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  • In 2009, some grey side-gilled sea slugs (Pleurobranchaea maculata) were found to be toxic following the deaths of several dogs on Auckland beaches. It was not previously known that these slugs contained tetrodotoxin (TTX) – the toxic substance that killed the dogs. The slugs are now considered New Zealand’s most toxic creatures.


    This slug can be up to 10 cm in length. It is pale grey and the mantle densely patterned with short, brown lines. It has rhinophores on the sides of the head. As its name suggests, the grey side-gilled sea slug has a feathery gill on the right side of its body. It has no shell, is soft bellied and slow moving. It is active when it is hungry – both day and night.


    These sea slugs are native to New Zealand and are found on both the North and South Island coastlines. Based on research conducted so far, it appears the North Island slugs are toxic in varying degrees, while the South Island ones are not. Scientists have yet to discover why. The slugs are found in a wide range of marine coastal habitats including rocky coastline. They are found subtidally to about 250 m water depth. They are most commonly observed during winter and spring when they lay egg masses.


    The grey side-gilled sea slugs are scavengers. Their diet includes sea anemones, marine worms and molluscs. They even eat each other!

    Toxicity of the slug

    Many species of slugs contain toxins to deter predators from eating them because they don’t have a hard shell for protection and are very vulnerable, but finding deadly TTX in this species was a surprise.

    Scientists from Cawthron Institute are working to determine the source of the toxin TTX. The source of TTX appears unrelated to the food chain – TTX has not been found in any of the sea creatures or algae that form the food chain the slugs are a part of. Scientists thought it may be produced by bacteria found in or on the slug, but the scientists have not yet been able to isolate any bacteria that produce the toxins. It is possible that the slugs are producing TTX themselves, and this is being investigated.

    Whatever the source, these slugs are deadly. Dogs can die from ingesting them and so can humans. One slug can contain enough TTX to kill at least four adults. The lethal dose of TTX to humans is 1–2 mg (about half a teaspoon of slug).

    Symptoms in humans from TTX poisoning include numbness and tingling around the mouth and nausea. Paralysis can occur. Medical attention should be sought immediately.

    It is not unusual to see slugs washed up on the beach. In the past, this has not been a concern, but with the discovery of TTX in these slugs, they should be avoided. The sighting of grey side-gilled sea slugs should be reported to your local council.

    The toxicity of these sea slugs is of national importance, and it may become an on-going issue. A number of organisations are involved in research and are monitoring these slugs, including the Cawthron Institute, the Auckland Council, the Auckland Regional Public Health Service, regional councils, Waikato University and the Hauraki Māori Trust Board.

    Nature of science

    The discovery of TTX in the grey side-gilled sea slugs in 2009 is an example of how science knowledge is not static but changes with time. It is also an example of how a number of scientists from different organisations collaborate to help communities monitor a potentially dangerous situation.

    Useful links

    View these video clips of the moving grey side-gilled sea slug.

    Listen to a RNZ audio clip about researching the grey side-gilled sea slug.

      Published 4 September 2012 Referencing Hub articles
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