordable for the octopus to bite every animal it encounters. That is why when facing potential predators, the octopus will first display its warning colour, the bright blue patterns. If it is not effective, it will have no choice but to deliver the deadly bite. So, next time when you meet a blue-lined or blue-ringed octopus, do not be afraid, you can keep a safe distance. Normally, it will not bite you unless you provoke it.

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Ocean Guardians Series - Embark on an underwater adventure in Hong Kong with Dr. Connie  

Follow Dr. Connie (Head of Marine Discovery Centre) and her marine friends as they dive into exciting underwater missions. Along the journey, Dr. Connie interacts with various local sea creatures and uses special magic tools to carry out rescue operations. The story highlights the impact of human activities on the marine environment, inspiring audiences to reflect and take action to protect our precious oceans.  

In this episode of Ocean Guardians Series, Dr. Connie encountered a Blue-lined Octopus on the run. However, with its camouflage skills, the octopus skilfully dodges and hides, leaving Dr. Connie, who just wants to pay a visit to her old friend, cannot find it too! Follow Dr. Connie and uncover the fascinating stories between her and Blue-lined Octopus!